Degree Days Calculator
The Degree Days Calculator converts a mean outdoor temperature into heating degree days (HDD) and cooling degree days (CDD) — the standard measure of how far, and for how long, the weather sits below or above a comfort balance point. Degree days let you compare seasons and estimate the energy a building needs for heating or cooling.
Enter Values
Minimum 0
How to use this calculator
- Enter the mean outdoor temperature for the period in °C.
- Set the base (balance-point) temperature — 18 °C is a common default.
- Enter the number of days, then read the HDD, CDD and dominant mode.
How it works
For each day, heating degree days are max(0, base − mean) and cooling degree days are max(0, mean − base). Multiplying by the number of days gives the totals in °C·days. Whichever total is larger sets the dominant mode; equal totals (typically both zero) are reported as Balanced.
Worked example
Worked example. With a mean of 10 °C, a base of 18 °C and 30 days: HDD = (18 − 10) × 30 = 240 °C·days and CDD = 0, so heating dominates.
Common mistakes
- Using a daily-detail base when only a single mean temperature is available — this simplified method assumes one mean per period.
- Leaving the base at 18 °C when the building's real balance point differs.
- Comparing degree-day totals across different base temperatures without noting the base.
Frequently asked questions
What base temperature should I use?
18 °C is a widely used default, but the true balance point depends on the building's insulation and internal heat gains — adjust the base to match your site.
What does °C·days mean?
It is degrees of temperature difference summed over days. 8 °C below base for 30 days equals 240 °C·days, the same as 16 °C below base for 15 days.
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Cite this tool
Zerdly. (2026). Degree Days Calculator. Retrieved from https://www.zerdly.com/tools/degree-days-calculator/
Tip: Enter any known values to calculate the remaining results.
Designed to run locally in your browser. Tool inputs are not intentionally stored by Zerdly unless stated otherwise.