Growing Degree Day Calculator
Enter one day per line as a maximum and a minimum, pick a base temperature, and choose which convention to apply. The calculator shows each day’s contribution and the running accumulation. Everything happens in the page — no request leaves the browser.
The values loaded above are an arithmetic example chosen so the sums can be checked by hand. They are not a recommendation for any crop, region or season — replace them with yours.
Units are yours, as long as they are consistent
The arithmetic contains no unit conversion and no fixed scale. Enter Celsius and the result is degree days Celsius; enter Fahrenheit and it is degree days Fahrenheit. The only requirement is that the daily temperatures, the base and the cutoff all use the same scale. Totals from the two scales are not interchangeable and there is no meaningful way to compare them without redoing the accumulation.
What the simple average method does
For each day the calculator takes the mean of the maximum and the minimum, subtracts the base temperature, and adds the result to a running total:
(max + min) / 2 − base
Two rules matter more than the formula.
A day below base contributes zero, not a negative number. If the mean falls below the base temperature the day adds nothing to the accumulation. It does not subtract previously accumulated heat. Development does not run backwards on a cold day, so an accumulator that goes down is modelling something else entirely. This is the single most common implementation error in a hand-built spreadsheet, and it is invisible in the output until a cold spell quietly erases a fortnight of accumulation.
The daily mean is a stand-in for an integral. What the model is really after is time spent above the base temperature, which would require the full temperature curve through the day. Two days with identical maxima and minima and completely different shapes — one a brief spike, one a long plateau — produce the same number here. Methods that fit a sine curve between the daily extremes exist precisely to address this, and they will disagree with this calculator, particularly on days that straddle the base or the cutoff.
What the horizontal cutoff changes
The plain method assumes that development keeps accelerating as it gets warmer, without limit. That is not how organisms behave: above some temperature the rate stops rising, and above a further temperature it falls.
The horizontal cutoff is the simplest correction. Any temperature above the cutoff is replaced by the cutoff before the average is taken — so on a hot day the maximum is pulled down to the ceiling, and if the minimum is also above the ceiling both are, and the day contributes exactly the cutoff minus the base. The assumption being made is that development continues at a constant rate above the threshold rather than stopping or reversing.
Selecting the horizontal cutoff in the panel applies it to both the daily maximum and the daily minimum. With the plain method the cutoff field is ignored entirely, and the calculator says so rather than silently using it.
Which convention are you actually using
This is where degree day figures go wrong in practice, and the failure is organisational rather than mathematical.
Several conventions are in circulation. Some agencies apply no upper limit at all. Some apply the horizontal cutoff described above. Some apply a vertical cutoff, where a day exceeding the upper threshold contributes nothing. Others use a variant that also raises the daily minimum up to the base temperature before averaging, which the calculator here does not do. And sine-based methods integrate an assumed daily curve rather than averaging two points.
Run the same daily temperatures through those conventions and you get different totals — not slightly different in edge cases, but systematically different across a warm season, because the divergence compounds every hot day. So a degree day total is not a portable number. It is a number attached to a base temperature, an upper treatment and a method, and quoting it without those three is quoting a figure that cannot be reproduced.
Neither convention offered here is the correct one. They are two of several, and which applies depends on the organism, the model being fed and the body whose guidance you are following. The calculator states which one it applied; make sure whatever you paste the answer into does the same.
The honest limits of the number
A degree day accumulation is a coarse model of development, and it is used because it is cheap and roughly right, not because it is accurate.
It ignores day length, which drives development in many species independently of temperature. It ignores water availability, nutrition, and stress, any of which can stall development while the accumulator keeps climbing. It uses air temperature as a proxy for the temperature of the organism, which can differ substantially under strong sun or heavy transpiration. And it assumes a linear response between base and cutoff, which is an approximation that is weakest exactly where most of the interesting variation sits — near the thresholds.
Used for what it is good at — comparing seasons, anticipating a stage a little before it arrives, timing a scouting visit rather than a decision — it earns its keep. Used as a precise predictor of a date, it will embarrass you in an unusual year.
Notes on the implementation
The arithmetic was written and checked in a second implementation before this one, against hand-worked cases including a day whose mean falls below base, a day where both readings exceed the cutoff, and a cutoff set above the day’s maximum so that it changes nothing. Those same cases are re-checked in your browser when the page loads, along with the input parsing, and the result is reported in the panel above. If the self-test fails, the calculator refuses to display a total rather than showing one that might be wrong.