Fertiliser is sold by weight and recommended by area, and those two numbers never meet on the pack: the back of the bag tells you grams per square metre, and nowhere does it tell you how many bags are in your year. Pick what you are feeding, give it the area, and this works out the annual tonnage and the whole number of bags — including how full the last one will be, which is what decides whether you actually need it.
The feed programme
Bed, border or lawn. Measure what gets fed, not the whole plot.
The figure on the back of the bag. The chips below set it if you have no label to hand.
The first of the three numbers: nitrogen, phosphate, potash.
Result
What the calculator works out
Three numbers, and only one of them is printed anywhere. The first is what a single pass over the ground weighs; the second is that figure multiplied by however many times you expect to feed in a year; the third is the number of bags you actually put in the trolley, which is the second figure rounded up past the bag size rather than rounded to it.
That last step is the only one most people get wrong, and it matters more than it sounds. A 4.2 kg year does not mean one 10 kg bag is wasteful and five 1 kg bags thrifty — it means you will either have half a bag left in the shed or be mid-season with nothing to hand. The shaded bag in the result panel is there so you can see exactly how much slack there is before you decide.
Reading the three numbers on the bag
Every compound fertiliser carries a label of three figures in the same order: nitrogen, phosphate, potash. A 10-5-5 product is 10% nitrogen by weight. That means 10 kg of it contains 1 kg of nitrogen and 9 kg of other material, most of which is there to make the nutrient spreadable rather than to feed anything.
A cheaper bag with a lower analysis is not cheaper per unit of nutrient. Two products both recommended at 35 g/m² deliver very different amounts of nitrogen if one is 20-5-5 and the other is 6-5-5: at that rate over 100 m², the first supplies 0.7 kg of nitrogen and the second 0.21 kg. The application rate is the same; the nutrition is not.
This calculator reports both, because the kg of product tells you what to buy and the kg of nitrogen tells you whether what you bought was sensible.
| Product | Typical analysis | Used for | Rate is usually |
|---|---|---|---|
| Lawn spring feed | 20-5-5 or 24-5-5 | Pushing leaf growth early in the season | 30–35 g/m² |
| Lawn autumn feed | 6-5-12 or 4-0-12 | Root and disease resilience, not top growth | 25–35 g/m² |
| General purpose | 7-7-7 | Mixed borders where nothing is being optimised | 50–70 g/m² |
| Rose and shrub | 10-7-10 | Flowering woody plants on a twice-a-year programme | 60–70 g/m² |
| Growmore | 7-7-7 | Vegetable beds before planting | 50–100 g/m² |
| Blood, fish and bone | 5-5-6 | Organic feeding, slower release | 50–70 g/m² |
Where the rate on the back of the bag comes from
Application rates are set so that a single pass supplies roughly the nutrient a crop can use in the interval before the next one, without pushing so much soluble salt into the soil that roots are damaged. That is why the number is not negotiable in the way people assume: doubling it does not double growth, it raises the salt concentration in the soil solution and burns fine roots.
Package rates assume an average soil. On light sand, nutrients leach through and a lower rate applied more often works better than one heavy pass. On heavy clay with plenty of organic matter, the same soil holds more and the standard rate is fine. Nothing about the label changes, so this is a judgement you make rather than one you read.
If you have never had the soil tested, the safe position is the lower half of the stated range applied closer together.
Granular, liquid or slow-release
The form changes how often you apply far more than how much you end up buying, and choosing between them is mostly a question about how much attention the plants are going to get.
The default
- Broadcast by hand or spreader, then watered in.
- Cheapest per unit of nutrient by some distance.
- Feeds over four to six weeks as it dissolves.
- Uneven spreading shows up as stripes of darker growth.
Fast and forgettable
- Applied with a watering can or a hose-end feeder.
- Available to the plant within days, useful for a crop in a hurry.
- Washes through quickly, so it needs repeating every two to four weeks.
- Easy to overdo because it feels weaker than it is.
Set and forget
- Coated granules that release with temperature and moisture.
- One application covers a whole season for containers and baskets.
- Most expensive per unit of nutrient.
- Release rate tracks warmth, so cool springs leave plants short early.
| Form | Applications per season | Cost per unit nutrient | Watch out |
|---|---|---|---|
| Granular | 2–4 | Lowest | Striping if spread unevenly |
| Liquid | 6–12 | Highest | Foliage scorch if applied in sun |
| Slow-release | 1–2 | High | Slow to start in a cold spring |
Feeding when the plant can actually use it
Nutrients do not wait politely in the soil. Nitrogen in particular moves with water, so a feed applied before the plant is growing, or before there is enough moisture to carry it to the roots, is largely gone before it can be used. Timing is why two gardeners following the same rate get different results.
Only once growth is visibly starting. Feeding dormant plants feeds the drainage, not the plant.
As leaves break and the soil has some moisture. This is the one pass that most rewards getting right.
Vegetables in active growth and containers, which have a limited volume of soil to draw from.
High-nitrogen feeds now push soft growth that the first cold night will damage. Move to the lower-nitrogen autumn products.
Woody plants need to harden off. Tomatoes, lawns and roses are the usual exceptions with their own late-season products.
How much nitrogen is too much
The failure mode of over-feeding is not obvious until it has happened: lush soft growth, a crop that looks better than ever and then lodges, fruits that split, lawns that need cutting twice a week and attract disease. It is far easier to add more later than to take it back out.
Leaf edges going brown and crispy while the middle of the leaf is still green is salt damage, not drought. Sudden wilting on a wet day is the same thing. A lawn that has gone dark green and then developed yellow patches in a stripe pattern is usually a spreader problem rather than a disease — the overlap strips got double the rate.
There is no product that undoes it. Water heavily to dilute, stop feeding, and wait.
