Seed packets quote a sowing rate in grams per square metre and a germination percentage, and neither one tells you how many bags to put in the trolley. This is a sowing planner: pick what you are sowing, give it the size of the ground, and it converts the packet’s small print into a whole number of packs — with the germination shortfall already built in.
Sowing planner
Purpose, then ground, then packs. Germination is already factored in.
1 · What are you sowing?
2 · How much ground?
3 · Sowing rate from the packet
4 · Germination on the packet
5 · Pack size you are buying
Density preview
One dot stands for a pinch of seed.
Why the packet rate is not the amount to buy
A sowing rate answers "how much seed goes on a square metre". It does not answer "how much of that seed becomes a plant". Germination is the gap between the two, and it is the number most people skip. At 85% germination you need roughly 18% more seed than the raw rate implies, because one seed in six will not come up.
The planner applies that gap as a division rather than a multiplication, so the answer can never come out below the label rate. It then rounds up to whole packs, because seed is sold in packs and not in grams. The strip of pack icons shows the rounding: solid packs are full, the dashed one is the part pack you carry home.
Getting the area right before you buy
The seed maths is simple enough that the area is where the error gets in. Two people measuring the same lawn rarely agree within 10%, and the difference between a 45 m² lawn and a 52 m² lawn is a whole extra kilogram of seed at lawn rates. The point is not to survey the garden like a boundary dispute; it is to be consistent, and to be honest about what you are not sowing.
Do
- Walk the length and the width and multiply. A stride of about 0.75 m is close enough for a first pass; you are choosing a packet size, not setting out a foundation.
- Sketch the shape and break it into rectangles before you touch a calculator. An L is two rectangles, a curve is the rectangle that contains it.
- Subtract what you are not sowing: paths, the shed footprint, the border that is already planted, the sandpit. Most gardens have 10–20% that is simply not grass.
- Round the area up to the next whole square metre, then leave the waste allowance to do its own job.
Avoid
- Using the plot size from a property listing. That number includes the house, the drive and the garage.
- Measuring the whole garden when you are only sowing one section of it.
- Measuring a slope on the flat. A 1-in-5 slope has about 2% more actual surface than its plan footprint, which is small, but the run-off is what kills seed on a bank.
- Doubling the rate to be safe. Overcrowded seedlings do not die individually; they produce a weak stand that lodges and then fails as a patch.
Most under-ordering starts with the area, not the rate. If you are working from a drawing or a property plan rather than from the ground itself, the garden area calculator handles the awkward shapes, and the lawn area calculator is quicker when it is one simple patch of grass. Either way, get this number right before you worry about the germination figure, because a 20% error in the area costs you 20% whatever the rate is.
If you are replacing a lawn rather than sowing one, the decision is seed or turf, and the arithmetic is quite different: turf is priced by the roll and laid in a day. The sod roll calculator runs that comparison, and the raised bed soil calculator covers the case where you are filling a bed rather than sowing into ground that is already there.
What the sowing rates actually mean
A rate is a target number of seeds per unit of ground, expressed in weight because nobody counts grass seed. That is why the same crop can be quoted at 15 g/m² by one supplier and 25 g/m² by another without either being wrong: the seed size differs, and so does the purity.
Sowing rates cross borders badly. An imperial packet quoted at 1 oz per 1,000 ft² is asking for about 3.05 g/m²; 2 oz per 1,000 ft² is roughly 6 g/m²; and 1 lb per 1,000 ft² is about 49 g/m². If you are reading a US packet over a metric garden, divide the oz figure by 1,000 and multiply by 3.05 rather than guessing that the numbers carry over.
The calculator takes either: switch the unit toggle and it converts both the area and the rate, so you can work in whichever system the packet is printed in.
| Use | Rate | Why it differs |
|---|---|---|
| New lawn from seed | 25–35 g/m² | Bare ground needs a full stand to close over |
| Overseeding an existing lawn | 15–25 g/m² | There is already grass there to fill the gaps |
| Wildflower meadow mix | 3–5 g/m² | Large seed, low density, wants air between plants |
| Shade mix | 30–40 g/m² | Thinner light means weaker establishment |
| Hard-wearing family lawn | 35–45 g/m² | Density is what survives traffic |
These are starting points drawn from typical packet guidance, not universal constants. Brands differ, and coated seed weighs more for the same number of seeds, so always defer to your own packet. Once the grass is up, the follow-on question is feeding it, which the fertilizer calculator handles by area and application rate.
