Degree-Day Heating Calculator
Enter heating degree-days, your home's UA (heat-loss coefficient) and fuel details to get seasonal fuel consumption and cost — the standard degree-day method.
Last updated: 2026-09-15
How the calculation works
- Each degree-day represents one day averaging 1 °F below 65 °F; total heat needed is UA × HDD × 24 hours.
- Fuel consumption divides heat by the fuel's energy content and the system's efficiency.
- Fuel contents: gas 100,000 BTU/therm, oil 138,500 BTU/gal, propane 91,300 BTU/gal, electric 3,412 BTU/kWh.
Formula
Heat (BTU) = UA × HDD × 24 Fuel = Heat ÷ (fuel content × efficiency)
| Symbol | Meaning | Unit |
|---|---|---|
HDD | Heating degree-days | °F·day |
UA | Whole-house heat-loss coefficient | BTU/h·°F |
η | System efficiency | — |
Worked example
Interpreting the result
The UA figure is the fingerprint of your envelope: halving it (insulation, air-sealing) halves the fuel bill at any HDD. The calculation also reveals payback comparisons — a heat pump with COP 3 divides the same heat requirement's cost by ~2.5 versus an 85% furnace, before fuel price differences.
Assumptions
- Linear heat loss with temperature (valid well below the balance point).
- HDD base 65 °F matches a 65–68 °F thermostat habit; colder habits should use HDD base 60.
- No internal or solar heat gains credited (they typically offset 10–20%).
Limitations
- Wind-driven infiltration varies more than the model assumes.
- Does not handle heat pumps' declining COP at low temperature — use their seasonal HSPF-based average.
Frequently asked questions
Where do I find degree-days for my location?
NOAA and degree-days.net publish HDD by station and base temperature. Most US climates run 2,000 (mild coastal) to 7,000+ HDD (upper Midwest) per year.
What is UA?
The whole-house heat-loss coefficient: how many BTU/h your home loses per °F of indoor-outdoor difference. Estimate it as design heating load ÷ design ΔT.