<== Degree-day home page

How degree days are computed

The calculation requires daily measurements of maximum and minimum outside air temperatures ( Tmax and Tmin ) and a 'base temperature' Tbase, nominated by the user as an estimate of the outside air temperature at which no artificial heating (or cooling) is required. In the UK Tbase for heating has commonly been set at 15.5oC, but other base temperatures can be adopted.

The degree-day figure for a given month or week is the accumulated total of daily results over the period in question.

The daily result for heating degree days, Dh, is selected from the following formulae, using the first one that matches:
Condition Formula used
Tmin>Tbase Dh=0
(Tmax+Tmin)/2>Tbase Dh=(Tbase-Tmin)/4
Tmax>=Tbase Dh=(Tbase-Tmin)/2-(Tmax-Tbase)/4
Tmax<Tbase Dh=Tbase-(Tmax+Tmin)/2

The daily result for cooling degree days, Dc, is selected from the following formulae, using the first one that matches:
Condition Formula used
Tmax<Tbase Dc=0
(Tmax+Tmin)/2<Tbase Dc=(Tmax-Tbase)/4
Tmin<=Tbase Dc=(Tmax-Tbase)/2-(Tbase-Tmin)/4
Tmin>Tbase Dc=(Tmax+Tmin)/2-Tbase

Degree Days Direct frequently-asked questions