Calculate steps unit rise and run in stairs. Related Topics. Miscellaneous - Engineering related topics like Beaufort Wind Scale, CE-marking, drawing standards and more; Related Documents. Air Change Rates in typical Rooms and Buildings - Fresh air - or make up air - requirements - recommended air change rates - ACH - for typical rooms and buildings - auditoriums, kitchens, churches and more. Mar 29,  · The minimum tread depth allowed by the IRC is 10 inches. There are several formulas for calculating the ratio between rise and run; the simplest is Rise + Run = 17 to 18 inches. Using a target run of 10 inches, the rise/run ratio is well within the “safe and comfortable” formula for this stair: 7 5/8 + 10 = 17 5/8 (Step 6). 4. Verify the. Measure Actual Stairway Total Run Length & Total Rise Height. Stair Calculations by Direct Layout on the Stringer. How to Measure Stair Rise & Run over Uneven Ground. Stairs built over both an uneven hillside & uneven bottom or top landing. Geometrical calculations for rise & run. Calculate the Total Number of Steps, treads, or Risers.