Even spreading beats extra quantity
Most feeding failures in a real garden are distribution failures. The difference between a border that got an even 50 g/m² and one that got 100 g/m² in the overlaps and nothing in between is not two different feeding programmes; it is the same amount applied badly. The calculator tells you the total, so it is worth being honest about how the total gets down.
Getting an even pass
- Split the quantity in half and apply it in two passes at right angles to each other.
- Measure the area rather than guessing it, ideally with the garden area calculator if the shape is anything other than a rectangle.
- Weigh or scoop into a marked container so each pass uses a known fraction of the total.
- Keep off the edge. Fertiliser thrown onto paving or into a drain is both wasted and a pollutant.
- Water it in if no rain is due, or the granules sit where they landed and scorch the leaves they touch.
Do
- Apply to moist soil, or water it in afterwards.
- Keep the product off leaves and off the crown of the plant.
- Sweep or wash any granules off hard surfaces immediately.
- Store the remainder sealed and dry so it does not set into a brick.
Avoid
- Doubling up because the plants looked pale last week.
- Applying in the middle of a hot day with foliage wet.
- Feeding into a dry, cracked surface and waiting for rain.
- Putting the same product on the same bed year after year without ever testing the soil.
A second worked example: four vegetable beds
Round numbers make the arithmetic look trivial. Here is a case with a real shape to it, where the beds are separated by paths and the answer is not simply the area of the plot.
- Measure the beds, not the plotFour beds, each 1.2 m by 2.4 m: 2.88 m² each, so 11.52 m² in total. The paths between them are not fed, so they never enter the calculation.
- Set the rate for hungry cropsGeneral purpose at 70 g/m²: 11.52 × 70 = 806 g per application, which is 0.81 kg.
- Multiply by the seasonVegetables on a three-application programme: 0.81 × 3 = 2.42 kg for the year.
- Check what nitrogen that actually isThe product is 7-7-7, so 2.42 × 0.07 = 0.17 kg of nitrogen across the season. For a plot this size that is a sensible amount, not a generous one.
- Convert to bagsIt is sold in 5 kg sacks. 2.42 ÷ 5 = 0.48, which rounds up to 1 bag — but that bag will be less than half used, which is exactly the situation where buying the smaller size or splitting with a neighbour is worth considering.
Storing what is left over
Fertiliser does not expire in the way seed does, but it does degrade into something much less useful: a bag left open in a damp shed absorbs water, the granules fuse, and what you are spreading next spring is a fraction of the analysis on the label.
Keep it sealed, off the floor and away from anything that gets damp. A plastic bin with a clip lid is enough. If it has already set solid, breaking it up works but the particles will not be uniform any more, so expect uneven spreading.
Never decant it into something that once held food or drink, and do not mix half-used bags of different products in one container — you no longer know the analysis of what you are applying.
How the arithmetic works
Per application: area in square metres, multiplied by the rate in grams per square metre, divided by 1,000 to get kilograms. Across the year, multiply that by the number of applications. For nitrogen actually applied, take the year figure and multiply by the first label number as a percentage. For bags, divide by the bag size and round up to the next whole one; the shaded figure is how full that last bag will be.
bags = ceil(year kg ÷ bag kg). The result never rounds down, because a plan that leaves you half a bag short in June is not a plan. The fill percentage on the last bag is there so you can see the size of the gap: at 95% you are choosing convenience, at 40% you should probably buy the smaller size.
Where this estimate stops being reliable
It assumes the rate you entered applies to your ground and your crop, which is the manufacturer's assumption rather than your own measurement. It does not account for what is already in the soil: a bed that has had compost worked into it every year for a decade may need nothing at all, and a tired one may need feeding twice as often at the same rate.
It also treats boundaries as clean. Plants at the edge of a bed draw from outside it, rows interplanted with a second crop share the same soil volume, and anything under glass behaves differently again. For planning a purchase these simplifications are fine. For diagnosing why a particular plant is not thriving, a soil test is worth more than any amount of arithmetic — and the compost bin volume calculator is the better next step if what your soil actually lacks is organic matter rather than nutrient.
Questions people ask
Can I use lawn feed on the vegetable beds?
Not usefully. A spring lawn feed is a high-nitrogen product designed to push leaf growth, which is exactly what you want on grass and less useful on a plant you want to fruit. The plants will not be harmed, but a general-purpose or tomato feed suits the crop better.
What happens if it rains right after I feed?
A light shower is helpful, because it carries the granules into contact with the soil. A week of heavy rain is not: soluble nitrogen moves down through the root zone and out. If the forecast is for persistent rain, wait — applying afterwards costs you nothing but a day.
Why does one brand recommend twice what another does?
Because the analyses differ. A 20-5-5 product at 35 g/m² and a 10-5-5 at 70 g/m² supply the same nitrogen per square metre. Always compare the nitrogen figure rather than the weight on the bag, which is what the fourth number in this calculator exists to tell you.
Should I feed before or after planting out?
Before, for most beds: work a base dressing into the top few centimetres a week or so ahead of planting, then top up through the season once the plants are established. Feeding into a planting hole puts a concentrated dose against bare roots, which is the classic way to lose a seedling you have already paid for.
Is more frequent feeding better than one big dressing?
Almost always. Three modest applications matching the crop's growth curve deliver more usable nutrient and less waste than one Dressing at three times the rate, because the soil can only hold so much in solution at once and the surplus simply leaves.
Do I need to feed at all if I mulch every year?
Mulch contributes organic matter and some nutrient as it breaks down, and it improves everything about how the soil behaves, but it is not reliably balanced. Most mulched beds need less fertiliser rather than none. The mulch volume calculator works out how much material that takes, if you are doing both.