Seed Quantity Calculator Guide
Use this seed quantity calculator to determine how many seeds to sow for your target number of plants. Enter desired plant count, expected germination rate, and a contingency buffer for pests or weak seedlings to get a rounded-up seed total.
How to use the seed quantity calculator
Enter how many healthy plants you want at transplant or maturity, the germination rate from your seed packet (or a conservative estimate), and an extra percentage for contingencies like damping-off, birds, or culls.
The result is total seeds to sow, rounded up to a whole number.
Seed quantity formula
Seeds to germinate target = Desired plants ÷ (Germination rate ÷ 100). Total seeds = Seeds to germinate × (1 + Extra % ÷ 100). Round up to the nearest whole seed.
Low germination or old seed requires sowing more to hit the same plant count.
- Adjusted plants = Desired ÷ (Germination % / 100)
- Total seeds = Adjusted × (1 + Extra % / 100)
- Typical packet germination: 80–95% for fresh seed
- Common contingency buffer: 10–25%
Worked example
You want 10 tomato plants. Seed packet lists 80% germination and you add 20% contingency for losses.
Adjusted = 10 ÷ 0.80 = 12.5 plants worth of seed. Total = 12.5 × 1.20 = 15 seeds (rounded up).
Germination rate and seed age
Seed packets state minimum germination percentage tested within the past year. Older seed, improper storage, or cool soil can lower real-world germination — use a lower rate or higher contingency when in doubt.
A simple germination test on a damp paper towel before sowing season confirms viability for leftover seed.
Sowing tips
Sow two seeds per cell in trays and thin to the strongest seedling, or sow extras and transplant the best. For direct-sown crops, plan succession plantings using our harvest date calculator.
- Label trays with variety and sow date.
- Match contingency to crop value and scarcity of seed.
- Pair with plant spacing to size beds before ordering seed.
- Store leftover seed cool and dry in airtight containers.