Seeds Needed Calculator (Bed Area & Spacing)
How many seeds or plants to sow for a bed or row from area, plant spacing and germination rate — with packets required, seeds per gram, and square-foot gardening equivalents.
How it works
Plants per row = floor(length ÷ spacing) + 1; rows = floor(width ÷ row spacing) + 1; seeds = plants × seeds per station ÷ germination rate
Seed packets are cheap but running out mid-row is annoying and over-sowing wastes plants and thinning time, so it pays to count. The number of final plants comes from the geometry — spacing along the row and between rows, with the extra plant at the end of each line — and the seed count inflates that for germination losses and for sowing two or three seeds per station to guarantee a plant at every spot. Germination rates on the packet are tested under ideal conditions; use 70–85% for older seed or cold soil. Square-foot gardeners will recognise the 1, 4, 9 and 16 plants per square that fall out of 30, 15, 10 and 7.5 cm spacings.
Worked example
A 3 × 1.2 m bed with plants 15 cm apart in rows 30 cm apart: 5 rows × 21 = 105 plants; at 85% germination sowing two seeds per station needs 248 seeds — one 250-seed packet.
Frequently asked questions
How much should I over-sow?
Two seeds per station for good seed, three for poor germinators like parsnip and carrot; for transplants, sow 20–30% more cells than plants you need.
Do I need the extra plant at the row end?
Only if you start planting right at the edge. Many gardeners start half a spacing in, which removes the +1 — the difference is small except on short rows.
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This calculator is for educational purposes. Double-check important results before acting on them.