Design code-compliant stairs with accurate riser, tread, and stringer calculations
Quick Answer:For a standard 9-foot (108-inch) floor-to-floor height, you need 14-15 risers at about 7.2-7.7 inches each, with 13-14 treads. Total run is approximately 11-12 feet. The stair angle should be 30-37 degrees for comfortable use.
Code Compliance
Calculating... IRC residential code
Number of Risers
--
actual height per riser
Number of Treads
--
at specified depth
Total Run
--
horizontal distance
Stringer Length
--
diagonal measurement
The "riser + tread = 17 to 18 inches" rule is your best friend for comfortable stairs. If your calculated riser height falls outside the 7-7.75 inch IRC range, try adjusting the number of risers by one. Also remember: the top riser is the floor of the upper level itself, so always account for finished floor thickness in your total rise measurement. Standard nosing of 0.75 to 1.25 inches extends beyond the riser face and is required by most codes.
Per the International Residential Code (IRC), the maximum riser height is 7.75 inches and the minimum is 4 inches. The ideal comfortable riser height is between 7 and 7.5 inches. All risers in a staircase must be uniform, with no more than 3/8 inch variation between the tallest and shortest riser. Commercial stairs have different requirements, typically 4-7 inch risers.
Divide the total rise (floor-to-floor height in inches) by the desired riser height (typically 7-7.5 inches). Round to the nearest whole number for the number of risers. The number of treads is always one less than the number of risers. For example, 108 inches total rise divided by 7.5 inches equals 14.4, rounded to 14 risers and 13 treads. Multiply treads by tread depth for the total run.
The riser plus tread rule states that the riser height plus tread depth should equal 17 to 18 inches for comfortable stairs. This is a time-tested formula used by builders. For example, a 7-inch riser with an 11-inch tread equals 18 inches, which is ideal. Some codes also use the formula: 2 x riser + tread = 24 to 25 inches. Both rules help ensure a comfortable walking pace on stairs.