Growing Degree Days Calculator & Heat Units & Maturity
Tracks heat units for maize
Turn daily temperatures into growing degree days — the heat units a crop actually accumulates — to estimate days to maturity and track progress far more reliably than the calendar.
Modified caps max at 30°C and floors min at 10°C before averaging — more realistic in heat.
At about 14 GDD a day, maize / corn needs roughly 193 days to accumulate the 2,700 °C·day it takes to reach maturity from these temperatures. Warmer days build heat units faster and shorten the season; cool spells stretch it out.
Next: track GDD from your actual sowing date to time scouting, fertilizer splits and harvest more reliably than calendar days alone. Above 30°C the crop gains little extra, which is why the modified method caps the high.
Base/upper thresholds and GDD-to-maturity are representative — exact values vary by variety and reference. Use local daily Tmax/Tmin for best results.
Growing degree days — key facts
- Simple formula
- ((Tmax + Tmin) / 2) − Tbase
- Floor
- negative GDD counts as 0
- Maize / rice base
- ≈ 10°C
- Wheat / barley base
- ≈ 0°C
- Cotton base
- ≈ 15.5°C
- Maize to maturity
- ≈ 2700 GDD
- Modified method
- caps Tmax at upper threshold
- Privacy
- Runs in your browser; nothing uploaded
How GDD predicts maturity
Crops are heat-driven: development tracks accumulated warmth above a base temperature, not the number of days on the calendar. Each day contributes ((Tmax + Tmin) / 2) − base heat units, and once the running total reaches the crop's GDD-to-maturity, it's ready to harvest. This tool computes the daily figure both ways — simple and modified — and divides the maturity target by the daily rate to estimate the season length under your temperatures.
The modified method matters in hot climates: above an upper threshold (around 30°C for many crops) extra heat does little for growth, so capping the high gives a truer count. Tracking real GDD from sowing lets you time fertilizer splits, pest scouting and harvest with far more confidence than fixed dates, which is why seed companies rate hybrids in heat units.
Predict harvest
Estimate days to maturity from your daily temperatures, then refine it as the weather unfolds.
Track progress
Enter accumulated GDD since sowing to see the percentage toward maturity and days remaining.
Time field work
Use heat units to schedule top-dressing, scouting and spraying at the right growth stage.
Compare hybrids
Match a variety's heat-unit rating to your season's GDD to pick one that will finish in time.
Frequently Asked Questions
What are growing degree days (GDD)?+
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.
How is GDD calculated?+
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.
What is the base temperature?+
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.
How do I estimate days to maturity from GDD?+
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.
What is the modified GDD method and when do I use it?+
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.
How do I track accumulated GDD?+
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.
Why use GDD instead of calendar days?+
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.
Does GDD account for frost or extreme heat?+
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.
Are the GDD-to-maturity figures exact for my variety?+
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.
Can I use °F instead of °C?+
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.