Degree-day calculator
Type a day's high and low and a base temperature to see the degree-days for that day by the modified average method that extension services print.
Checked against the sources at the bottom of this page on October 8, 2026. Published tables are revised: the source has the last word.
Work it out
How the answer is worked out
If the low is below the base temperature, the base temperature stands in for the low. If an upper cutoff is typed and the high is above it, the cutoff stands in for the high.
Degree-days = (high + low) divided by 2, minus the base temperature, with the stand-in values used where they apply. A result below zero counts as zero.
The three temperatures use one scale, Fahrenheit or Celsius, and the answer is in degree-days of that scale. Both readings are shown, since a Celsius degree-day is 5/9 of a Fahrenheit one.
What the sources say
Penn State Extension's page on growing degree days describes the same steps: add the high and low and divide by two, call the result zero if it is at or below the base temperature, otherwise take the mean minus the base, use the base in place of a low that falls below it, and use a cutoff in place of a high that rises above it. Its worked examples include a high of 80 and a low of 60 with a base of 50 giving 20 degree-days, and a high of 93, a low of 74, a base of 50 and a cutoff of 86 giving 30.
Utah State University Extension's page on degree days for insect pests calls this the Modified Average Method: the lower threshold is used in place of a daily low below it, and the upper threshold, or cutoff, in place of a daily high above it. It calls the starting point for adding up the days the biofix, which can be a biological event or a calendar date.
The University of California Statewide IPM Program's page on degree-days says several methods exist, that all are approximations, and that the method used should be the one used when the organism's growth figures were developed. It describes triangle and sine methods; this page uses the averaging method of the extension pages above. It also says it takes nine Fahrenheit degree-days to make five Celsius degree-days.
The page holds no base temperature, threshold or date for any plant or pest. Those belong to the organism and are published with its research.
Arithmetic only. The answer is only as good as the numbers entered. It is not a plan or a safe working load for any job.