This seed starting calculator works out when to sow seeds indoors and when to transplant out — from your last spring frost date and each crop’s standard weeks-to-transplant.
Sow Date = Last Frost − Crop Weeks — the Gardener’s Backward Calendar
Everything in spring gardening hangs off one local statistic (your average last frost) minus a species constant. Whether the seed is worth sowing at all is our Germination Rate Calculator‘s test. Part of our Ecology Calculators collection.
Worked Example (Verified Against the Calendar)
- Last frost May 1, 2027, tomatoes (defaults): sow indoors Saturday, March 13, 2027 (7 weeks prior); transplant out May 8 (+1 week of soil warming) — dates checked day-by-day
- Peppers are the patience crop: 9 weeks back = late February for the same frost date, and transplant holds to +2 weeks — starting them with the tomatoes is the classic zone-pushing error
- Cool crops invert the caution: broccoli transplants at the frost date itself and tolerates light frost — its 5-weeks-back sow date lands mid-March with far lower stakes
- Squash/cucumber: only 2–4 weeks indoors (they resent transplanting) — many gardeners direct-sow after frost instead and lose nothing
Calendar Craft
- Find your frost date from local extension/USDA (or almanac by ZIP) — microclimates (urban heat, frost pockets, altitude) shift it ±2 weeks within one town.
- Succession: sow lettuce/carrots in waves every 2–3 weeks; one date is a harvest spike, waves are a season.
- Fall gardens run the film backwards: first fall frost minus days-to-maturity minus ~2 weeks of slowing autumn light.
- Label with sow dates, not intentions — March optimism forgets which tray is which by April.
Related Tools
- Germination Rate Calculator — test the seed lot first
- Plant Spacing Calculator — where transplants land
- Ecology Calculators — the full collection
Frequently Asked Questions
When should I start tomato seeds indoors?
6–8 weeks before your last frost date, transplanting about a week after it (soil warming). For a May 1 last frost: sow ~March 13, transplant ~May 8. Starting earlier doesn’t buy earlier tomatoes — it buys leggy, rootbound seedlings that stall after transplant while correctly-timed ones sail past.
What is my last frost date?
Your area’s average final spring frost (50% probability date), published by agricultural extensions and almanacs by ZIP/postcode. Tender plants use a safer standard still: transplant 1–2 weeks after it, per the crop offsets in this calculator. Frost dates are statistics, not promises — watch the forecast in transplant week regardless.
Why do peppers need starting so early?
They germinate slowly (10–21 days, wanting 25–30 °C soil) and grow slowly as seedlings — 8–10 indoor weeks to reach transplant size, plus they demand warm soil outside (nights above ~13 °C). The calculator’s +2 week transplant offset encodes the warmth wait.
What does “hardening off” mean?
Acclimating indoor seedlings to sun, wind and temperature swings over 7–10 days: an hour of shade, then progressively more sun and exposure, then nights outside. Skip it and transplant day delivers sunscald and wilt to plants that looked perfect on the windowsill.
Can I start seeds too early?
Yes — the commonest spring error. Oversized seedlings exhaust their pots, flower prematurely (peppers), yellow, and suffer transplant shock; right-sized 6-week transplants routinely out-yield 10-week veterans by midsummer. The calendar here exists to restrain February enthusiasm with March arithmetic.
Is this seed starting calculator free?
Yes — free, instant, no sign-up.