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Heating Degree Days

Free Heating Degree Days calculator with clear step-by-step results.

About Heating Degree Days

Heating degree days accumulate the shortfall of outdoor temperature below a base temperature, which sits below room temperature because occupants, appliances and solar gain provide free heat. Multiplying by the building's heat loss coefficient gives delivered heat, and dividing by system efficiency gives the fuel actually burned.

Formulas

Heating degree days

HDD = max(0, base temp - mean outdoor temp) x days

Fuel consumed

kWh fuel = UA x HDD x 24 / 1000 / system efficiency

Worked example

With the default values below, Heating Degree Days returns Heating degree days of 294.5. Change any input above and every figure here is recalculated in your browser.

Inputs used

Mean outdoor temperatureaccepts -40–30 C
6 C
Base temperatureaccepts 5–25 C
15.5 C
Number of daysaccepts 1–366
31
Building heat loss coefficientminimum 1 W/K
220 W/K
Heating system efficiencyaccepts 20–400 %
88 %
Fuel priceminimum 0 $/kWh
0.09 $/kWh

Results

Heating degree days
294.5 Kd
Heat delivered to the building
1,555.0 kWh
Fuel consumed
1,767.0 kWh
Heating cost for the period
$159.03

Step by step

  1. 1Values used
  2. 2Heating degree days
  3. 3Fuel consumed
  4. 4Heating degree days= 294.5 Kd
  5. 5Heat delivered to the building= 1,555.0 kWh
  6. 6Fuel consumed= 1,767.0 kWh
  7. 7Heating cost for the period= $159.03

Assumptions and limits

  • The result is derived only from the 6 inputs listed above. Anything not among them is not part of the calculation.
  • In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
  • Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

Why is the base temperature 15.5 C?

It is the long-standing convention for UK-style degree day series: internal gains typically supply the last few degrees, so heating is only needed once outdoors falls below roughly 15.5 C. Well-insulated buildings justify a lower base.

Can efficiency exceed 100%?

For a heat pump, yes - it moves heat rather than generating it, so enter its seasonal COP as a percentage, such as 340.

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