Why You Cannot Just Count Cells
If you want 72 tomato seedlings, sowing exactly 72 cells almost guarantees a shortfall. Two things eat into your count: not every seed germinates, and not every sprout survives transplanting. A typical fresh-seed germination rate is 80 to 90 percent, and transplant survival often runs 85 to 95 percent depending on how gently you pot up. Stack those together and a single seed per cell at 85 percent germination and 90 percent survival yields only about 77 usable plants per 100 cells.
How the Math Works
This calculator works backward from the number of plants you actually want. It accounts for sowing more than one seed per cell, which sharply raises the odds that at least one sprouts in each cell, then thins to the strongest.
per-cell success = 1 - (1 - germ%)^seedsPerCell
cells = ceil(target / (per-cell success x survival%))
trays = ceil(cells / cells-per-tray)
Picking a Tray Size
Standard 1020 trays come in common densities: 72-cell for veggies and flowers, 128-cell plug trays for high-volume starts, and 32- or 18-cell jumbo trays for plants that hate root disturbance like squash and cucumbers. Bigger cells mean fewer per tray but stronger, longer-lived seedlings. If you sow two seeds per cell at 85 percent germination, your odds of an empty cell drop from 15 percent to roughly 2 percent, so you can confidently fill far fewer cells to hit the same target.
Frequently Asked Questions
Should I sow one seed per cell or two?
Two seeds per cell is the safe default for most home gardeners, especially with older or bargain seed. It dramatically reduces empty cells, and you simply snip the weaker sprout at the soil line once both are up so you do not disturb the keeper's roots.
Where do I find my germination rate?
Seed packets usually print a germination percentage and a packed-for year. Fresh seed from a reputable supplier is often 85 to 95 percent, while seed more than two or three years old can drop well below 70 percent. When in doubt, run a quick paper-towel germination test on ten seeds.
What counts as transplant survival?
It is the share of healthy sprouts that make it through potting up or moving to the garden. Gentle handling, hardening off, and avoiding root-bound cells keep this above 90 percent. Rushed transplants, cold soil, or damping-off can pull it down toward 70 percent.
Why does the calculator round up to whole trays?
You cannot buy a fraction of a tray, so it rounds cells up to the next full tray and shows the spare cells you will have. Those extras are useful as backups or for succession sowing a week or two later.
Practical Guide for Seed Tray & Cell Count Calculator
Start by being honest about how many plants you truly have room for. Gardeners routinely sow three times what their beds can hold, then scramble to give away leggy seedlings. Working backward from a realistic target keeps your seed budget, shelf space, and light setup in proportion.
Germination and survival are levers you control. A seedling heat mat can lift germination of warm-season crops by 20 points or more, and bottom watering plus good airflow slashes damping-off losses. Every point you add to those rates means fewer cells to sow and less seed spent for the same harvest.
Build in a small succession buffer. Rather than over-sowing one giant batch, many growers fill their calculated cells now and reserve a partial tray to sow a week later. That second wave covers any early failures and staggers your harvest so everything does not ripen the same week.
Quick Checklist
- Measure your bed space first so your seedling target fits where it will actually grow.
- Check the germination percentage and pack date on each seed packet.
- Sow two seeds per cell for old or low-germination seed, then thin to one.
- Keep a spare partial tray for backups and succession sowing.