Why a wildflower mix needs a fraction of the seed
The reason a meadow mix is sown at a tenth of a lawn rate is arithmetic about seed size. A rate expressed in grams is a proxy for a number of seeds, and the number of seeds in a gram varies by more than an order of magnitude between species. Sowing the same weight of a large-seeded wildflower mix as you would of ryegrass is not a generous margin; it is ten times the intended plant population.
The jump from 30 g/m² for a lawn to 4 g/m² for a meadow looks like a misprint, and it is the single thing that makes people distrust the packet. It comes down to how many seeds are in a gram. Grass seed is small and fine; a gram of perennial ryegrass holds roughly 500 individual seeds. Meadow mixes contain larger, heavier seed — a gram of a typical wildflower mix might hold 50 to 150 seeds depending on how much inert carrier is blended in.
| Seed type | Approximate seeds per gram | Consequence |
|---|---|---|
| Perennial ryegrass | 400–600 | Dense sward from a modest weight |
| Fine fescue | 800–1,200 | Very fine, goes further per gram |
| Meadow grass mix | 300–700 | Blended, so treat the packet as the reference |
| Wildflower meadow mix | 50–200 | Large seed, so a little weight covers a lot of ground |
| Coated or pelleted seed | Roughly halved | The coating adds weight but not seeds |
This is also why you should never swap one mix's rate onto another packet. Coated seed in particular can weigh nearly twice as much for the same number of seeds, so a rate that worked last year can leave this year's sowing thin. When in doubt, count out five grams, count the seeds, and compare against the figures above.
Sowing method changes what you actually need
The same seed sown two different ways gives two different results, so the waste allowance is not a single number you can look up. Broadcast by hand on a breezy day loses more than a spreader does; a drill meters seed into a slot and loses least of all. If you are honest about which one you are doing, the allowance stops being a guess.
Before you open the first packet
- Firm the surface and break up the top 20–30 mm. Seed needs contact with soil, not with a crust.
- Remove stones and the previous crop's thatch. Seed resting on dead material germinates and then dies.
- Check the forecast for a settled few days: heavy rain in the first 48 hours moves seed into the hollows.
- Decide how you will sow before you measure. The method sets the waste allowance, and the allowance is applied to a number you have not worked out yet.
- Mark out the area physically — string, sand, or a hose on the ground. It makes the subtraction step obvious instead of theoretical.
Packet rates assume broadcast sowing: seed scattered evenly across the whole surface. That is the most wasteful method, and it is the right baseline because it is what most people do. Change the method and the quantity changes with it.
- Broadcast by hand is the reference case. Expect to use the full rate, and sow half in one direction and half at right angles, or you will get stripes.
- Broadcast with a spreader distributes more evenly, so you can sit at the lower end of the range. Set the spreader to half the rate and make two passes crossing over.
- Drilled rows concentrate seed where the plants will be, so the effective rate drops sharply — often by half for vegetables and annuals.
- Cell trays and plugs use the least seed of all by weight, because you sow one or two seeds per cell and discard the failures.
- Hydroseeding and other slurry methods sit near the label rate but add a carrier, which changes how the seed settles on a slope.
The planner assumes the broadcast case, which is why it can look generous if you are drilling rows. That margin is deliberate: broadcast sowing loses seed to wind, birds and uneven spread, and the cost of a thin lawn is a second season of work.
Seed, turf or plugs: three routes to the same lawn
Seed is the cheapest route and the slowest, and that trade-off is worth stating plainly before you buy anything. The calculator on this page is about the seed route; if you are weighing the three up, the honest comparison is not price per packet but price per square metre of usable surface, plus how many weeks of watering you are signing up for.
Cheapest, slowest, neediest
- Lowest cost per m² by a wide margin, especially above about 40 m².
- Four to eight weeks before it carries foot traffic, longer in cool weather.
- Needs the surface kept damp every day for the first two to three weeks.
- Best when the ground is prepared properly and you can keep off it.
Instant, expensive, unforgiving
- A finished lawn the day it is laid.
- Several times the cost of seed, and it must be laid the day it arrives.
