How to use the seed quantity calculator
- Choose how to work out the plants: enter the plants you want directly, describe rows in a bed, or enter plants per square foot.
- For rows, enter the planted area, the row spacing in inches and the plants per foot of row. For an area, enter the area and the plants per square foot.
- Enter the germination rate from your seed packet or your own test.
- Optionally enter the seeds in one packet to see how many packets to buy. Read the seeds to sow.
Formula
Row length (ft) = planted area / (row spacing in inches / 12)
Plants from rows = row length x plants per foot (or area x plants per square foot)
Seeds to sow = plants wanted / germination rate, rounded up to a whole seed
Germination is treated as the only loss. A rate of 80% is entered as 80 and used as 0.80.
100 plants at 80% germination
To end up with 100 plants when 80% of seeds sprout, you need 100 / 0.80 = 125 seeds. About 100 of them should sprout. A packet of 50 seeds means 3 packets.
For a 100 square foot bed with rows 12 inches apart, the rows add up to 100 feet. At 2 plants per foot that is 200 plants, so 200 / 0.80 = 250 seeds. At 70% germination, 100 plants need 143 seeds because 142.86 is rounded up.
Why round up
Seeds come in whole numbers, so any fraction is rounded up. Rounding down would leave you short of the plants you planned. The tool shows the number of seeds you can expect to sprout so you can see how close it lands.
A packet rate comes from the seed seller, and cold soil, depth and watering can change what happens in your garden, so some growers sow extra. If you want a safety margin, enter a lower germination rate than the packet shows and read the result as a larger sowing.
Spacing and layout
Plants per foot and row spacing come from your seed packet or plan. The plant spacing calculator shows how many plants fit a bed in a grid, and the planting schedule calculator turns your frost date into dates. For soil to fill a raised bed, see the raised bed soil calculator.
For lawns, which use a rate per 1,000 square feet, use the grass seed calculator instead.
Assumptions and limits
- The germination rate is the share of sown seeds that become plants.
- Row spacing is the distance between rows and plants per foot is along a row.
- Thinning, transplant losses, pests and weather are not modeled.
- Follow the sowing depth and spacing printed on your seed packet.
Frequently asked questions
How many seeds do I need for 100 plants?
Divide 100 by the germination rate. At 80% you need 125 seeds, at 70% you need 143, and at 100% you need 100.
How do I find the germination rate?
It is printed on most seed packets. You can also test a sample by sprouting 10 seeds between damp paper towels and counting how many sprout.
Why does the tool round up?
A fraction of a seed cannot be sown, and rounding down would give fewer plants than you wanted, so the result is always rounded up.
What does plants per foot mean?
It is how many plants you want in each foot of row after thinning. Multiply it by the total row length to get the plants in a bed.