Teaching · For Teachers · Life and earth science

Heat, counted the
way plants and
insects count it.

Log each day’s high and low from a school weather station or a forecast. The calculator works out growing degree days by the method you choose, keeps the running total and marks the day a target is reached.

Start a logWhat you get

The degree day calculator

The worked example is made up: eight days of May temperatures for a corn field, with a target of 120. Replace them with your own readings. Nothing you type leaves this browser.

Loading the calculator. If nothing appears, turn on JavaScript.

What you get

Teaching · For TeachersFree

What you finish with
Growing degree days for every day in your log, the running total, the average a day and the date a target is reached, with notes on any day that was capped or raised. It prints as a log and downloads as a spreadsheet for graphing.
Who it is for
Science and agriculture teachers and their students, school and home gardeners, and families tracking a garden or a class planting.
Format
Browser calculator for a phone, tablet or computer, by touch or keyboard. Fahrenheit or Celsius. Up to 200 days. Prints a log; downloads a spreadsheet (.csv); export and import a file.
Access
Free. No account, no sign-in and nothing to install. Everything you type stays in this browser; nothing is sent to Caveat.
Where it lives
The For Teachers space in Caveat Teaching.
Support
Questions, a correction or a request: team@caveatacademic.com.

How to use it

  1. Choose units and the method, and set the base temperature (and the cutoff for the modified method).
  2. Add a line for each day with its high and low. Days can go in any order; the page sorts them.
  3. Type a target if you are tracking a stage, such as emergence, from a seed company or extension guide.
  4. Read each day’s degree days and the running total.
  5. Print the log or download the spreadsheet to graph it.

Start a log

How degree days are worked out

The day’s average temperature above the base.

A mild dayHigh 74°F → 74°FLow 51°F → 51°F(74 + 51) ÷ 2 − 5012.5degree daysA hot dayHigh 88°F → 86°FLow 63°F → 63°F(86 + 63) ÷ 2 − 5024.5degree daysA cool dayHigh 61°F → 61°FLow 42°F → 50°F(61 + 50) ÷ 2 − 505.5degree daysA mild dayHigh 74°F → 74°FLow 51°F → 51°F(74 + 51) ÷ 2 − 5012.5degree daysA hot dayHigh 88°F → 86°FLow 63°F → 63°F(86 + 63) ÷ 2 − 5024.5degree daysA cool dayHigh 61°F → 61°FLow 42°F → 50°F(61 + 50) ÷ 2 − 505.5degree days
Three days from the worked example, base 50°F and cutoff 86°F, modified method. A high over the cutoff is counted at the cutoff; a low under the base is raised to the base.

The arithmetic

A day’s growing degree days are the average of its high and low minus the base temperature, and never less than zero. The modified method used for corn first caps the high at the cutoff (86°F, or 30°C) and raises a high or low below the base to the base, because growth slows above the cutoff and does not run backwards below the base. The running total adds the days in date order.

Why the totals matter

Plants and insects develop on heat, not the calendar: a cool May delays corn emergence and a warm one speeds it. Extension services publish degree-day targets for crop stages and for pest life cycles, each with its own base. Use the base and method a target was written for, and keep Fahrenheit and Celsius totals apart: a Celsius degree day is 1.8 Fahrenheit degree days.

Sources

  1. McMaster, G. S., and Wilhelm, W. W. (1997). Growing degree-days: one equation, two interpretations. Agricultural and Forest Meteorology, 87(4), 291–300.
  2. NOAA National Centers for Environmental Information and the Climate Prediction Center (cpc.ncep.noaa.gov): degree day definitions and daily temperature data.
  3. Iowa State University Extension and Outreach, and other land-grant university extension services: the modified 86/50 method for corn growing degree days and stage targets.
  4. U.S. Department of Agriculture, National Agricultural Statistics Service, Crop Progress reports: crop stages tracked through the season.

The worked example is made up, including its target. Degree days are only as good as the temperatures entered. Your log stays in this browser. Support: team@caveatacademic.com.