Growing Degree Days Calculator & Heat Units & Maturity

Growing degree days, also called heat units or thermal time, measure the accumulated warmth a crop experiences above a base temperature below which it doesn't grow. Crops develop in step with heat rather than calendar days, so GDD predicts stages like flowering and maturity more reliably than dates alone.

The simple method is GDD = ((Tmax + Tmin) / 2) − Tbase for the day, set to zero if negative. The modified method first caps Tmax at the crop's upper threshold and raises any Tmin below the base up to the base before averaging — this avoids over-counting heat on very hot days. This tool shows both.

The base (or threshold) temperature is the lowest temperature at which a crop grows; below it, development effectively stops. It's crop-specific — about 10°C for maize, rice and soybean, 0°C for wheat and barley, and around 15.5°C for cotton. The tool uses the right base for the crop you pick.

Divide the crop's GDD-to-maturity by the average GDD it accumulates per day. For example, maize needing 2700 GDD at 15 GDD/day reaches maturity in about 180 days. Warmer weather raises daily GDD and shortens that estimate.

The modified method caps the daily high at the crop's upper threshold (e.g. 30°C) and the low at the base before averaging. It's preferred for heat-tolerant summer crops because growth doesn't keep increasing once it's very hot, so the simple method would overestimate heat units.

Add up each day's GDD from your sowing date. Enter the running total in the tool's progress field and it shows the percentage toward maturity and the days remaining at the current rate — handy for timing scouting, top-dressing and harvest.

Two crops sown on the same date can mature weeks apart depending on temperature. GDD captures the actual warmth a crop received, so it aligns field operations and harvest predictions with the season's real conditions rather than the calendar.

Not directly — GDD only sums warmth above the base and (in the modified method) ignores heat above the upper threshold. It doesn't penalise frost damage, heat stress, drought or daylength, so treat it as a development clock rather than a complete model.

They're representative values for each crop. Specific varieties and hybrids publish their own GDD or 'comparative relative maturity' ratings, and references differ on base temperatures, so use your seed company's figure when you have it.

This tool works in °C. To convert a Fahrenheit base or temperature, use °C = (°F − 32) × 5/9. GDD totals in °F and °C are not interchangeable, so keep your base, temperatures and maturity target all in the same unit.