Understanding your result
The headline is the fuel the house needs for a heating season, with the cost, the season’s heat and the degree days used in the sub-line. The stats give the cost, the heat in million BTU and the fuel per degree day. The table shows the climate zone and design temperature, the degree days and whether they are the zone’s typical figure or yours, the design heat loss and the heat loss per degree, the correction factor, then the heat, the fuel, the cost, and the average cost per month over a seven-month season.
The fuel per degree day is the figure to take to your bills. Divide any winter month’s fuel by that month’s degree days; if the result is close to the calculator’s, the estimate is right and the house behaves as designed. If it is much higher, the house is losing more heat than its design load says, and the duct leakage calculator and a blower-door test are the next steps.
The design heat loss comes from the furnace size calculator or a Manual J; the heating cost by fuel calculator runs the same season through every fuel at once; and the thermostat setback calculator shows what a lower night setting takes off the result.
How we calculate this
This is the variable-base degree-day method of the ASHRAE Handbook, Fundamentals, in its simplest form: the heat loss coefficient times the degree-hours, scaled by an empirical correction for internal and solar gains and part-load behaviour. The design temperatures and typical degree days for each zone are on the BTU per square foot table, drawn from ASHRAE design data and NOAA normals and rounded. The fuel energy contents and default prices are on the fuel energy content and cost page. For a heat pump enter the seasonal COP times 100 as the efficiency.
The assumptions behind the numbers
| Assumption | Value | Where it comes from |
|---|---|---|
| Degree-day base | 65 °F | NOAA and ASHRAE convention |
| Correction factor | 0.65 | ASHRAE degree-day method, typical for modern houses (0.6 to 0.8) |
| Typical HDD by zone | 300, 1,500, 2,800, 4,500, 6,300, 7,800, 9,500 | NOAA normals for representative cities, rounded |
| Design temperatures | 50, 35, 25, 15, 0, −10, −20 °F | Typical 99% heating design temperatures, rounded |
| Indoor temperature | 70 °F | Standard design condition |
| Energy content | EIA figures | US Energy Information Administration |
Assumptions last reviewed October 8, 2026.
The estimate is for a typical season; a cold winter can have 15% more degree days than normal and a mild one 15% fewer. It does not model thermostat setbacks, unusual solar gain, wood stoves supplementing the furnace, or the extra fuel a boiler with an indirect water heater uses for hot water in winter. The guide to heating and cooling degree days explains the method in detail, and the guide to comparing fuels per million BTU covers the fuel side.
Two worked examples
A gas-heated house in Chicago
Design heat loss 60,000 BTU/hr, zone 5 (0 °F design, 6,300 HDD), natural gas, 95% AFUE, $1.50 per therm.
- Heat loss per °F: 60,000 ÷ 70 = 857 BTU/hr per °F
- Season heat: 857 × 24 × 6,300 × 0.65 = 84.2 million BTU
- Gas: 84.2 million ÷ 0.95 ÷ 100,000 = 887 therms
- Cost: $1,330, about $190 a month over a seven-month season
- Fuel per degree day: 0.141 therms
If last January’s bill showed 190 therms in a month with 1,300 degree days, that is 0.146 therms per degree day, within 4% of the estimate; the house is behaving as its design load says.
An oil-heated house in Minnesota
2,400 sq ft, zone 6 (−10 °F design, 7,800 HDD), good insulation, heating oil at 85% AFUE, $3.70 per gallon.
- Design heat loss: 2,400 × 52 × 0.85 = 106,080 BTU/hr
- Heat loss per °F: 106,080 ÷ (70 − (−10)) = 1,326 BTU/hr per °F
- Season heat: 1,326 × 24 × 7,800 × 0.65 = 161.3 million BTU
- Oil: 161.3 million ÷ 0.85 ÷ 138,500 = 1,371 gallons, $5,071
- Fuel per degree day: 0.176 gallons
At $5,000 a season this house is the kind where a cold-climate heat pump or a dual-fuel system pays back quickly; the heat pump versus furnace calculator runs the same 161 million BTU through both.
Where to find your inputs
Design heat loss. From a Manual J report, the furnace size calculator, or estimated here from the floor area and insulation.
Degree days. On your gas or electric bill, from NOAA for your nearest station, or the zone’s typical figure if you leave the field at zero.
Efficiency. AFUE from the furnace or boiler label; for a heat pump, the seasonal COP × 100 (HSPF2 ÷ 3.412 × 100).
Price. The all-in price per unit from your bill or fuel delivery ticket.
Common mistakes
- Using Celsius degree days. They are five-ninths of the Fahrenheit figure and halve the estimate.
- Skipping the correction factor. Without it the estimate is about half again too high.
- Using the furnace’s size as the heat loss. Furnaces are oversized; use the house’s load.
- Comparing one month with the season. Compare fuel per degree day, month to month.
- Forgetting water heating. A gas bill includes the water heater; subtract the summer baseline.
- Expecting precision in a mild winter. Degree days vary 15% from year to year.
Questions people ask
- What is a heating degree day?
- One degree Fahrenheit by which a day's average outdoor temperature falls below 65 °F. A day averaging 30 °F contributes 35 heating degree days; a day at 65 °F or warmer contributes none. Adding them up over a year gives the climate's heating demand, about 6,300 for Chicago, 4,500 for Washington DC, 2,700 for Atlanta and 130 for Miami. The base of 65 °F reflects that internal gains keep a house comfortable down to about that outdoor temperature.
- How do I estimate heating fuel from degree days?
- Multiply the house's heat loss per degree (the design heat loss divided by the design temperature difference) by 24 hours and by the degree days, multiply by the ASHRAE correction factor of about 0.65, then divide by the equipment's efficiency and the fuel's energy content. A house losing 60,000 BTU per hour at 0 °F loses 857 BTU per hour per degree; over 6,300 degree days that is 84 million BTU, or 887 therms of gas in a 95% furnace.
- Why is there a 0.65 correction factor?
- Because the simple degree-day formula overstates fuel use. The design heat loss is calculated without the heat from people, appliances, lights and sunshine, which supply a meaningful share of the heat in real houses, and equipment runs more efficiently at part load than the formula assumes in some cases and less in others. ASHRAE's degree-day method applies an empirical factor of 0.6 to 0.8; 0.65 is a typical value for well-insulated modern houses.
- How do I check the estimate against my bills?
- Divide a winter month's fuel use by that month's heating degree days, which most utility bills print and NOAA publishes. If a month with 1,000 degree days used 140 therms, the house uses 0.14 therms per degree day, which matches the default example. A figure well above the calculator's means the house loses more heat than the design load suggests, through air leaks, missing insulation or duct losses.
- Can degree days estimate cooling costs too?
- Cooling degree days (degrees above 65 °F) describe the cooling climate, and the calculator shows the zone's typical figure for reference, but cooling energy depends more on sun, humidity and internal gains than heating does, so the degree-day method is much less reliable for it. For cooling, use the AC running cost calculator with run hours from a thermostat, or equivalent full-load hours.
- Where do I find degree days for my town?
- NOAA's National Centers for Environmental Information publishes monthly and annual heating and cooling degree days for thousands of US weather stations, and 30-year normals. Most gas and electric bills print the month's degree days next to the usage, and websites that compile weather-station data offer degree days for any base temperature. Use base 65 °F and Fahrenheit degree days; Celsius degree days are five-ninths as many.