Seed packets give you a spacing figure and leave you to do the geography. This turns a bed measurement into a plant count: how many go along the row, how many rows fit across, and what that leaves you with in total — drawn out as the grid it will actually be, edges included. It also answers the question that follows five minutes later, which is whether buying a tray of 24 plugs or a packet of seed gets you closer to a finished bed.
The bed
Centre to centre, not the gap between leaf tips.
Plants against a path or a wall need room to grow outwards.
Result
What the calculator is counting
It builds an imaginary grid inside your bed. Along one edge it keeps the margin you asked for, then measures how many spacings fit across the remainder, and adds one — because a row with ten gaps in it has eleven plants, which is the single most common off-by-one in garden planning. It does the same down the length, and multiplies.
Everything follows from that pair of numbers. The total tells you whether the six-pack is enough. The density tells you whether the spacing you chose is realistic for what you are growing. The grid drawn in the result panel is the same answer as a picture, which is worth having because an arrangement that looks fine as two numbers can look very crowded as dots.
Spacing is a compromise with time
A packet that says thin to 20 cm is describing where the plant should end up, not where it should start. Most vegetables are sown closer than their final spacing and thinned, because germination is never complete and pulling every third seedling is cheaper than nursing gaps for a month. Understanding which of the two numbers you have been given changes the answer substantially.
The sowing distance is where seeds go in the ground. The final spacing is where the survivors should stand once thinned. If you enter the final figure and sow to it, you will have gaps wherever a seed failed, which is why sowing thinly and thinning properly beats spacing generously from the start.
If you are working out seed rather than plant numbers, the seed quantity calculator picks up from here and converts the spacing into grams, which is what you actually buy.
| Crop | Sow at | Thin to | Notes |
|---|---|---|---|
| Carrot | 2–3 cm | 5–8 cm | Do not transplant; thin in the evening to avoid carrot fly |
| Beetroot | 5 cm | 10 cm | Clusters of seeds, so several seedlings share one slot |
| Lettuce | 25 cm | 25–30 cm | Sow a pinch and thin to one plant |
| Cabbage | n/a | 45–60 cm | Almost always raised as plugs and planted out |
| Leeks | n/a | 15 cm | Grown in modules or a seed bed, then transplanted |
| Spinach | 5 cm | 15–20 cm | Cut-and-come-again can stay unthinned |
Square versus staggered
Put plants on a square grid and the distance between diagonal neighbours is 1.41 times the spacing. Offset every other row by half a spacing and the plants sit equidistant from six neighbours instead of four, which fills the space more evenly and leaves fewer visible channels of bare soil. The cost is around 14% more plants for the same bed.
This matters most for ground cover, borders and anything you want to knit together and hide the soil. It matters least for crops you need to walk between, because the row structure is doing useful work: it is where you hoe, water and harvest from, and a staggered layout takes that away from you.
The margin at the edge
Plants at the boundary behave differently from plants in the middle. Against a path they get brushed and trodden on; against a wall they get reflected heat and no rain; against a lawn they compete with grass roots for some centimetres sooner than you expect. The margin field exists because the obvious approach — filling the rectangle right to its edges — leaves you with a row that never thrives.
Getting the layout out of the calculator and onto the ground
- Measure the bed inside any edging or retaining boards, not outside them.
- Mark the two working dimensions with string lines or a sand line before planting anything.
- Count out your plants into rows before you start, so you discover a shortfall while standing up.
- Work from a board or plank if the soil is wet; kneeling repeatedly compacts exactly what you are about to plant into.
- If the shape is awkward, split it into rectangles and run the garden area calculator on each before you come back here.
Buying the right quantity
Plug plants and bedding are sold in trays of six, twelve, twenty-four or thirty, and seed in packets that rarely specify a count. Neither lines up with the number you worked out, so it is worth knowing which way to round.
Do
- Round up to the next tray, and use the spares for the gaps that appear in the first fortnight.
