How to use

Choose what you want to find: seeds for the trays you have, or trays and seeds for a number of seedlings.
Pick the tray size by cells per tray, or choose "Other" and type the number of cells.
Enter the number of trays, or the number of seedlings you want.
Enter seeds per cell. One is enough for big, reliable seed; two or three for small seed.
Enter the expected germination rate. If you have done a germination test, use that figure.
Read the seeds needed and the seedlings to expect.

How it's calculated

Cells = cells per tray × trays, and seeds = cells × seeds per cell.
The chance that a cell gives at least one seedling is 1 − (1 − germination)^(seeds per cell). With 80% germination and 2 seeds, that is 1 − 0.2² = 0.96.
Expected seedlings = cells × that chance (you keep one per cell), rounded down.
Seeds expected to sprout = seeds × germination — this is before you thin to one per cell.
Reverse: cells to sow = seedlings wanted ÷ chance per cell, rounded up, trays = cells ÷ cells per tray, rounded up.

Worked example

Two 72-cell trays, 2 seeds per cell, 80% germination:

72 × 2 = 144 cells, 144 × 2 = 288 seeds
Chance per cell = 1 − 0.2 × 0.2 = 96%
Expected seedlings = 144 × 0.96 = 138.24 → 138, and about 6 cells may stay empty
Seeds expected to sprout = 288 × 0.8 ≈ 230

Reverse: you want 100 seedlings with the same settings. 100 ÷ 0.96 = 104.2 → sow 105 cells → 2 trays of 72 and 210 seeds, with 39 cells left spare for another crop.

Which tray size?

As a rough guide, bigger cells suit seedlings that stay longer in the tray or have bigger seeds: 32 or 50 cells for cucumber, gourds, okra, chili, tomato and brinjal; 72 or 105 cells for choy sum, pak choy, lettuce and kailan; 128 or 200 cells for small seedlings you transplant early. Small cells dry out quickly in our heat, so check them every day.

Tips

Label every tray with the crop and sowing date — many seedlings look alike at first.
Thin by snipping extra seedlings at soil level with scissors. Pulling them disturbs the roots of the one you keep.
Sow a few more cells than you need. Some seedlings are lost when thinning and transplanting, and spares let you pick the strongest.
Keep trays under a roof in heavy rain; big drops wash seeds out and waterlog the cells.

GuidesHow to Start Vegetable Seeds at Home (Beginner's Guide for Malaysia)Sow vegetable seeds in trays at home - the right mix, sowing depth, watering, light, preventing damping-off, hardening off and transplanting.

CalculatorsGermination Test & Seeds-to-Sow CalculatorTurn a simple paper-towel germination test into a germination rate, then work out how many seeds to sow for the plants you want and the chance each hole gets a seedling.

CalculatorsSowing Density & Seed CalculatorSowing density calculator: how many plants fit your bed or pot and how many seeds to sow for kangkung, choy sum, lettuce, chili, tomato and more.

FAQ

Why does the calculator say some cells will stay empty? Even good seed does not germinate 100%. With two seeds per cell, a cell is empty only when both fail. The calculator shows how many cells that is likely to be.

Should I sow 3 seeds per cell to be safe? For fresh seed with good germination, 2 is usually plenty. Use 3 for older seed or slow, uneven crops like coriander or chili, or for seeds you know germinate poorly.

What germination rate should I use? Use your own test result if you have one. If not, start with 80% and adjust after your first tray.

Can I put two seedlings in one cell and keep both? For most vegetables, keep one per cell so roots are not tangled at transplanting. Spring onion and some herbs can be grown as small clumps.