Stair calculator

Rounded up ensures no riser exceeds the 7.75-inch code cap. If you rounded down, at least one riser would be taller than allowed, and steps would be uneven. Rounding up gives a whole number of equal-height risers that all fit under the limit.

The vertical part of a step, including its height, is the riser. A tread is the flat part you put your foot on. The final surface, whether the top landing or the floor, accounts for the missing tread, so the tread count is one less than the riser count.

Divide the stair width by 16 inches, round down to a whole number, then add one for the starting stringer. The count therefore depends on the width. The resulting spacing is typically about 16 inches on center but can be wider; a 36-inch-wide stair takes 3 stringers, which puts them 18 inches apart.

The 7-11 rule for stairs is a comfort guideline that suggests a riser height of about 7 inches and a tread depth of about 11 inches. The residential code limits behind this calculator are a maximum riser of 7.75 inches and a minimum tread run of 10 inches, so the 7-11 rule fits within those bounds.

For a 9 foot rise, you need 14 risers and 13 treads. A 9 foot rise equals 108 inches, and dividing that by 14 gives 7.71 inches per riser. That leaves 13 treads between the risers, and at a 10 inch tread run the flight runs 130 inches.

For 7 steps with 7.5 inch risers and 10 inch treads, the stringer diagonal is about 80 inches. The length comes from the square root of the total rise squared plus the total run squared, which with a 52.5 inch rise and a 60 inch run gives roughly 80 inches. The board you buy must be longer than that cut diagonal so you have material for the cuts and the bearing ends.