Seed Starting Date Calculator
Work out when to sow indoors and when to move seedlings outside, counting back from your last spring frost by each crop's lead time. Enter your frost date, pick a crop, and get both dates plus the indoor growing window.
Look this up for your ZIP or region — it anchors the whole schedule.
Weeks before last frost to sow, and weeks after frost to plant out — a negative transplant value means before frost (hardy crops). Check your seed packet.
Formula & how it works
Sow indoors = last frost − (start weeks × 7 days). Transplant = last frost + (transplant weeks × 7 days), where a negative transplant value falls before the frost date.
Indoor growing time = start weeks + transplant weeks, the span from sowing to planting out.
Worked example
With a last frost of May 15 and tomatoes (start 6 weeks before, transplant 1 week after), sow indoors around April 3 and transplant around May 22 — about 7 weeks under lights. Hardy broccoli, transplanted 2 weeks before frost, would instead go out around May 1 from the same early-April sowing.
Timing seeds well
Everything hangs off the frost date
The last spring frost is the anchor for the whole schedule, and it's local — it can differ by weeks between nearby areas and swings with elevation. Look up the average for your ZIP or region and enter it here. Remember it's an average, not a guarantee, so keep an eye on the forecast and be ready to protect or delay tender transplants if a late cold snap threatens.
Tender versus hardy changes the plan
Tender crops like tomatoes, peppers, and basil can't take frost, so they transplant after the last frost date. Hardy crops like broccoli, cabbage, and lettuce tolerate light cold and can go out a couple of weeks before it, which is why their transplant timing is negative here. Getting this direction right matters more than a few days on the sowing date.
Follow the packet, and don't start too early
These lead times are typical, but varieties vary and the seed packet is the authority — enter its numbers when they differ. And resist starting too early: seedlings held too long indoors get leggy and root-bound, and often do worse than ones sown on schedule. Later is usually recoverable; too early rarely is.