65 degrees F. minus the days average temperature (in degrees F.) = Heating Degree Days (HDD)
The easiest way to calculate it (not exact) is:
65 - [(Days High Temp + Days Low Temp)/2] = HDD
Days high temp = 20 degrees F.
Days low temp = 5 degrees F.
65 - [(20 + 5)/2] = 52.5 HDD's
I find that the calculation as done above is improved overall by multiplying by a kludge (or fudge) factor of roughly 1.06, and I believe this is so because in the depths of the winter there are fewer daylight hours than dark hours in a day, and you are trying to estimate the days average temperature from only two data points (days high and days low).
Therefore, an improvement to the above formula (with admittedly an empirical kludge, also not exact) is:
(65 - [(Days High Temp + Days Low Temp)/2]) x 1.06 = HDD
The kludge bumps up the heating degree days for the example day above to 52.5 x 1.06 = 55.7 HDD's
In my case I would expect to burn 1.57lbs/HDD x 55.7 HDD's = 87.5 lbs. of coal for the example day.
Or you can just go here: http://www.degreedays.net/
***The company I work for uses HDD's to estimate its monthly heat bill, and to keep the energy company honest.***
*** Your energy company uses monthly HDD's to compute your bill when they send you an estimated rather than an actual monthly bill. ***