“To determine freezing degree days and how fast your favourite lake will build up ice, start by taking the average temperature (in degrees Fahrenheit) over the past 24 hours. So, let’s say that the daytime high yesterday was 30 F and the night time low was 20 F. This means the average temperature was 25 F. Now, subtract the average temperature (25 F) from the freezing point of water (32 F) and we get 7 freezing degree days.
If the day time high was 20 F, on the other hand, and the night time low was 0 F, the average temperature would have been 10 F, giving us 22 freezing degree days (32 F – 10 F = 22 FDDs).
See how simple it is?
Now, based on the studies that have been done, once a thin sheet of ice has formed on a lake, it will typically increase at the rate of one inch / 15 freezing degree days (FDDS).
So, if we go back to our examples, this means that on the relatively warm day, when there were 7 FDDs, the lake made about one half-an-inch of ice over the 24-hour period. But, on the much colder day, when we experienced 22 FDDs, the lake made about 1.5-inches.”
https://myfishingpartner.com/how-fast-does-your-lake-make-ice/