Sow for the plants you want, not the seeds you have
If you want twelve tomato plants, sowing twelve seeds gets you nine or ten. Germination on a fresh packet might be 85%, on two-year-old seed considerably less, and of those that come up one or two will be weak enough that you would not want to give them a season of bed space.
The calculator works backwards from the number of plants you actually want, dividing by the germination rate and then adding roughly 15% for culling. Twelve plants at 80% germination means sowing eighteen seeds. That feels wasteful until the first year you sow exactly enough and end up with gaps you cannot fill because it is too late to sow again.
Seed is the cheapest input in the whole garden. A packet costs less than a single potted plant from a garden centre, and the difference between sowing twelve seeds and eighteen is measured in pennies. Bed space and season length are the scarce resources, not seed.
| Plants wanted | At 85% germination | At 70% | At 50% |
|---|---|---|---|
| 6 | 9 seeds | 10 seeds | 14 seeds |
| 12 | 17 seeds | 20 seeds | 28 seeds |
| 24 | 33 seeds | 40 seeds | 56 seeds |
| 50 | 68 seeds | 83 seeds | 115 seeds |
Earlier is not better
The strongest temptation in February is to start everything, and it is almost always a mistake. A seedling that outgrows its cell before conditions outside are fit for it does not wait politely — it becomes root-bound, checks its growth, and often never fully recovers.
A tomato sown ten weeks before the last frost instead of six spends four extra weeks in a pot on a windowsill with insufficient light. It ends up tall, pale and weak-stemmed, and a stocky six-week plant will overtake it within a fortnight of both going out. The extra month bought nothing.
Light is the real constraint indoors, not warmth. Most windowsills provide far less light than a seedling wants, which is what produces the long pale stems people call leggy. If you are starting more than a tray or two, a simple LED grow light does more for plant quality than any amount of extra time.
The offsets in this calculator are the ones that produce a plant ready to go out at the right moment — not the earliest a seed will germinate.
Hardening off is not optional
A plant raised indoors has thin leaves, no wind conditioning and no UV tolerance. Moved straight outside it can be scorched or wind-damaged within a day, and the setback can cost a fortnight even when the plant survives.
Hardening off means putting plants out for a couple of hours in shade on day one, and extending the time and exposure over seven to ten days until they are out overnight. It is tedious and it is the difference between plants that keep growing after transplanting and plants that sit still for two weeks.
Build it into the plan rather than treating it as an afterthought. The plant-out dates the calculator gives you are when plants should be going into the ground, which means hardening off starts a week to ten days before that.
Common questions
How many seeds should I sow for 12 plants?
About 18 at a typical 80% germination rate, which allows for the seeds that do not come up and the weakest seedlings being culled.
Can I start seeds too early?
Yes, and it is a common mistake. A seedling that outgrows its cell becomes root-bound and checks its growth. A stocky six-week tomato overtakes a leggy ten-week one within a fortnight of planting out.
Why are my seedlings tall and pale?
Not enough light. Most windowsills give far less than a seedling wants, and the plant stretches looking for more. A basic LED grow light fixes it more effectively than a sunnier window.