Degree Days Calculator
Reviewed by Hamza Gujjar, Algorithm Architect & Software Developer
Last updated: July 27, 2026How energy analysts normalize weather-driven energy demand
Degree days quantify how much heating or cooling demand a given day creates relative to a reference base temperature (commonly 65F/18C, the point below which most buildings need heating and above which they need cooling).
Worked example
For a day averaging 45F outdoor temperature against a 65F base temperature (heating mode):
HDD = max(0, 65 - 45) = 20.0 Heating Degree Days (HDD)
Frequently asked questions
Why cap the result at zero? A day warmer than the base temperature (in heating mode) or colder than it (in cooling mode) contributes no degree days at all for that mode - there's no heating demand on a hot day, so negative values are floored to zero rather than allowed to offset other days.
How are degree days actually used? Utilities and energy analysts sum daily degree days across a billing period or season to normalize energy usage comparisons - this lets a building's energy consumption be fairly compared across different weather conditions, locations, and years, isolating actual efficiency changes from simple weather variation.