About the Heating Degree Days Calculator
This heating degree days calculator converts a list of daily temperatures into heating degree days (HDD) and cooling degree days (CDD). Paste one day per line, either as a high and a low (which are averaged) or as a single daily mean, choose a base temperature and the tool adds up how far each day’s mean fell below the base (heating) or rose above it (cooling).
Degree days are the standard way energy managers, utilities and building engineers measure how cold or hot a period was. Because a building’s heating fuel use rises roughly in proportion to HDD, you can compare bills between winters fairly, check whether an efficiency upgrade really worked, or forecast next month’s heating cost. If you know how much fuel your home uses per degree day (bill usage ÷ HDD for the billing period), enter it to turn degree days into an estimated cost.
Choose °F or °C and the base switches to match: in the US the usual base is 65 °F; in the UK and much of Europe 15.5 °C or 18 °C is common. A lower base suits well-insulated buildings with large internal gains. Lines can include a date or label before the numbers (for example "2026-01-05, 41, 27"); text tokens are ignored. Use the same temperature unit throughout; degree days in °C are 5/9 the size of °F degree days, so an energy-per-HDD factor must be in the same unit.
With the default inputs, the heating degree days is 163. Change any value above to recalculate instantly.
How to use the heating degree days calculator
- 1Paste daily temperatures, one day per line, as "high, low" or a single daily mean.
- 2Pick °F or °C and set the base temperature (65 °F is the US standard).
- 3Optionally enter your heating energy per degree day and its price.
- 4Read the total HDD and CDD and the per-day breakdown.
- 5Compare periods or bills by dividing energy use by HDD.
Formula and method
For each day, the mean temperature is the average of the high and low (or the single value you enter). A day contributes heating degree days equal to how many degrees its mean is below the base temperature, and cooling degree days equal to how many degrees it is above. A day can never have both. The period totals are the sums over all days.
This is the simple mean-temperature method used by NOAA and most utilities. The optional energy estimate multiplies total HDD by your home’s energy use per degree day, which assumes heating use is proportional to HDD — a good approximation for heating-dominated months, but it ignores water heating and cooking, so derive the factor from heating-only usage where possible.
- T_base
- Base (balance-point) temperature, e.g. 65 °F
- T_mean
- Daily mean temperature
- HDD, CDD
- Heating and cooling degree days
Worked examples
A cold week (°F, base 65)
The daily means are 38, 43, 31, 35, 48, 52 and 45 °F. Subtracting each from 65 gives 27 + 22 + 34 + 30 + 17 + 13 + 20 = 163 HDD. At 0.15 therms per HDD the week’s heating needs about 24.5 therms, roughly $36.68 at $1.50 per therm.
Early summer means (°F)
With single daily means, five days are above 65 °F and contribute 7 + 15 + 20 + 13 + 3 = 58 cooling degree days. The 64 °F day adds 1 heating degree day.
Celsius with dates, base 18 °C
Dates are ignored and the last two numbers on each line are averaged: 0.5, 2.5, −2, 6 and 8 °C. Against an 18 °C base that is 17.5 + 15.5 + 20 + 12 + 10 = 75 HDD, with the coldest day contributing 20.
Frequently asked questions
What are heating degree days?+
Heating degree days measure how much, and for how long, the outdoor temperature was below a base temperature. A day with a mean of 50 °F against a 65 °F base counts 15 HDD. Adding up the days gives a single number that tracks heating demand for a week, month or season.
Why is 65 °F used as the base temperature?+
Historically, US homes needed little heating when the outdoor mean was about 65 °F, because sunlight, people and appliances supplied the remaining heat. NOAA and US utilities still publish degree days at 65 °F. Modern well-insulated homes often have a lower balance point, so some analysts use 60 °F or 55 °F.
How do I use degree days to compare energy bills?+
Divide the heating fuel used in each billing period by the HDD for the same dates. The result, such as therms per HDD, removes the effect of weather, so if it falls after you add insulation or a new furnace, the upgrade is working regardless of whether the winter was mild or harsh.
What is the difference between HDD and CDD?+
Heating degree days count degrees below the base and indicate heating demand; cooling degree days count degrees above the base and indicate air-conditioning demand. Both use the same daily mean temperature, and any single day contributes to only one of them.
Where can I get daily temperature data?+
National weather services publish daily highs and lows for weather stations. In the US, NOAA’s Climate Data Online provides daily records and ready-made degree-day summaries; many airports and local weather sites also list daily history you can paste into this tool.