Frost Date Calculator
The published date is usually a coin flip.
Work out Frost Date. The published date is usually a coin flip. Names the misconception directly.
Growing season
170 days
30 April to 17 October · plant out from 14 May
A last frost date is a percentile of a historical distribution, not a promise. The commonly published one is usually the 50% date — meaning frost falls after it in half of all years — which is not what most people assume when they read it. That is why the hardiness buffer exists. Two weeks past the 50% date gets most tender crops through most years; waiting for the 10% date does the same more conservatively at the cost of a shorter season. Microclimate beats the map. A south-facing wall, a slope that drains cold air, or a city centre can shift the effective date by a fortnight in either direction, and a thermometer in your own garden over two seasons is worth more than any published figure.
How the Frost Date Calculator works
Growing season length, a safe planting date after a hardiness buffer, and when to start seeds indoors — with the risk percentile of your frost dates stated rather than hidden.
Also known as: when is my last frost date · when can i plant out · how long is my growing season · when to start seeds indoors
Frequently asked questions
What does a last frost date actually mean?
It is a percentile of a historical distribution. The commonly published figure is usually the 50% date, meaning frost falls after it in half of all years — not what most people assume.
How long should I wait after the last frost date?
Two weeks is a common buffer for tender crops, or wait for the 10% date instead. Either trades a shorter season for a much lower chance of losing the planting.
How do I know if a crop fits my season?
Growing season minus your buffer minus days to maturity. If that is negative, the crop needs starting indoors, a shorter variety, or protection at one end.
How far ahead should I start seeds indoors?
Count back from the safe planting date by the weeks the variety needs — typically six to eight for tomatoes and peppers, four for brassicas.
Does my garden match the published dates?
Often not. A south-facing wall, a slope that drains cold air, or an urban location can shift the effective date by a fortnight, and a thermometer over two seasons beats any map.
Put this calculator on your own site
Free to use, on any site, commercial or not. Paste this where you want it to appear. It is a plain iframe, so it works in WordPress, Squarespace, Wix, Webflow, Ghost and anything else that accepts HTML.
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<p style="font:13px/1.5 system-ui,sans-serif;margin:6px 0 0;color:#64748b">Powered by <a href="https://www.thecalclibrary.com/frost-date-calculator" style="color:#64748b">Frost Date Calculator</a> from The Calc Library</p>The only condition is that the credit line below the frame stays in place. That one line is what pays for the tool being free — it is how anyone else finds it.