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Know the maximum allowed riser height to determine the rise and run of stairs. For a standard straight run of stairs in a residence, the maximum stair riser height is 7 3/4 inches, and the minimum tread depth is 10 inches. The risers is the vertical part of a step, and a tread is the horizontal part that you walk on. To calculate how many risers are needed, measure the floor-to-floor height in inches, then divide the height by 7 3/4. The result will likely be a fraction, so always round up.
Since in one step on a staircase a foot travels by two risers and a tread, Blondel arrived at the formula two times the unit rise, plus the unit run = 25.5, or unit rise = (25.5 - unit run) over 2. This formula works well only for moderate values for unit rise (or unit run).ncreasing the unit run even an inch or two can greatly increase the size of the staircase. With a rise of 12 feet and a unit rise of 7.2 inches, increasing the unit run from 9â³ to 11â³ makes the staircase two feet longerâprobably 6 square feet of floor area carved out of the living room.