Enter the maximum temperature, minimum temperature, and base temperature into the calculator to determine the signed degree-day value (positive = cooling, negative = heating). This calculator can also evaluate any of the variables given the others are known.

Degree Day Calculator

Enter any 3 values to calculate the missing variable

Degree Day Formula

The following formula is used to calculate the signed degree-day value for a single day (average temperature departure from the base temperature). A positive result corresponds to cooling conditions; a negative result corresponds to heating conditions.

DD = (Tmax + Tmin) / 2 - Tbase

Variables:

  • DD is the signed degree-day value (°C-days or °F-days)
  • Tmax is the maximum temperature for the day (°C or °F)
  • Tmin is the minimum temperature for the day (°C or °F)
  • Tbase is the base temperature (°C or °F)

To calculate the degree days, add the maximum temperature for the day to the minimum temperature for the day. Divide the result by 2 to get the average temperature for the day. Subtract the base temperature from the average temperature to get the signed degree-day value (DD).

What is a Degree Day?

A degree day is a measurement used in weather and energy management to quantify how much (and for how long) outdoor air temperature is above or below a chosen base temperature. A common base temperature used in the U.S. is 65°F (≈18.3°C), while some datasets use 18°C (≈64.4°F) depending on the application and region.

In standard reporting, degree days are usually split into:

  • Cooling Degree Days (CDD) = max(0, ((Tmax + Tmin)/2) − Tbase)
  • Heating Degree Days (HDD) = max(0, Tbase − ((Tmax + Tmin)/2))

This calculator uses the signed form DD = ((Tmax + Tmin)/2) − Tbase so that you can solve for any of the variables. If DD > 0, then CDD = DD and HDD = 0. If DD < 0, then HDD = −DD and CDD = 0.

How to Calculate Degree Day?

The following steps outline how to calculate the Degree Day using the formula: DD = (Tmax + Tmin) / 2 – Tbase

  1. First, determine the maximum temperature for the day (Tmax) in °C or °F.
  2. Next, determine the minimum temperature for the day (Tmin) in °C or °F.
  3. Next, determine the base temperature (Tbase) in °C or °F.
  4. Use the formula DD = (Tmax + Tmin) / 2 – Tbase to calculate the signed Degree Day value (DD).
  5. Interpret the sign: if DD is positive, it corresponds to cooling (CDD = DD). If DD is negative, it corresponds to heating (HDD = −DD).

Example Problem:

Use the following variables as an example problem to test your knowledge.

Tmax = 25°C

Tmin = 15°C

Tbase = 10°C