- Buy a few extra for anything you will be harvesting young and repeatedly.
- Check whether the plants are sold as a tray or loose before you work out the shortfall.
- Pot on any spares rather than squeezing them in — an overcrowded bed underperforms as a whole.
Avoid
- Ordering the exact number and discovering losses a week later.
- Planting a few too many in the hope of a fuller bed; they compete and none of them reach size.
- Ignoring the row when buying, then trying to fit thirty plants into a bed planned for twenty-four.
- Buying large plants to save time in a bed that was spaced for plugs.
A worked example on awkward ground
Here is the same calculation for a bed nobody can plan in their head: a corner plot that is a rectangle with a tree in it.
- Measure what is plantableThe plot is 6.0 m by 8.0 m, but a mature apple takes out a circle roughly 2.4 m across in one corner. Rather than subtract an awkward shape, treat the bed as a rectangle running past it: 6.0 m by 5.6 m of usable bed.
- Choose the crop and its spacingPerennial border stock at 40 cm between plants and between rows, offset, with a 25 cm margin at the edge.
- Across the bed(6.0 − 0.5) ÷ 0.40 = 13.75, which is 13 full spacings, so 14 plants across.
- Down the bed(5.6 − 0.5) ÷ 0.40 = 12.75, so 13 rows down it.
- Count and adjust14 × 13 = 182 plants square. Offset rows need roughly 14% more: 207. Sold in trays of 24, that is nine trays, with a handful left for losses in the first month.
Where the numbers stop being true
The grid assumes every position gets a plant and every plant survives. Neither happens. Expect a few percent of losses in transplanting even with careful handling, and more if the plants were rootbound in their tray or sat dry for a day before planting.
It also assumes a flat rectangle. Sloping ground has more actual surface than its plan footprint, so plant counts from a plan will undershoot slightly on a bank. And it assumes one crop over the whole bed: once you start interplanting a fast salad between slow brassicas, the useful number is not the count but how long each position is occupied, which no calculator can tell you from measurements alone. For that, the row spacing planner is the better frame, because it thinks in beds and pathways rather than points on a grid.
How the arithmetic works
Plants across: take the bed width, subtract twice the margin, divide by the spacing, take the whole number, and add one. Plants down: the same for the length and the row spacing. Total is the two multiplied, density divides that by the bed area. The staggered figure multiplies the total by 1.14, which is the practical difference between a square grid and equilateral spacing rather than an exact geometric result.
Ten gaps across a bed means eleven plants, because there is one at each end as well as one in each gap. This is why a 6 m bed at 30 cm is twenty-one plants across and not twenty: (6 − 0.4) ÷ 0.3 = 18.67, giving eighteen full spacings and nineteen plants.
Questions people ask
Should I measure centre to centre or edge to edge?
Centre to centre. Packet spacing always means the distance between one plant's growing point and the next, so measuring to the leaves will give you a bed that is too tight once anything has grown.
What if the bed is not a rectangle?
Split it. An L is two rectangles, a curve can be treated as the rectangle that contains it with a few plants knocked off, and a circle is a rectangle 78% of its width by the full diameter as a rough approximation. Run each piece separately and add them up.
Can I plant closer for a bigger harvest?
Up to a point, no. Closer planting means smaller individual plants, more competition for water and nutrients, and worse airflow, which is how fungal problems begin. The total yield per square metre often falls even though there are more plants in it.
Do plants against a wall need a bigger margin?
Usually yes. The soil there is drier, because the roof keeps rain off, and the wall reflects heat. Thirty centimetres rather than twenty is a reasonable starting point, and keep an eye on watering through the first summer.
What about square-foot gardening figures?
Those recommendations — one plant per square foot, four per square foot, and so on — already include their own assumptions about mature size. Enter 30 cm spacing for one per square foot and you will get a very similar answer, so either method works as long as you do not mix the two.