Estimate first and last frost dates for your area.
Frost dates are climate averages, not forecasts. The length of the frost-free season falls away with latitude — roughly four to five days for every degree you move poleward — and again with altitude, because air cools about 6.5 C for every thousand metres of height. Local terrain then shifts the dates: cold air is denser than warm air, so it drains downhill and pools on valley floors, which is why a frost pocket can be several degrees colder than a slope a few metres above it, while a city or coastline stores enough heat to add a fortnight at each end. Centring the estimated season on midsummer gives an average last spring frost and first autumn frost, and adding a safety margin to the spring date gives a planting date that survives most years rather than the average one.
Frost-free season and frost dates
Frost-free days = 330 - 4.6 x (latitude - 25) - 3.2 x (elevation / 100) + site shift; last spring frost = day 200 - frost-free days / 2; first autumn frost = day 200 + frost-free days / 2
Frost dates here are modelled climate averages, not a forecast, and roughly half of all years will see frost outside them. Do not rely on these figures alone to protect crops, livestock, irrigation lines or unheated plumbing — check the current forecast and local station records before acting.
Cold air is denser and drains downhill overnight, pooling in hollows and valley floors. These frost pockets can run 2-4 C colder than sloping ground only a few metres higher, which is why this calculator subtracts nearly two weeks from the season for them.
No — it is an average, so roughly half of years have a later frost. That is what the safety margin is for: adding ten to fourteen days moves you from a coin flip to a date that holds in most seasons.
It gets you within a couple of weeks for temperate inland sites, but it cannot see lakes, coastlines, prevailing winds or your particular hollow. Always check a published frost-date table for your nearest weather station before committing a crop.