Growing Degree Days Calculator
Daily corn heat units and season progress from high, low, and maturity target.
What it does
Growing degree days bank daily heat for crops: corn grows between 50 and 86 F, so each day contributes its capped mean minus 50. An 80 over 60 day banks 20 while a 93 over 74 day banks 30 after the cap.
The tool above takes the daily high and low with a maturity target in either system. It returns today's GDD with the capped inputs and season progress beside them.
The default of 80 over 60 gives 20 GDD with 2,680 of 2,700 left. A cool 75 over 45 day raises the low to 50 first for 12.5, and a hot 93 over 74 caps the high at 86 for 30.
The method follows extension corn development guides with a second extension program matching each cutoff. Both cap highs at 86, floors at 50, and track near 2,700 to black layer maturity.
How to use
- Pick imperial (F) or metric (C).
- Enter the day high and low temperatures.
- Enter the hybrid maturity target in matching GDD.
- Read today's GDD first for the daily bank.
- Sum daily values across the season for staging.
Formula
- base
- No growth below: 50 F corn, 10 C metric.
- cap
- No extra growth above: 86 F corn, 30 C metric.
- 2,700
- Typical F-GDD to black layer maturity.
Caps apply before averaging, not after: a 93 high enters as 86. Metric mirrors with base 10 and cap 30, so 20 F-GDD days read about 11.1 C-GDD.
Worked example
Example: high 80, low 60
Imperial defaults: high 80 F, low 60 F, target 2700.
- Caps: 80 under 86 stands, 60 above 50 stands.
- Mean: (80 + 60) / 2 = 70.
- GDD: 70 - 50 = 20.
- Season: 2,700 - 20 = 2,680 left to maturity.
- Tool shows 20 GDD, the capped inputs, 2680 of 2700 to maturity.
Reference table
| High / low | Capped mean | GDD |
|---|---|---|
| 80 / 60 | 70 | 20 GDD |
| 75 / 45 | 62.5 (low to 50) | 12.5 GDD |
| 93 / 74 | 80 (high to 86) | 30 GDD |
Limits
- Corn cutoffs only; other crops use other bases.
- Heat units ignore water, pests, and fertility.
- Hybrids vary hundreds of GDD around 2,700.
- Single days never stage a field alone.
Common mistakes
- Averaging before capping the high and low.
- Reading calendar days instead of heat units.
- Mixing F targets with C daily values.
- Counting sub base days as negative.
- Staging hybrids off generic maturity tables.
Try it yourself
Quick checks. Answers sit tucked under each question.
High 75, low 45?
Low rises to 50: mean 62.5, 12.5 GDD.
High 93, low 74?
High caps at 86: mean 80, 30 GDD.
Metric 26.7 over 15.6, target 1500?
Mean 21.1 minus 10: 11.1 GDD, 1489 to go.
Questions
What are growing degree days?
Banked daily heat above a crop base: corn counts mean minus 50 F.
Why cap at 86?
Corn gains nothing past it, so hotter highs enter as 86.
Why floor at 50?
No corn growth below it, so colder lows enter as 50.
How many GDD to maturity?
Near 2,700 F-GDD (about 1,500 C-GDD) for full season corn to black layer.
Daily or seasonal?
Both: days bank, seasons sum. Staging reads the running total.
Other crops?
Same math, different base: wheat near 40 F, alfalfa near 41 F.
Sources
- Penn State Extension: Understanding growing degree days
- Purdue Extension: Corn GDD and maturity (NCH-40)