Seed Starting Calculator
Enter your ZIP code, tick the crops you are growing, and get the date to sow each one indoors, counted back from your own last frost date at the level of risk you are willing to take.
Seed starting calculator
How the sowing date is worked out
One subtraction, done properly:
sow indoors = last frost date - (weeks a seedling needs indoors)
A tomato wants six to eight weeks indoors, so with a last frost of May 15 the window runs from May 15 - 8 weeks = March 20 to May 15 - 6 weeks = April 3. Both ends are shown because both ends are real. Sow at the early end and you get a bigger plant but risk a leggy, root-bound one if the weather turns cold. Sow at the late end and you lose a week or two of harvest.
The part other calculators leave out: "last frost date" is not one number. It is the middle of a distribution. NOAA publishes freeze dates at several probability levels and the gap between the 10% and 90% columns runs about four weeks at most stations. Pick a threshold and everything downstream moves. That is why two well-known garden sites can print last frost dates a week apart for the same town and both be right: one publishes the 30% date, the other the 50% date, and neither tells you.
We model the spread as a normal distribution of freeze dates with a standard deviation of 12 days, which reproduces that four-week gap. The shift applied is round(z × 12) days: 0 days at 50%, 6 days later at 30%, 15 days later at 10%, and 15 days earlier at 90%.
Where the frost date comes from: the same regional table the frost date lookup uses, keyed on the first three digits of your ZIP code, so the two pages can never disagree.
Where the crop numbers come from: the weeks-indoors figures are the consensus published by US cooperative extension services. Where they differ, they differ by about a week, which is why every crop carries a range instead of a single number. The back of your own seed packet always beats a table, including this one.
Seed starting chart: weeks before last frost
The numbers this calculator runs on. Works anywhere in the world once you know your own last frost date, which is why this is the part worth saving.
| Crop | Weeks indoors before last frost | Set out | Days to maturity |
|---|
Crops not listed here are the ones that resent transplanting: beans, corn, carrots, beets, radishes, turnips, parsnips, peas, dill and cilantro all do better sown straight into the ground. Cucumbers, squash and melons can be started indoors three to four weeks early, but they catch up so fast that direct sowing is usually the easier choice.
Keep planning
All of these read the same frost date table, so nothing here contradicts anything there.
Common questions
What happens if I start seeds too early?
You get leggy, pale, root-bound seedlings that sulk for weeks after transplanting and are often overtaken by plants sown a month later. Indoor light is far weaker than sunlight, so a plant held indoors too long stretches for it. Starting late costs you a little harvest. Starting far too early can cost you the plant.
Why does the calculator give a range instead of one date?
Because the honest answer is a range. Extension services publish tomato as six to eight weeks, not seven. A single date would look more authoritative and be less true. Sow at the early end if you have strong light and can pot the seedlings up once, and at the late end if you are working on a windowsill.
Which risk setting should I choose?
It depends on what a loss costs you. If you have row cover, spare seedlings and time to resow, the 50% or even the 90% setting buys you weeks of season. If you drove two hours for one heirloom tomato plant, take the 10% setting and wait. The tool shows exactly how many days of season each choice costs, so it is a real decision rather than a guess.
Do I count from sowing or from germination?
From sowing. The weeks-indoors figures already allow for germination time, which is why slow starters like pepper, celery and rosemary carry such long windows. Do not add extra days on top.
Can I start cucumbers, squash and melons indoors?
Yes, three to four weeks before your last frost, and it is worth it in a short season. They hate root disturbance though, so use a large cell or a soil block, do not let them get past the second true leaf, and be gentle. In a long season, direct sowing catches up within a couple of weeks anyway.
How long before my last frost do I harden seedlings off?
Give them seven to ten days. Start with an hour outdoors in shade and shelter, build up to a full day of sun and wind, and bring them in overnight until the final few nights. Plants moved straight from a windowsill to open ground get scorched and windburned, and the setback can be worse than sowing two weeks late.
What soil temperature do the seeds actually need?
Warmer than most rooms. Peppers and eggplant want soil around 80°F to germinate reliably and will sit for three weeks at 65°F. Tomatoes are happy around 75°F. Lettuce and brassicas germinate perfectly well at 60 to 65°F. A cheap heat mat is what makes the long pepper window actually work.
How accurate is my frost date?
Accurate to within about one to two weeks, which is the honest resolution of any ZIP-level frost table. Elevation, slope, pavement, walls and nearby water all shift it. That uncertainty is exactly why the risk setting exists rather than being hidden in a footnote.