Old seed and seed you saved yourself rarely match the rate printed on a packet, so sowing a small test and measuring the real rate is far more reliable. Enter how many seeds went into the test and how many came up, and this page works out the germination rate, then reverses it to show how many seeds you actually need for the number of plants you want.
Germination rate is seeds germinated divided by seeds sown, and the sowing amount is the target plant count times one plus the margin, divided by the rate, rounded up. Rounding is applied once at the end and the expression used is printed in the results. The table underneath shows how the sowing amount moves as the margin changes, so a tight seed supply is easy to spot.
No official figure exists for the spare margin, so you set it. Losses during thinning or transplanting and variety-specific germination conditions are not modelled, and the page does not work out sowing dates. If germinated seeds outnumber the seeds sown, or none came up, both figures are reported and no result is shown. Current as of October 2026.
Frequently Asked Questions
There is no set figure, but a larger test gives a steadier rate. Enter whatever number you actually used and the page calculates from that.
No official value is published, so the choice is yours. The table compares sowing amounts across margins to make that choice easier.
Because the margin is added and the result is then rounded up. Setting the margin to 0 gives the sowing amount closest to the bare target.