Growing Degree Days Calculator (GDD) with Base Temp
Growing degree days (GDD) measure how much useful heat a day gave a crop or insect. Enter the day's high and low plus your base temperature to get the daily value, total several days, or see how many days a target will take. Base and upper values are typical examples, so use the figures for your own crop. After switching units, re-enter your measurements.
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Formula
Daily growing degree days use the horizontal-cutoff method:
GDD = (min(high, upper) + max(low, base)) / 2 - base, and never below 0.
- Accumulated GDD = daily GDD x number of days (for a run of similar days).
- Days to reach a target = target / average GDD per day, rounded up.
Base and upper temperatures differ by crop and pest. The defaults here (50 F or 10 C base, 86 F or 30 C upper) are typical values, not a rule for every plant.
Worked example
High 78 F, low 56 F, base 50 F, upper 86 F: (78 + 56) / 2 - 50 = 17 GDD.
- Ten days like that: 17 x 10 = 170 GDD.
- To reach 1,200 GDD at 17 a day: 1,200 / 17 = 70.6, so 71 days.
- A hot day with a high of 95 F and a low of 40 F is capped to 86 and lifted to 50: (86 + 50) / 2 - 50 = 18 GDD.
- In Celsius, 30 C high, 15 C low, base 10 C: (30 + 15) / 2 - 10 = 12.5 GDD.
When to use it
Degree days track development better than calendar days because plants and insects respond to heat. Use them to time sowing, to estimate when a crop reaches a stage, or to compare seasons. Published GDD targets for a crop are typical values and depend on variety.
Common mistakes
- Using a base that belongs to a different crop or pest.
- Forgetting that a cold day gives 0, not a negative number, so you cannot subtract from a running total.
- Mixing Fahrenheit and Celsius between the high, the low and the base. After switching units, re-enter all values.
How it differs from nearby calculators
The planting date calculator counts calendar days from a frost date, while this one counts accumulated heat. The germination rate calculator deals with seed counts, not temperature.
Frequently asked questions
How do you calculate growing degree days?
Add the daily high and low, divide by 2, then subtract the base temperature. This calculator first caps the high at the upper cutoff and lifts the low to the base, and never returns less than 0. For a high of 78 F, a low of 56 F and a base of 50 F: (78 + 56) / 2 - 50 = 17 GDD.
What base temperature should I use?
It depends on the plant or pest. A base of 50 F (10 C) is a common value for corn and many warm-season crops, and 40 F (about 4.5 C) is often used for cool-season crops. These are typical values only, so check the figure published for your crop or insect.
Why is there an upper cutoff?
Growth slows or stops in extreme heat, so a very hot afternoon should not count as extra development. With an upper cutoff of 86 F, a 95 F high is treated as 86 F: with a low of 40 F lifted to 50 F, the day gives (86 + 50) / 2 - 50 = 18 GDD.
How many days will it take to reach my GDD target?
Divide the target by the average GDD per day and round up. To reach 1,200 GDD at 17 GDD a day takes 1,200 / 17 = 70.6, so 71 days. Real weather varies, so treat the result as an estimate.
How do I calculate growing degree days in Celsius?
Use the same method with Celsius inputs. A high of 30 C, a low of 15 C and a base of 10 C give (30 + 15) / 2 - 10 = 12.5 GDD for the day. A typical Celsius base is 10 C and a typical upper cutoff is 30 C.
Can growing degree days be negative?
No. If both the high and low are below the base, the day contributes 0. For example a high of 45 F and a low of 30 F with a base of 50 F gives 0 GDD, because heat below the base does not subtract from your total.
How many days to reach 2,700 GDD?
At an average of 17 GDD a day, 2,700 / 17 = 158.8, so 159 days. A figure like 2,700 GDD is a typical order of magnitude for a long-season warm crop; check the number published for your own variety.