Frost Date Calculator
Frost date calculator works out Days between the frost dates and Frost-free days between them from Average last spring frost, Average first autumn frost and Weeks to start seed indoors before the last frost. The default figures on this page give days between the frost dates of 143 days; change any entry and the result, the working and the sources update together.
Your figures
Updates as you typeHow this is worked out
4 steps
- 2026-05-15 to 2026-10-05 is 143 days, of which 142 lie strictly between the two frost dates.
- Sowing indoors 6 weeks (42 days) before the last frost gives 2026-04-03.
- Transplanting 2 weeks after the last frost gives 2026-05-29, leaving 129 days before the first autumn frost.
- An autumn crop needing 60 days plus a 14-day fall factor must be sown 74 days before 2026-10-05: 2026-07-23.
Its 4 worked examples are re-run on every release, last on 15 September 2026. What changed
Sources and references
3 sources
What the frost date calculator calculates
Turns your local average last spring frost and first autumn frost into a planting calendar: the length of the frost-free growing season, when to sow indoors (counted back from the last frost), when it is safe to transplant, and the last date to sow a crop for a autumn harvest (counted back from the first frost as days to maturity plus a fall factor for shortening days). The frost dates themselves are station-specific climate data and are taken as inputs.
How to use this frost date calculator
Enter Average last spring frost, Average first autumn frost, Weeks to start seed indoors before the last frost and Weeks after the last frost before setting out. Days between the frost dates updates as you type, and “Use a worked example” loads checked figures you can adjust. Change one figure at a time to see what drives the result.
How the result is worked out
Open “How this is worked out” beside the result for the step-by-step calculation behind Days between the frost dates, Frost-free days between them and Growing season.
The method follows NOAA National Centers for Environmental Information, Freeze/Frost Occurrence Data (Climate Normals), USDA Agricultural Research Service, Plant Hardiness Zone Map and University extension planting-calendar practice (e.g. Johnny’s Selected Seeds fall harvest planner).
A 15 May to 5 October season example
With Average last spring frost: 2026-05-15, Average first autumn frost: 2026-10-05, the calculator returns Days between the frost dates: 143, Frost-free days between them: 142. This example is checked against an independent source in the test suite.
What you can compare
Change one input at a time to see how it affects Days between the frost dates, Frost-free days between them and Growing season. The inputs you can vary are Average last spring frost, Average first autumn frost, Weeks to start seed indoors before the last frost, Weeks after the last frost before setting out, Days to maturity of the autumn crop and Fall factor.
Understanding Days between the frost dates, Frost-free days between them and Growing season
Average frost dates are probabilities, not promises: a 50% date is exceeded in half of years. Check the current forecast before setting tender plants out.
Results are general information, not financial, tax, legal, medical or engineering advice. Check any decision that matters with a qualified professional.
See something wrong? Report a problem with this calculator
Frequently asked questions
How to determine frost date?
Turns your local average last spring frost and first autumn frost into a planting calendar: the length of the frost-free growing season, when to sow indoors (counted back from the last frost). Open “How this is worked out” beside the result to see each step for your own figures.
Will it frost at 40 degrees?
Turns your local average last spring frost and first autumn frost into a planting calendar: the length of the frost-free growing season, when to sow indoors (counted back from the last frost). Put your own figures in above and the working updates beside the result. For scale, the default figures on this page give 143 days.
What is the last frost date for Ohio in 2026?
On the figures this page opens with, the days between the frost dates is 143 days. No single number fits everyone: it follows Average last spring frost, Average first autumn frost and Weeks to start seed indoors before the last frost, so enter your own above and the answer updates as you type.
What is the last frost date for Florida?
It needs Average last spring frost, Average first autumn frost, Weeks to start seed indoors before the last frost, Weeks after the last frost before setting out, Days to maturity of the autumn crop and Fall factor. Nothing else is required, and the figures stay in your browser.
What information do I need for the frost date calculator?
You need Average last spring frost, Average first autumn frost, Weeks to start seed indoors before the last frost, Weeks after the last frost before setting out, Days to maturity of the autumn crop and Fall factor. The page opens with default figures that give days between the frost dates of 143 days, so you can see the scale before entering your own.
What does this frost date calculator show?
It shows Days between the frost dates, Frost-free days between them, Growing season, Start seed indoors on, Safe to transplant from, Direct sowing can begin, Last date to sow the autumn crop and Harvest the autumn crop by, with the working and any warnings for your entries.