Rex says
Everyday utility math — the kind you'd otherwise pull up four browser tabs for. I keep it to one clean answer.
Try a scenario
Click to load — tweak from there.Inputs
Result
Total heating degree days
810
Estimated fuel units needed
792.4
Estimated heating cost
$1,069.73
Average HDD per day
27.0
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One click, a permanent link with your numbers baked in.
How to use this
- 1Enter average outdoor temperature over period (°F).
- 2Enter number of days.
- 3Enter base temperature (°F).
- 4Enter fuel units per hdd (from past bill).
- 5Enter system efficiency (afue) (%).
- 6Enter fuel price per unit (therm/gallon/kwh) ($).
- 7Read your total heating degree days on the right — it updates as you type.
- 8Hit Share to keep the scenario or send it to someone.
About this calculator
Heating degree days quantify how much and how long you need to heat a building — one HDD accrues for each degree the daily average temperature falls below a 65°F base, the point most buildings need no heating or cooling. Utilities and HVAC contractors use accumulated HDD over a billing period or season to normalize energy usage across different weather, letting you compare this January's gas bill to last January's fairly. This calculator converts an average outdoor temperature and duration into total HDD, then estimates heating fuel cost using your system's efficiency and fuel price, which is the same logic behind utility 'weather-normalized billing' comparisons.
Worked example
Using the values the calculator loads with:
Inputs
- Average outdoor temperature over period: 38 °F
- Number of days: 30
- Base temperature: 65 °F
- Fuel units per HDD (from past bill): 0.9
- System efficiency (AFUE): 92 %
- Fuel price per unit (therm/gallon/kWh): 1.35 $
Results
- Total heating degree days: 810
- Estimated fuel units needed: 792.4
- Estimated heating cost: $1,069.73
- Average HDD per day: 27
What each field means
Inputs
- Average outdoor temperature over period (°F)
- The average outdoor temperature over period used in the calculation, measured in °F. Starts at 38 °F so you have a working example on load. Accepted range: -20–65 °F.
- Number of days
- The number of days used in the calculation. Starts at 30 so you have a working example on load. Accepted range: 1–365.
- Base temperature (°F)
- The base temperature used in the calculation, measured in °F. Starts at 65 °F so you have a working example on load. Accepted range: 55–70 °F.
- Fuel units per HDD (from past bill)
- The fuel units per hdd (from past bill) used in the calculation. Starts at 0.9 so you have a working example on load.
- System efficiency (AFUE) (%)
- The system efficiency (afue) used in the calculation, measured in %. Starts at 92 % so you have a working example on load. Accepted range: 50–100 %.
- Fuel price per unit (therm/gallon/kWh) ($)
- The fuel price per unit (therm/gallon/kwh) used in the calculation, measured in $. Starts at 1.35 $ so you have a working example on load.
Results
- Total heating degree days
- Returned as a whole number and shown as the headline result. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Estimated fuel units needed
- Returned as a whole number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Estimated heating cost
- Returned as a money amount in US dollars. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Average HDD per day
- Returned as a whole number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
FAQ
Why is 65°F the standard base temperature?
It reflects a historical assumption that indoor comfort sits around 70°F, with roughly 5°F of 'free heat' coming from occupants, lighting, and appliances, so a building needs no active heating until outdoor temps drop below 65°F. Utilities still use it because decades of billing data are built on that base.
Where do I find my building factor?
Divide fuel used (therms, gallons, or kWh) on a past bill by the HDD accumulated over that exact billing period, which you can get from NOAA climate data or your utility's usage history — this back-calculates your specific building's efficiency and size into one number.
How is this different from cooling degree days?
Cooling degree days (CDD) flip the comparison — they accumulate when average temperature exceeds the base, measuring air conditioning demand instead of heating demand. Together HDD and CDD are used to size HVAC systems and forecast seasonal energy budgets.
Can I use a different base temperature?
Yes, well-insulated modern homes sometimes use 60-62°F bases since they need less outdoor-driven heating, while older or larger buildings may use 65-68°F; check what base your utility uses for consistent comparisons.
Accuracy and limitations
- Estimates assume standard, average conditions — local rules, pricing, and materials vary.
- Results are rounded for readability; add a buffer before ordering, booking, or committing.
- Double-check anything with a real cost attached against a local quote.
Related tools
Cite this calculator
Writing about this topic? Grab a citation — every link helps keep these tools free.
RevenueLab. (2026). Heating Degree Days Calculator. Retrieved from https://www.revenuelab.fyi/toolbox/heating-degree-days
<p>Source: <a href="https://www.revenuelab.fyi/toolbox/heating-degree-days" target="_blank" rel="noopener">Heating Degree Days Calculator — RevenueLab</a> (2026).</p>
Source: [Heating Degree Days Calculator — RevenueLab](https://www.revenuelab.fyi/toolbox/heating-degree-days) (2026).