Degree Days Calculator
Calculate heating and cooling degree days from a base temperature and average outdoor temperature — the standard measure of heating and cooling demand.
How it works
HDD/day = max(0, base − avg temp) · CDD/day = max(0, avg temp − base) · Total = per-day × days
Degree days measure how far, and for how long, the outdoor temperature sits below (heating) or above (cooling) a comfort base — usually 65°F or 18°C. Each degree the daily average falls below the base counts as one heating degree day; sum them over a period and you have a number that tracks heating demand far better than temperature alone. Utilities and energy audits use degree days to compare bills across months and to size heating and cooling systems.
Worked example
With a 65°F base and a 45°F average over 30 days: each day is 65 − 45 = 20 heating degree days, so the month totals 20 × 30 = 600 HDD. Because the average is below the base, cooling degree days are zero.
Sources: Degree day — Wikipedia
Frequently asked questions
What base temperature should I use?
The traditional default is 65°F (18°C), the outdoor temperature below which buildings typically need heating. Well-insulated or internally warm buildings sometimes use a lower base.
What are degree days used for?
They normalize energy use for weather. Dividing your heating bill by heating degree days lets you compare a mild month to a cold one, or this year to last, on equal footing.
Can a day have both heating and cooling degree days?
Using a single daily average, no — the average is either above or below the base. More detailed methods using hourly data can register both on a day with big temperature swings.
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This calculator is for educational purposes. Double-check important results before acting on them.