Heating Degree Day Energy Calculator
Estimate a season’s heating energy and cost from your home’s heat-loss rate and local heating degree days — and what more insulation or air sealing would save.
Method reviewed October 26, 2026. Version 1.0.0. How it works
This is an estimate. It uses a simplified method and typical assumptions, all listed below. Use it to plan and to check quotes; equipment should be sized from a full room-by-room load calculation (ACCA Manual J) by a qualified professional.
Result
- Heat delivered per season
- 88.16MMBtu
- Heating cost per season
- $1,336
About 88.2 MMBtu of heat per season, costing about $1,336.
- The 65 °F degree-day method counts heat loss below 65 °F outdoors. Internal and solar gains, thermostat setbacks and a well-insulated home’s lower balance point make real use differ — compare with your bills.
- Heat-loss rate (UA)
- 816.3BTU/h·°F
- Fuel input per season
- 92.8MMBtu
Upgrade
- Reduction in UA
- 32.22BTU/h·°F
- Heat saved per season
- 3.48MMBtu
- Cost saved per season
- $53
Show the working
- UA = design heat loss ÷ (65 °F − design temperature)40,000 ÷ (65 − 16) = 816.3 BTU/h·°F
- Season heat = UA × 24 h × heating degree days816.3 × 24 × 4,500 = 88,163,265 BTU = 88.16 MMBtu
- Cost = heat ÷ efficiency × price88.163 ÷ 0.95 × $14.4 = $1,336
- ΔUA = area × (1/R before − 1/R after)32.223 BTU/h·°F → 3.48 MMBtu per season
The degree-day method
Heating degree days add up how far, and for how long, the daily mean temperature falls below 65 °F over a season. A home’s heat-loss rate, UA, is the heat it loses per hour for each degree of temperature difference. The season’s heat need is then UA × 24 hours × heating degree days.
If you know your design heat loss, the calculator finds UA by dividing it by the difference between 65 °F and your design temperature. Dividing the heat need by your heating system’s efficiency gives the fuel bought.
Estimating an upgrade
Insulation lowers UA by area × (1 ÷ R before − 1 ÷ R after). Air sealing lowers it by 0.018 × volume × the reduction in natural air changes per hour. Multiplying the change in UA by 24 × degree days gives the heat saved each season.
The 65 °F method ignores internal and solar gains and setbacks, so treat it as an estimate and compare with past bills.
Worked example
These are the example values the calculator opens with, and the result the engine calculates for them.
Inputs
- Heat-loss rate from
- Design heat loss and design temperature
- Design heat loss
- 40,000 BTU/h
- Outdoor design temperature
- 16 °F
- Heating degree days (base 65 °F)
- 4,500 °F·day
- Heating system efficiency
- 95 %
- Fuel price per MMBtu of input
- 14.4 $
- Estimate an upgrade
- Insulation (area and R-values)
- Area upgraded
- 1,000 ft²
- R-value before
- 19 h·ft²·°F/BTU
- R-value after
- 49 h·ft²·°F/BTU
Result
- Heat delivered per season
- 88.16 MMBtu
- Heating cost per season
- $1,336
- Heat-loss rate (UA)
- 816.3 BTU/h·°F
- Fuel input per season
- 92.8 MMBtu
About 88.2 MMBtu of heat per season, costing about $1,336.
Assumptions
- Seasonal heat = UA × 24 × heating degree days (base 65 °F); UA from your design heat loss when you enter one.
- Upgrades change UA by A × (1/R₁ − 1/R₂), or by 0.018 × volume × ΔACH for air sealing.
Limitations
- An estimate: gains, setbacks and the base temperature affect real use. Calibrate against past bills where possible.
Frequently asked questions
Where do I find heating degree days?
From NOAA, EIA or many utilities, for your location and for a typical or a specific year.
Why might my bills be lower than this?
Heat from people, appliances and the sun covers part of the loss, and a well-insulated home needs no heat until it is colder than 65 °F outside. The method tends to overestimate for efficient homes.
How do I enter electricity as the fuel?
Price per MMBtu of electricity is the price per kWh × 293.07. For a heat pump, enter its seasonal COP × 100 as the efficiency.
Sources
- Units and calculators explained: Degree-days — U.S. Energy Information Administration. Degree days compare the daily mean temperature with a 65 °F base; heating degree days accumulate when the mean is below 65 °F.
- Calculating heat loss by infiltration (factsheet, 1988) — Washington Energy Extension Service (via AIVC). Infiltration heat loss Q = 0.018 × ACH × volume × ΔT.
- Heat Loss from Buildings — The Engineering ToolBox. Conduction heat loss Q = U·A·ΔT with U = 1/ΣR.
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