2017/6/15/ · Day 7 & day 9 The compost heap should reach its maximum temperature on these days. As a simple guideline. if you can put your arm into the compost up to your elbow it is not at 50 degrees Celsius! And is, therefore, not hot enough. Probably best to use a compost or cake thermometer! You need an optimum temperature of 55-65 degrees Celsius.