Heating Degree Days Calculator
Heating degree days measure how cold a period was, in one number. Enter the days and the average temperature and compare winters, months or cities on equal footing.
Your results
Degree-days are floored at zero: days warmer than the base temperature add no heating demand. A proper degree-day total sums each day individually; a period average is a close approximation for a single stretch of weather.
Worked example
January cold snap: 31 days averaging -5 C
31 × (18 - (-5)) = 31 × 23 = 713 degree-days. The same weather in Fahrenheit (average 23 F, base 65 F): 31 × 42 = 1302 F-degree-days, and 1302 ÷ 1.8 = 723 C-degree-days (slightly different because 65 F is 18.3 C, not exactly 18 C).
How this calculator works
degree-days = days × max(0, base temperature - average temperature). Fahrenheit degree-days convert to Celsius degree-days by dividing by 1.8, because a Fahrenheit degree is 5/9 the size of a Celsius degree.
Heating fuel use tracks degree-days closely: if this January has 10 percent more degree-days than last January, expect roughly 10 percent more space-heating fuel, all else equal. Utilities and Environment Canada publish official daily degree-day totals; this tool lets you run your own period and base.
Common questions
What base temperature should I use?
18 C in Canada (65 F in the US). It approximates the outdoor temperature above which most buildings need no heating. Greenhouses, warehouses and industrial users often track other bases, which is why the base here is editable.
Can I compare two cities with this?
Yes: enter the same period length and each city's average temperature for that period. The city with more degree-days needed proportionally more heating energy for a similar building.
Why is the floor at zero?
Degree-days only count heating demand. A warm spell does not create negative heating use; cooling degree days are the mirror-image measure for summer, using the same formula above the base.
Related calculators
Ready to turn degree days into dollars? Compare systems with the Heating Cost Comparison Calculator. For a single appliance, use the Appliance Electricity Cost Calculator, and for heater sizing, the Heating BTU Calculator.
Estimates only, based on the average temperatures you enter. Official degree-day records come from Environment and Climate Change Canada.