- Needs the same daily watering for the first fortnight to root in.
- Best for small areas, slopes that would wash seed away, or when the lawn is needed immediately.
The middle path
- Plants spaced 15–30 cm apart that knit together over a season.
- Costs more than seed, less than turf; fewer failures on difficult ground.
- Weeds have room to move in during the gap between plugs.
- Best for warm-season grasses and for repairing worn patches.
| Route | Cost index | Time to walk on | Where it wins |
|---|---|---|---|
| Seed | 1.0 | 4–8 weeks | Large areas, prepared ground, flexible timing |
| Plugs | 2.5–4.0 | 10–16 weeks | Warm-season grasses, erosion-prone edges |
| Turf | 6.0–10.0 | Immediately | Small areas, slopes, deadlines |
If you go the turf route, the arithmetic changes shape entirely: you are counting rolls, not grams. The sod roll calculator works that out from the same two measurements, and it has its own waste allowance because cut offcuts at the edges are a much bigger loss than a few stray seeds.
Spacing is a separate decision from quantity
Quantity and spacing get confused constantly. Sowing rate controls how much seed goes down; spacing controls where the resulting plants end up. For a meadow or a vegetable bed you usually care about the second, and thinning afterwards wastes the seed you over-bought. Work the layout out first with the plant spacing calculator, or with the row spacing planner for drilled rows, then come back here for the weight.
Timing matters more than the last 5%
A perfect quantity sown into cold or bone-dry ground still fails. Seed needs warmth and consistent moisture to germinate, which is why early autumn and spring are the usual windows. If you are unsure whether your date is realistic, check it against the frost date and seeding planner before you open the packet — seed sitting in cold wet soil rots rather than waits.
Mixing species without wrecking the rate
Blends are where the rate-on-the-packet stops being a single number. If you mix two species half and half by weight, the rate you should sow is not the average of their recommended rates — it is the weighted blend of them, and the weighting is by the proportion of seed in the mix, not by the proportion of the final sward you want.
Practically: work out how many grams of each component you are putting on per square metre, add those, and use the total as the rate. A mix that is 70% ryegrass at 30 g/m² and 30% fescue at 20 g/m² is sown at 21 + 6 = 27 g/m², not at the 25 g/m² you would get by averaging the two recommendations.
| Component | Share of mix | Own rate | Contribution |
|---|---|---|---|
| Perennial ryegrass | 70% | 30 g/m² | 21 g/m² |
| Red fescue | 30% | 20 g/m² | 6 g/m² |
| Blend total | 100% | — | 27 g/m² |
| For 100 m² | — | — | 2,700 g |
Two cautions. First, the germination correction applies to the blend as a whole, and if the components were bought in different years, use the worse figure — germination in a blend is governed by the weakest component, because that is the one that leaves the gap. Second, species that need different sowing depths should not be blended and sown in one pass; the small-seeded component will end up too deep and the large-seeded one too shallow. For those, sow separately, which means running the plant spacing calculator for each and treating them as two jobs on the same ground.
Measuring ground you can walk across
You rarely need a tape measure to get an area good enough for buying seed. A comfortable adult pace is close to 0.75 m, so four paces is about three metres. Walk the length and the width, count the paces, and you have the two numbers the planner wants. The error from pacing is usually a few percent, which matters far less than the difference between sowing at 85% germination and sowing at 60%.
Shapes that are not rectangles are worth a minute of thought. An L-shaped lawn is two rectangles added together. A bed with a curved edge is best treated as the rectangle that contains it, because the seed you save by trimming the curve is less than the seed you lose to a thin edge. A circle is the radius squared times 3.14. A slope is longer than it looks on a plan — measure down the slope, not across it on the flat, or you will under-order by the difference.
Subtract what is not being sown: an established border, a path, a shed base, a tree canopy that will shade grass out anyway. People rarely bother, and then wonder why they have half a pack left over. On a typical suburban lawn the non-lawn area is 10 to 20% of the plot, which is a whole pack on a medium job.
The six weeks after sowing decide the result
The quantity decision is made before you sow, but whether it was the right quantity only becomes visible weeks later. Most people who conclude they under-ordered actually lost the seed after it went in the ground: dried out in the first fortnight, washed into a hollow, or eaten.
