Stair Calculator

Under the IRC (residential), the maximum riser height is 7 3/4 inches. Under the IBC (most commercial occupancies), it is 7 inches. Both codes also require that risers be uniform: the difference between the tallest and shortest riser in a flight may not exceed 3/8 inch. This calculator divides your floor-to-floor height into equal risers and flags any that exceed the limit.

The IRC requires a minimum tread depth of 10 inches (measured nosing to nosing); the IBC requires 11 inches. When the tread is less than 11 inches, a nosing projection of 3/4 to 1 1/4 inch is required so the effective walking surface is deeper. Treads, like risers, must be uniform to within 3/8 inch.

Divide the total floor-to-floor height by a comfortable target riser of about 7 inches and round to the nearest whole number. That is your riser count. Then divide the floor-to-floor height by that count to get the exact, equal riser height. The number of treads is always one less than the number of risers, because the top riser lands on the finished upper floor.

Both the IRC and IBC require a minimum clear headroom of 6 feet 8 inches (80 inches), measured vertically from the tread nosing line to any construction above. This calculator returns the minimum stairwell opening length needed to hold that clearance for your floor assembly thickness, the number that most often forces a straight stair to become a longer opening or an L-shape.

Comfortable stairs fall between about 30 and 37 degrees. Two time-tested proportioning rules confirm the feel: twice the riser plus the tread (2R + T) should land near 24 to 25 inches, and riser plus tread (the Blondel rule) near 17 to 18 inches. This calculator reports both figures alongside the angle so you can tune the riser and tread to the sweet spot.

Set the riser count from the floor-to-floor height and the spiral riser limit of 9 1/2 inches, then divide the total rotation (usually 360 degrees) by the number of treads for the angle each tread turns. Code requires each tread to be at least 6 3/4 inches deep measured at the walkline, 12 inches in from the center column, with headroom of at least 6 feet 6 inches. Switch this calculator to spiral mode for the riser height, angle per tread and walkline tread depth at any diameter.