🌡️ Enter Daily High & Low Temperatures

Enter the high and low temperature for each day. We calculate the mean and subtract from the base temperature (65°F standard). Each degree below base = 1 HDD.

65°F is the US industry standard — heating rarely needed above this

📅 Day-by-Day Temperatures (up to 14 days)
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— HDD
Total heating degree days (live)
Enter daily temperatures above
Avg: —°
0 days with HDD
Coldest: —°

Calculating your heating degree days…

🌡️ Your Heating Degree Days
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Total HDD
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Heating Degree Days
Average Temp
—°
Mean daily temperature
Days Below Base
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out of — days entered
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Coldest Mean
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Warmest Mean
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HDD
Avg HDD/day
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65°
base
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Heating Load Severity
Mild Moderate Severe —

📅 Day-by-Day HDD Breakdown

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🏠 What These HDD Mean for You

Heating Degree Days — Explained Simply

HDD is a simple but powerful measure of how cold a period was. It tells you how hard your heating system had to work. Here’s everything you need to know.

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What Are Heating Degree Days?

A Heating Degree Day (HDD) measures how much the outdoor temperature fell below the base temperature (65°F). If the mean daily temperature was 45°F, you had 20 HDD that day. Over a week or month, you add them all up. More HDD = colder period = more heating fuel used.

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The Formula Is Simple

Mean daily temperature = (high + low) ÷ 2. Then: HDD = maximum of (0, base temperature − mean). So if mean was 55°F and base is 65°F, you get 10 HDD. If mean was 70°F, you get zero HDD — no heating needed above the base temperature.

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Why 65°F Is the Base

At 65°F (18°C) outdoor temperature, most buildings with normal internal heat gains (people, appliances, sunlight) don’t need active heating. Below 65°F, heat starts escaping faster than it’s generated internally, so the furnace kicks on. This is why 65°F became the international standard base temperature.

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HDD and Fuel Bills

Your heating fuel use is roughly proportional to HDD. If you used 100 therms of gas in a month with 500 HDD, expect about 120 therms in a month with 600 HDD (20% more HDD = 20% more fuel). This makes HDD a powerful tool for comparing different heating periods or different locations.

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Using HDD for Energy Planning

Utility companies use HDD to normalize energy consumption data. If you see your bill was “weather-normalized,” they used HDD to remove the effect of temperature variation. You can do the same — divide your fuel use by HDD to get a “fuel per HDD” number that lets you track true efficiency over time.

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CDD vs HDD

Cooling Degree Days (CDD) measure cooling need — when temperature is above the base. Heating Degree Days measure heating need — when temperature is below the base. Miami has lots of CDD and few HDD. Minneapolis has lots of HDD and few CDD. Most US cities have both, with the balance shifting by season.

📐 The HDD Calculation Formula

Mean Daily Temp = (Daily High + Daily Low) ÷ 2 Daily HDD = MAX(0, Base Temperature − Mean Daily Temp) Period HDD = Sum of all daily HDD values in the period Estimated Fuel Use = (HDD × Home Area × Insulation Factor) ÷ (Efficiency × Fuel Energy Content)

The base temperature is 65°F (18.3°C) in the US and Canada, 60°F (15.5°C) in the UK. Fuel energy content: natural gas ≈ 100,000 BTU/therm; heating oil ≈ 138,500 BTU/gallon; propane ≈ 91,600 BTU/gallon; electricity ≈ 3,412 BTU/kWh. The insulation factor adjusts for how leaky your home is.

Annual HDD by US City — Reference Table

Base 65°F. Source: NOAA 30-year Climate Normals.

CityAnnual HDDAvg Winter TempHeating SeasonClimate
Miami, FL~20068°FDec–Jan onlyVery mild
Los Angeles, CA~1,27557°FNov–MarMild
Atlanta, GA~2,99044°FNov–MarMild-Moderate
Dallas, TX~2,31046°FNov–MarMild-Moderate
Denver, CO~6,02033°FOct–AprModerate
Chicago, IL~6,53028°FOct–AprModerate-Cold
New York, NY~4,75035°FOct–AprModerate
Boston, MA~5,63030°FOct–AprCold
Minneapolis, MN~8,38016°FSep–MayVery Cold
Anchorage, AK~10,86013°FSep–JunExtreme

Frequently Asked Questions

It’s straightforward. For each day: find the mean temperature by adding the high and low, then dividing by 2. Subtract that from 65°F (the base temperature). If the result is positive, that’s your HDD for the day. If it’s zero or negative, you have zero HDD. Add up all the daily values to get your period HDD. This calculator does all that automatically — just enter your daily highs and lows.
The 65°F base was established by research showing that at this outdoor temperature, the heat generated by occupants, appliances, and sunlight is usually enough to maintain comfortable indoor temperatures without active heating. At 65°F outdoors, most homes naturally stay around 68–70°F inside. The 65°F standard was adopted by NOAA, DOE, and energy utilities in the US, making it easy to compare data across different sources.
Once you have your HDD for a period, use the Fuel Cost tab above. You’ll need your home’s square footage, your fuel type, and your fuel rate (from your utility bill). The calculator applies a thermal load factor based on your home size and insulation level, then divides by your heating system’s efficiency to estimate actual fuel consumption. It’s a close estimate — actual fuel use also depends on duct losses, thermostat settings, and occupant behavior.
It varies widely by location. Mild climates like Southern California see 1,000–2,000 HDD annually. The national average for the continental US is roughly 4,500–5,000 HDD. Cold northern climates like Minnesota or upstate New York see 7,000–10,000+ HDD. The higher your annual HDD, the more you spend on heating and the more you benefit from home insulation upgrades or a higher-efficiency furnace or heat pump.
Yes. The UK and Europe commonly use a base of 15.5°C (equivalent to 60°F). In that system, a day where the mean temperature is 5°C gives 10.5 HDD (Celsius). The calculation is identical — just subtract the mean temperature from the base temperature. This calculator supports Celsius input and base temperatures. The results (HDD values) will be in °C-days instead of °F-days, and the numbers will be roughly 55% of the equivalent Fahrenheit HDD.
Insulation and air sealing directly affect how fast heat escapes your home. A poorly insulated home can use 30–60% more fuel per HDD than a well-insulated one of the same size. Improving insulation effectively lowers your “fuel per HDD” number permanently. This is why energy audits compare fuel use against HDD — if your fuel-per-HDD ratio drops after an insulation upgrade, you know the upgrade is working.
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