Light watering two or three times a day in warm weather. A single heavy soak is worse than three light ones: it crusts the surface and moves the seed.
Ryegrass is up in 5–10 days, fescues take 14–21. Do not conclude the seed has failed because nothing has moved by day nine.
A light starter feed at half the rate you would use on established grass. The fertilizer calculator gives the quantity for the area you actually sowed, not the whole garden.
Cut at 6–8 cm and take off no more than a third of the blade. Cutting earlier or lower pulls seedlings that have not rooted properly.
Patch the thin spots at about half the original rate. Full-rate patching into existing grass crowds both the new seedlings and the old plants.
Germination is not instant and it is not uniform. Ryegrass comes up in seven to ten days in warm soil; fescues and meadow species can take three weeks or more and are easy to mistake for failure. Until the seedlings are established, the surface needs to stay damp — a dry day in the first fortnight kills more seed than any amount of under-ordering.
Water lightly and often rather than heavily and rarely. A crust forms on the surface if it bakes between soakings, and seedlings cannot push through it. If you used a mulch or a light covering of soil, that crust is less likely. Keep off the area until the first cut, which is usually when the grass reaches 5–7 cm, and set the mower high for the first two or three cuts so you are not pulling young plants out.
Bare patches after six weeks are a distribution problem rather than a quantity problem in most cases. Rake the patch, reseed at the overseeding rate rather than the full rate, and keep it damp. This is what the leftover pack is for, and it is a good reason not to be too precise about buying exactly the calculated amount.
Testing germination before you commit
The germination figure on a packet is a laboratory result from when it was printed. If the packet has been open in a shed for two summers, that number no longer describes what is in the bag, and no calculator can recover it. The test takes ten minutes and a sheet of kitchen paper, and it turns the most uncertain input in the whole calculation into one you measured.
- Count out 20 seedsTake them from the middle of the bag, not the top, and do not pick the plump-looking ones.
- Lay them on damp kitchen paperFold the paper over the seeds, slide it into a resealable bag, and leave it somewhere at room temperature out of direct sun.
- Wait the packet's stated germination timeTypically 7–21 days depending on species. Check every few days that the paper has not dried out.
- Count what has a rootIf 14 of the 20 have germinated, you have 70% — put that number into the calculator instead of the printed figure.
- Decide from the gapA result within about 10 points of the packet is normal variation. Twenty points below it means either a bigger order or a fresher bag.
Seed loses roughly 10–20% germination per year in ordinary shed storage, and much faster in heat or damp. Sealed, cool and dry is the whole prescription; a sealed container in a refrigerator is better still for small quantities left over from a big job. Label the bag with the year you bought it, because an unlabelled packet is an unknown germination rate wearing a printed one.
What the seed actually costs
Two things surprise people about seed pricing: the per-gram price moves in the opposite direction to the packet size, and the cheapest option on the shelf is frequently the most expensive per square metre once you account for what does not grow.
Seed is not priced linearly. A 500 g pack might cost two and a half times a 250 g pack, and a 5 kg sack might cost six times a 1 kg pack, so the cheapest way to buy is not always the size your calculation lands on. The planner shows the leftover precisely so you can judge this: if it reports three full packs plus one at 20%, a smaller pack size may cost more per kilogram but less in total. It is worth pricing both before you commit.
There is also a shelf-life argument against buying the largest sack. Germination falls year on year, and a 5 kg sack that takes three seasons to use is worse value than fresh 1 kg packs, even at a higher price per kilogram, unless you are sowing a genuinely large area in one go.
Where this estimate stops being reliable
- Germination on a packet is a laboratory figure. It falls with age, and faster in warm or damp storage.
- Soil temperature, moisture and time of year change establishment far more than the last few grams per square metre.
- Mixes containing very different seed sizes use a compromise rate: the small seed is over-sown and the large seed under-sown.
- Overseeding into an existing sward is governed by competition with the grass already there, not by the rate.
- Birds, surface wash on a slope, and a crusted surface all remove seed after you have sown it.
A second worked example: the awkward corner bed
Rectangles are easy. The case that actually goes wrong is the L-shaped bed with something already growing in it, where people multiply two numbers, forget the third, and then buy for the wrong area. Here is that case worked through end to end.
