Quick answer
Heating degree days measure how far each day's mean temperature falls below a base: HDD = Σ max(0, base − (high + low) ÷ 2). Five cold days with a 65 °F base accumulate 117.5 HDD.
Enter the base temperature and the daily highs and lows. The calculator returns the heating degree days and days counted.
Related tools: Cooling Degree Days Calculator: Cooling degree days from daily high and low temperatures. Snow Water Equivalent Calculator: Depth of water in a snowpack from snow depth and density. Heat Index Calculator: Apparent temperature from air temperature and relative humidity (°F).
How to use the Heating Degree Days Calculator
- Enter base temperature.
- Type daily highs, for example 50, 40, 30.
- Type daily lows, for example 30, 25, 20.
- Press Calculate to see the result, and use Reset to start again.
| Input | What to enter | Allowed values |
|---|---|---|
| Base temperature | A number | — |
| Daily highs | Text, such as 50, 40, 30 | — |
| Daily lows | Text, such as 30, 25, 20 | — |
Formula and method
Formula
HDD = Σ max(0, base − (high + low) ÷ 2)
IEEE-754 double precision; shown to the decimals indicated.
Use the same temperature unit for everything; a common base is 65 °F (18 °C).
Worked examples
These examples are verified: each one is an automated test case that the live calculator must reproduce exactly.
Example 1
Inputs: Base temperature = 65; Daily highs = 50, 40, 30, 60, 70; Daily lows = 30, 25, 20, 40, 50.
| Result | Value |
|---|---|
| Heating degree days | 117.5 |
| Days counted | 5 |
Example 2
Inputs: Base temperature = 18; Daily highs = 10, 12, 5; Daily lows = 0, 2, -4.
| Result | Value |
|---|---|
| Heating degree days | 41.5 |
| Days counted | 3 |
The first line is the headline result (heating degree days). Below it the heating Degree Days Calculator lists days counted, which describe the same case from other angles. IEEE-754 double precision; shown to the decimals indicated.
Worked example
A day with a high of 50 and a low of 30 has a mean of 40, contributing 25 HDD. Colder days add more. Five days total 117.5 here.
Why it is used
Heating energy use is roughly proportional to HDD, so utilities and building managers compare consumption per degree day to separate weather from efficiency. A base of 65 °F (18 °C) is typical.
How the result changes with base temperature
Holding every other input at the first worked example, this table changes only base temperature. It shows how sensitive the answer is to that one value; each row was produced by running this calculator.
| Base temperature | Heating degree days | Days counted |
|---|---|---|
| 32.5 | 7.5 | 5 |
| 65 | 117.5 | 5 |
| 130 | 442.5 | 5 |
| 260 | 1,092.5 | 5 |
What happens with invalid input
These messages are produced by the calculator itself, so you know what to fix.
- Daily lows: entering "30" shows "Enter one low for every high." beside that field and no result.
- Base temperature: left empty shows "Enter base temperature." beside that field and no result.
Choosing between the Heating Degree Days Calculator and similar calculators
Each row shows what a related calculator on this site asks for and what it returns, so you can pick the one that matches your question.
| Calculator | You enter | It returns |
|---|---|---|
| [Cooling Degree Days Calculator](/science/cooling-degree-days-calculator/) | base temperature, daily highs and daily lows | Cooling degree days |
| [Snow Water Equivalent Calculator](/science/snow-water-equivalent-calculator/) | snow depth and snow density | Snow water equivalent |
| [Heat Index Calculator](/science/heat-index/) | air temperature and relative humidity | Heat index |
| [Sunrise and Sunset Calculator](/science/sunrise-sunset-approximation-calculator/) | date, latitude and longitude | Sunrise |
| [Days Until Calculator](/date-calendar/days-until-calculator/) | target date | Days until |
Background: environment and acoustics
Decibels are a logarithmic scale, so sound levels cannot be added arithmetically: two equal sources raise the level by about 3 dB, not double it. Environmental and emissions results depend on the factors behind them (for example the US EPA's equivalency factors), which are averages for a country, fuel and year.
Use the result to compare scenarios rather than as a measured value.
Assumptions and limitations
- Lists must be the same length
If a required value is empty, not a number, or outside its allowed range, the form marks that field with a message and shows no result instead of a misleading number. The automated checks deliberately try invalid entries for daily lows and base temperature to confirm this.
Frequently asked questions
What is a typical annual HDD?
About 2,500 to 8,000 °F-days in the US depending on climate.
Can HDD be negative?
No. Each day contributes max(0, base − mean temperature), so warm days add zero rather than a negative value.
How do CDD and HDD differ?
CDD measures warm days.
What does the Heating Degree Days Calculator calculate?
Heating degree days from daily high and low temperatures. It returns Heating degree days and Days counted.
What formula does the Heating Degree Days Calculator use?
HDD = Σ max(0, base − (high + low) ÷ 2). Use the same temperature unit for everything; a common base is 65 °F (18 °C).
What do I need to enter in the heating Degree Days Calculator?
You enter Base temperature, Daily highs and Daily lows.
What is an example of a heating Degree Days result?
With base temperature 65, daily highs 50, 40, 30, 60, 70 and daily lows 30, 25, 20, 40, 50, the heating degree days is 117.5.
Where does the method come from?
The method follows: Standard degree-day definition (NOAA / ASHRAE).
Related calculators
- Cooling Degree Days Calculator — Cooling degree days from daily high and low temperatures.
- Snow Water Equivalent Calculator — Depth of water in a snowpack from snow depth and density.
- Heat Index Calculator — Apparent temperature from air temperature and relative humidity (°F).
- Sunrise and Sunset Calculator — Approximate sunrise, sunset, solar noon and day length for a date and location.
- Days Until Calculator — Days remaining until any date (generalizes “days until June 15”, “days until Christmas” and similar pages).
- Days Old Calculator — How many days old you are.
Sources
- Standard degree-day definition (NOAA / ASHRAE)
Content last reviewed 2026-11-04. Formulas and worked examples are checked by automated tests against independently computed values.