Why use this calculator
Crops develop on temperature, not on the calendar. A wheat crop sown into a cold June takes weeks longer to reach flowering than the same variety sown into a mild May, and getting flowering into the right window is one of the biggest yield drivers in Australian grain growing: too early risks frost, too late risks heat and a dry finish.
Growing degree days, also called thermal time, turn temperature into a development clock. This calculator gives daily and accumulated thermal time with a base temperature and an upper cut-off, shows how close the crop is to a target stage, and projects the days left so you can time fungicides, nitrogen, irrigation and harvest logistics against development rather than dates.
How to use it
- Enter the average daily maximum temperature for the period from your weather station or the Bureau of Meteorology.
- Enter the average daily minimum for the same period.
- Enter the base temperature for your crop, such as 0 for wheat or 10 for maize.
- Enter the upper cut-off, or a high value if your crop has none. Enter the days since sowing or emergence.
- Enter the target thermal time for the stage you are tracking, from a variety guide or seed company.
What each result means
- Daily degree days
- Thermal time gained in an average day at the temperatures entered, after holding each temperature between base and cut-off.
- Date the target is reached
- The calendar date the crop should reach the target stage, counting the days to target forward from today at the same average temperatures.
- Accumulated degree days
- Thermal time gained since sowing or emergence, assuming every day matched the average temperatures you entered.
- Share of target reached
- Accumulated thermal time as a percentage of the target, showing how far through this development phase the crop is.
- Degree days still needed
- Thermal time still to accumulate before the crop reaches the target stage at the same temperatures.
- Estimated days to target
- Calendar days left to the target stage if the same average temperatures continue, rounded up to whole days.
The formula
Each daily temperature is first held between the base and upper cut-off:
- Tmax' = min(max(Tmax, base)
- Upper)
- Tmin' = min(max(Tmin, base), upper), in °C
Daily degree days = (Tmax' + Tmin') / 2 minus base, never below zero
Accumulated degree days = daily degree days x days since sowing
Share reached = accumulated / target x 100
Degree days still needed = target minus accumulated
Days to target = degree days still needed / daily degree days, rounded up
How this number is used in practice
Australian grain agronomy uses thermal time mainly to match sowing date and variety to the flowering window. GRDC and state department research publishes optimal flowering periods for each district, and variety guides describe how fast each wheat or barley variety develops. For wheat and barley the base temperature is usually 0 °C, which is why thermal time in these crops is often quoted in the thousands of degree days from sowing to flowering.
Vernalisation and daylength also affect winter crops, so thermal time alone is a guide rather than a guarantee for them.
Summer crops such as maize, sorghum and cotton are more strictly driven by heat. Maize commonly uses a 10 °C base with a 30 °C cut-off, and cotton growers track day degrees on a 12 to 15 °C base to predict squaring, flowering and boll opening, which then drives irrigation and defoliation timing.
Using averages over a period gives a fair estimate, but daily records are better because a run of very hot or cold days is not the same as the average. The Bureau of Meteorology publishes daily maximums and minimums for most stations to do this properly.
Worked examples
Maize: 28 °C max, 15 °C min, 10 °C base, 30 °C cut-off, 30 days after sowing, 1,500 target
11.5 degree days a day, 345 accumulated, 23.0% of target, 1,155 still needed, about 101 days to go.
Wheat in winter: 16 °C max, 4 °C min, 0 °C base, 30 °C cut-off, 60 days, 1,300 degree days to flowering
10.0 degree days a day, 600 accumulated, 46.2% of target, about 70 days to flowering.
Hot spell on maize: 38 °C max, 22 °C min, 10 °C base, 30 °C cut-off, 70 days, 1,500 target
16.0 degree days a day (the cut-off holds the 38 °C maximum at 30), 1,120 accumulated, about 24 days to go.
When people use it
- Predicting flowering of wheat or barley to judge frost and heat risk
- Timing fungicide or nitrogen applications to a growth stage
- Estimating harvest dates for contractors and grain storage
- Comparing development of a crop this season with last season
- Tracking insect development, which also follows thermal time
Tips
- Use daily records rather than period averages when you have them; hot and cold spells skew averages.
- Check the base temperature your variety guide used. Mixing bases gives meaningless comparisons.
- Count from the same starting point as the target, either sowing or emergence.
- Winter cereals also respond to vernalisation and daylength, so treat thermal time as a guide for them.
- Rerun the projection each fortnight with actual temperatures as the season develops.
What it assumes, and where it stops being accurate
- Uses the averaging method with both temperatures held between base and cut-off. Other methods, such as sine curves, give slightly different totals.
- Every day in the period is assumed to have the average maximum and minimum entered.
- The projection assumes the same temperatures continue until the target is reached.
- Vernalisation, daylength, water stress and nutrition, which also change crop development, are not modelled.
- Target thermal times vary by variety and region, so always use a figure published for your variety.
Terms used on this page
- Thermal time
- Accumulated temperature above a base, measured in degree Celsius days, used to predict crop development.
- Base temperature
- The temperature below which a crop makes no developmental progress.
- Upper cut-off
- The temperature above which extra heat is not counted toward development.
- Phenology
- The timing of crop growth stages such as emergence, flowering and maturity.
Questions people ask
- What base temperature should I use?
- Use the base temperature from the source of your target figure. Common values are 0 °C for wheat, barley and oats, 5 °C for canola in some models, 10 °C for maize and sorghum, and 12 to 15 °C for cotton. A target published on one base cannot be compared with accumulation on another.
- What does the upper cut-off do?
- Above a certain temperature, development stops speeding up and heat can even slow it. The cut-off caps the maximum used in the calculation, so a 38 °C day counts no more than a 30 °C day for maize. If your crop's model has no cut-off, enter a high value such as 45 °C.
- Where do I find target degree days for my crop?
- Seed companies publish thermal time to maturity for maize, sorghum and cotton hybrids. For winter cereals, GRDC and state department variety guides and flowering time tools describe phase lengths by variety and sowing date. Your agronomist can also help translate a variety's maturity rating into thermal time for your district.
- Why use degree days instead of calendar days?
- Because development speed depends on temperature. A crop in a cool season takes more calendar days to reach the same stage than the same crop in a warm one, but a similar amount of thermal time. That makes thermal time a better predictor of flowering and maturity, especially when comparing sowing dates or seasons.
- Is growing degree days the same as thermal time?
- Yes, in practice. Growing degree days, heat units, day degrees and thermal time all describe accumulated temperature above a base, measured in degree Celsius days. Australian grains research tends to say thermal time, while North American material and seed companies more often say growing degree days or heat units.
Use this with
Growing Degree Days Calculator answers one part of the question. These pick up where it stops: Rainfall & Soil Moisture Calculator, Crop Yield Estimator, Seed Rate Calculator and Harvest Yield Estimator.
Where these figures come from
This calculation is jurisdiction-neutral arithmetic. No country-specific rules are applied.
- Grains Research and Development Corporation. Spray application, farm budgeting, crop water use and phenology research for Australian grain growers
- Bureau of Meteorology. Daily temperature, rainfall and evaporation records for Australian stations
- NSW Department of Primary Industries. Farm budgets, drought feeding, fencing, farm dam and soil water guidance
Growing Degree Days Calculator produces an estimate for general information. It is not financial, tax, medical, legal or engineering advice. read the full disclaimer. Spotted a wrong number? Report it and it gets fixed. The form picks up this calculator and the inputs you used.