ℹ About This Calculator
The degree-day method is a simple, widely used way to estimate a building's annual heating (or cooling) energy from climate data and the building's heat-loss characteristics. This calculator estimates the annual energy load and fuel input from the heating degree days for your location, the building's overall heat-loss coefficient (UA), and the heating system efficiency, in US units. It is ideal for quick energy estimates, fuel budgeting, comparing efficiency upgrades, and sanity-checking a more detailed energy model.
Heating degree days (HDD) measure how cold a location is over a year by summing, for each day, the number of degrees the average temperature falls below a base (usually 65°F). Multiplying the degree days by 24 hours and the building's UA (the sum of U·A for the whole envelope plus infiltration) gives the annual heating energy the building loses, and dividing by the system efficiency gives the fuel that must be purchased. The same approach with cooling degree days (CDD) estimates cooling energy. It assumes internal gains offset the load below the base temperature.
Frequently Asked Questions
What are heating degree days?
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Heating degree days (HDD) are a measure of how cold a location is over time, calculated by summing, for each day, the number of degrees the day's average temperature falls below a base temperature — usually 65°F in the US. A day averaging 45°F contributes 20 heating degree days. Adding them over a year gives the annual HDD, a single number that captures the heating demand of the climate and is published for cities by ASHRAE and weather services.
How do I estimate annual heating energy from degree days?
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Multiply the annual heating degree days by 24 hours and by the building's overall heat-loss coefficient UA (in BTU/hr·°F): annual load = 24 × DD × UA. This gives the heat the building loses over the year in BTU. Divide by the heating system's efficiency to get the fuel input required. For example, UA = 500 with 5,000 degree days and 90% efficiency needs about 667 therms per year.
What is the building UA value?
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UA is the building's overall heat-loss coefficient — the sum of U-value times area (U·A) for every part of the envelope (walls, roof, windows, floor) plus the equivalent conductance of air infiltration and ventilation, in BTU/hr·°F. It represents how much heat the whole building loses per degree of temperature difference. A lower UA means a tighter, better-insulated building that uses less energy for the same climate.
What base temperature is used for degree days?
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The traditional base temperature is 65°F, on the assumption that internal gains from people, lights and appliances offset the heat loss until the outdoor temperature drops below 65°F, at which point heating is needed. Modern, well-insulated or high-internal-gain buildings may have a lower balance-point temperature, so degree days to other bases (60°F, 55°F) are sometimes used for a better match. Always use degree days computed to the base that fits your building.
Can I use degree days for cooling too?
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Yes — cooling degree days (CDD) sum the degrees each day's average temperature rises above the base, and the same method estimates cooling energy: cooling load ≈ 24 × CDD × UA, divided by the cooling system's efficiency (expressed as COP or SEER). Cooling estimates are less accurate than heating because solar gain, humidity and internal loads play a larger role, so degree-day cooling estimates are rougher and often supplemented by a full load model.
How accurate is the degree-day method?
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The degree-day method is a good first-order estimate, typically within 10-20% for heating in a steady-occupancy building, and it's excellent for comparing before-and-after energy for efficiency upgrades since the same assumptions cancel out. It is less accurate where solar gain, occupancy schedules or internal loads vary a lot, or for cooling. For detailed design or code compliance, an hour-by-hour energy model is used instead, with degree days serving as a quick check.
Is this degree days calculator accurate?
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It applies the standard degree-day energy equation (24 × DD × UA ÷ efficiency), so it gives a reliable first-order annual energy estimate given accurate inputs. Use published degree days for your city at the correct base temperature, a UA that reflects the actual envelope and infiltration, and the real seasonal efficiency of the heating system. For precise design, confirm with a detailed energy model.
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