Not every seed sprouts, and not every seedling makes it to the garden. Sowing a few extra keeps you from coming up short. Enter how many tomato plants you want, and this calculator works out how many seeds to sow after allowing for germination and seedling losses.
Try a preset:
Your entries stay in your browser. Nothing is sent to a server or saved to an account.
What Your Result Means: the number is how many seeds to sow so that, after some fail to sprout and a few seedlings are lost, you still end up with the plants you want. Fresh seed with a high germination rate needs little padding; old seed needs more.
How Many Tomato Seeds Should You Sow?
Sow a few more than the plants you want, because some seeds will not sprout and a few seedlings will not make it. Enter your target number of plants and, if you know them, your germination and survival rates, and the tool works out the seeds to sow.
- Enter Plants Wanted, the number you plan to grow on.
- Set the Rates, germination and seedling survival, or use the defaults.
- Read the Result, seeds to sow and cells to fill.
What Each Result Means
| Result | What It Tells You |
|---|---|
| Seeds to sow | How many to plant to hit your target |
| Cells to fill | One seed per cell, or fewer cells at two seeds each, thinned to one |
| Rates used | The germination and survival you entered |
Seeds by Germination Rate
Seeds to sow for six wanted plants at 90 percent seedling survival.
| Germination Rate | Seed Quality | Seeds to Sow for 6 Plants |
|---|---|---|
| 90 percent | Fresh, up to about 1 year old, stored cool and dry | about 8 |
| 75 percent | About 3 to 4 years old, or 2 to 3 years at room temperature | about 9 |
| 55 percent | About 5 to 6 years old, or poorly stored | about 13 |
How Is the Seed Count Worked Out?
The tool divides your target plants by the germination rate, then by the seedling survival rate, and rounds up to a whole number of seeds.
Single-sow: one seed per cell, so cells = seeds to sow
Double-sow cells = plants wanted / (1 minus (1 minus germination rate) squared) / survival rate, rounded up; sow two per cell and thin to one
Dividing by each rate builds in the losses at both stages, so the seeds that do sprout and survive still meet your target.
Worked Example
Say you want 6 plants, your seed germinates at 85 percent, and about 90 percent of seedlings survive to transplant. That is 6 divided by 0.85, then by 0.90, which rounds up to 8 seeds, one per cell in 8 cells. If you double-sow instead, the chance that at least one of two seeds sprouts is 1 minus 0.15 squared, or about 97.75 percent; 6 divided by 0.9775 and then by 0.90 rounds up to 7 cells (14 seeds), each thinned to the strongest seedling.
Factors That Change the Count
- Seed age: older seed germinates at a lower rate, so sow more.
- Storage: seed kept cool and dry holds its germination longer.
- Warmth: tomato seed sprouts fastest at 75 to 85°F and needs at least 70°F for reliable germination; cool mix lowers the rate.
- Sowing depth: about a quarter inch is ideal; too deep cuts emergence.
- Damping off: overwatering and poor airflow kill seedlings, lowering survival.
Understanding Your Outputs
| Output | Meaning | Best Used For |
|---|---|---|
| Seeds to sow | Total to plant | Checking your seed packet has enough |
| Cells to fill | Single-sow or double-sow cell count | Setting up trays |
| Rates used | Germination and survival applied | Checking your assumptions |
When to Use It
Use it before you buy seed or start a tray, so you sow enough without wasting a whole packet. Pair it with the germination time calculator for timing and the seed viability checker to judge old seed.
Common Mistakes to Avoid
- Sowing exactly your target: some seeds will not sprout, leaving you short.
- Over-sowing wildly: a huge surplus wastes seed, mix, and space.
- Ignoring old seed: last year’s packet may germinate well below its printed rate.
- Never thinning: two seedlings crowding one cell both grow weak.
- Cold mix: sowing into cool soil drops germination; use warmth.
Accuracy and Limitations
This is a planning estimate based on the rates you enter.
What It Calculates: seeds to sow and cells to fill for your target plants.
What It Does Not Know: your exact seed lot or conditions. If you are unsure, sow a couple extra, or run a quick germination test first.
Methodology
Method: the target number of plants is divided by the germination rate and then the seedling survival rate to find the seeds needed, rounded up. Inputs Used: plants wanted, germination rate, seedling survival. Defaults: 85 percent germination (about right for seed two years old kept cool and dry) and 90 percent seedling survival, both adjustable. Cells: single-sowing uses one cell per seed; double-sowing divides the plants wanted by the chance that at least one of two seeds sprouts (1 minus (1 minus germination) squared) and by survival, then each cell is thinned to one seedling. Rounding: seeds and cells round up so you never fall short. Note: a quick home germination test gives you a real rate for old seed.
Sources: The Old Farmer’s Almanac – Tomatoes, Iowa State University Extension – Growing Tomatoes, Royal Horticultural Society – Tomatoes. Last reviewed: September 26, 2026 · Reviewed by Aiza Anwar.
Frequently Asked Questions
How many tomato seeds should I plant per plant I want?
Sow a little more than one. For good seed at about 85 percent germination and 90 percent seedling survival, roughly one and a third seeds per wanted plant, so about 8 seeds for 6 plants.
How many seeds should I sow per cell?
Two is common. Sow two per cell or pot, then thin to the single strongest seedling once they have their first true leaves.
What is a normal germination rate for tomato seeds?
Fresh, well-stored tomato seed often germinates at 85 to 95 percent. Older seed drops off, which is why the calculator lets you lower the rate.
How do I test old tomato seed?
Roll ten seeds in a damp paper towel, keep it warm, and count how many sprout in a week or so. Six of ten is about 60 percent germination.
Do I really need to sow extra seeds?
Yes, a few. Even good seed does not all sprout, and some seedlings are lost to damping off, so a small surplus keeps you from coming up short.
Why is my germination so low?
Usually cool mix, sowing too deep, or old seed. Tomato seed sprouts fastest at 75 to 85°F (keep it at least 70°F), sown about a quarter inch deep in moist, not soggy, mix.
Can I sow tomato seeds directly in the garden?
In long, warm seasons yes, but most gardeners start indoors for an earlier, more reliable crop. Direct sowing needs warm soil and a longer season.
How long do tomato seeds stay viable?
Stored cool and dry, tomato seed often stays usable for four to six years, with germination slowly falling. Test older seed before you rely on it.
How many seedlings survive to transplant?
With good care, most do, often 85 to 95 percent. Losses come mainly from damping off, so use clean mix, water carefully, and give good airflow.
Related Tomato Tools
Work It Out with the Plants Needed Calculator
This calculator gives a planning estimate from the rates you enter. Real germination and survival depend on your seed, mix, warmth, and care. When in doubt, sow a couple extra or run a quick germination test. Spotted an error? Tell us here.