The stair run is the horizontal distance between the leading edge of one tread to the leading edge of the next tread. The rise is the vertical distance from the surface of one tread to the surface of the next one. The dimensions for stairs are based on an established ratio between the tread and riser that allows comfortable and safe use of the stairs. The total run, or horizontal distance, and total rise, or vertical height, of the staircase is variable, but the treadriser ratio still applies. Typically for interior stairs the relationship is 1 tread + 1 riser = 17 inches minimum or 18 inches maximum.
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Calculate the vertical distance from the finished floor landing to the finished floor surface of the upper floor to find the total rise. Divide the distance by the desired height of the riser. Round the result up or down to the nearest whole number if the result is a fraction.

Divide the vertical distance by the rounded up whole number. The result is the required number of risers.

Find the tread width  stair run  by using the risertread ratio formula: 1 riser plus 1 tread = 17 inches minimum. If the riser is 7 inches, the tread is 10 inches. Make sure any adjustments are less than the maximum 18 inches.

Make adjustments to calculations, not individual risers and treads, to fit within the minimum and maximum dimensions in the treadriser relationship. Each riser and each tread must be consistent for the finished staircase.
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