- Split the shape into two rectanglesThe long leg is 8.0 m by 2.4 m: 19.2 m². The short leg is 3.6 m by 1.8 m: 6.48 m². Together that is 25.68 m², which rounds to 26 m².
- Subtract what is already plantedThere is an established shrub in the corner with a clear mulched circle about 1.2 m across. Treated as a square that is 1.44 m², leaving 24.2 m² to sow.
- Apply the rate for what you are sowingIt is a wildflower and grass mix at 4 g/m²: 24.2 × 4 = 96.8 g.
- Correct for germinationThe packet says 72%: 96.8 ÷ 0.72 = 134.4 g. This is the step people skip, and it is the difference between a bed that fills in and one that looks sparse for a year.
- Add the waste allowanceBroadcast by hand on a windy day, so 8%: 134.4 × 1.08 = 145.2 g.
- Convert to packsThe supplier sells 100 g packs. 145.2 ÷ 100 = 1.45, which rounds up to 2 packs. The half-empty second pack is not waste if you store it well — see the germination section below.
Notice that the two adjustments people argue about — germination and waste — moved the answer by about 50 g between them, while forgetting the shrub would have moved it by 4 g. The area is still the dominant term, and the area is the part you can get right with a tape measure.
How the arithmetic works
Seed at the label rate is area multiplied by rate. The adjusted figure divides by the germination fraction, which can only ever increase it. Packs are the ceiling of the adjusted weight divided by the pack size, so you always have enough. The effective rate divides the adjusted weight back by the area, and is the number worth comparing against your packet when the result looks surprising. Everything runs in your browser; no input is sent anywhere.
Questions people ask
Can I use seed left over from last year?
Yes, but expect germination to have dropped, particularly if it was stored in a shed that gets warm. Raise the rate toward the top of the range, or test a small patch first and count what comes up.
Why is my lawn patchy when I sowed the right amount?
Distribution and moisture, almost always. Sow half the seed walking one way and half at right angles, rake it in lightly, and keep the surface damp until the seedlings are established. The right quantity sown unevenly produces the same patchiness as too little seed.
Should I sow heavier for a shady lawn?
Yes, and use a mix bred for shade. Shade mixes sit at the top of the rate range because weaker light means more seedlings fail to establish. Sowing a standard mix thickly in shade does not fix the underlying problem.
Is a wildflower mix really sown that thinly?
It is, and this is the most expensive mistake in the category. Applying a lawn rate of 30 g/m² to a meadow mix is roughly eight times too much seed: the seedlings crowd each other, the tall species collapse, and you get a thin, disappointing stand from a great deal of seed.
Do I need to rake the seed in?
A light raking to about 5 mm, or a thin covering of sieved soil, improves contact with the ground and stops birds taking it. Burying it deeper than about 10 mm prevents most grass seed from emerging at all, so shallow is right and deep is worse than leaving it on the surface.
How much should I add for a slope?
Measure down the slope rather than across it on the flat, which gives the true surface area. On anything steeper than about one in five, also expect some seed to wash before it roots — sow at the top of the range and consider a light mulch or an erosion blanket rather than simply sowing more.
Can I mix two part-used packets?
Yes, as long as they are the same type of mix and both are reasonably fresh. Germination of the blend will be somewhere between the two figures, so use the lower percentage in the planner — it errs on the side of sowing slightly more.
Can I sow into existing grass without digging it up?
Yes — that is overseeding, and it uses roughly half the rate of a new lawn, because you are thickening rather than establishing. Scarify or rake first so the seed reaches soil, and accept that the result depends more on that contact than on how much you put down. Sowing a full new-lawn rate into existing grass mostly feeds the established plants.
Why does the packet's coverage figure disagree with the rate?
Coverage on a packet is a marketing number calculated at the lowest rate anyone would reasonably use, on perfectly prepared ground, with fresh seed. The rate is the honest figure. If the two disagree, work from the rate and the area you measured — the coverage claim is what you get if everything goes right, and the calculator is what you need if it does not.
Should I sow more on a slope?
Not much more, and not for the reason people think. A 1-in-5 slope only adds about 2% of actual surface, so the area barely changes; the loss is from seed washing down before it roots. Fix that with a light raking-in, a mulch or an erosion blanket rather than by raising the rate, which mostly feeds the bottom of the slope.