Foxmoth is right.
Rate of climb for given conditions is not calculated by a formula, but by reference to charts or tables like the ones he has posted. These are derived from flight test results adjusted for varying conditions, and can be found in your Flight Manual or Pilot's Operating Handbook. (POH)
Can I ask, are you studying for a PPL or do you have one already?
If you are studying for the Performance and Planning exam, then graphs, charts and tables will be provided for you to work out things like rate of climb.
MJ
