Stair Calculator
Calculate riser height, tread depth, stringer length, and total run from your floor-to-floor rise — with code compliance checked as you go.
How to use this calculator
- 1Measure the total rise from finished floor to finished floor, including the upper floor's thickness.
- 2Enter a preferred riser height — 7 to 7.5 inches is comfortable for most people.
- 3Enter your tread depth, measuring the walking surface without the nosing.
- 4Check the compliance table before cutting anything.
How it works
Stair layout
risers = total rise ÷ preferred riser, rounded to a whole number riser height = total rise ÷ risers treads = risers − 1 total run = treads × tread depth stringer = √(total rise² + total run²) angle = arctan(total rise ÷ total run)
The riser count must be a whole number, so you choose the count nearest your preferred height and then divide the total rise by it — the actual riser height is whatever that division gives, not the number you started with. There is always one fewer tread than riser because the upper floor serves as the final tread. The stringer is the hypotenuse of the rise and run.
Worked example
A 109-inch floor-to-floor rise with a 7.5-inch target gives 109 ÷ 7.5 = 14.53, rounded to 15 risers. Each riser is 109 ÷ 15 = 7.267 inches. With 14 treads at 10.5 inches, the total run is 147 inches and the stringer is 183 inches — a 36.6° stair.
Stair Calculator: the complete guide
Why the riser height is never the number you picked
You cannot have 14.53 risers. The count has to be a whole number, so the actual riser height is the total rise divided by that count — which is why entering 7.5 inches above returns 7.267.
This is the step most DIY stair failures skip. Building to a nominal 7.5-inch riser and letting the last step absorb the remainder produces one riser of a different height, which is both the leading cause of stair falls and an automatic inspection failure.
The code tolerance is tight: the largest and smallest riser in a flight may differ by no more than 3/8 of an inch, and the same applies to tread depths. Measure the total rise precisely, including finished flooring at both ends, before dividing.
The comfort rules
Three empirical rules describe a stair that feels right underfoot. Rise plus run should total 17 to 18 inches. Twice the rise plus the run should total 24 to 25 inches — the oldest of the three, attributed to François Blondel in 1675, and derived from the geometry of a natural stride. Rise multiplied by run should land near 75.
They generally agree, and a 7.25-inch rise with a 10.5-inch tread satisfies all three. Steeper stairs are tiring to climb; shallower ones cause people to break stride awkwardly.
The hard code limits are separate: a maximum 7.75-inch riser and a minimum 10-inch tread for residential stairs. Comfort rules are advisory, the limits are not.
Headroom, width, and handrails
Headroom is measured plumb from a line touching the nosings, and the residential minimum is 6 feet 8 inches. It is the requirement most often missed on a basement stair, where a joist or duct sits right where the stair passes under it — and discovering it after the stringers are cut is expensive.
Minimum width is 36 inches above the handrail, and the handrail itself may project no more than 4.5 inches into that width. Handrails are required on at least one side of any flight with four or more risers, mounted 34 to 38 inches above the nosing line, graspable, and continuous for the full flight.
Guards are required where a walking surface is more than 30 inches above the floor below, and openings must reject a 4-inch sphere — the rule that keeps a child's head from fitting through.
Cutting the stringers
Stringers are usually 2×12 lumber, marked with a framing square set to the rise and run using stair gauges. Cut them with a circular saw, finishing each corner with a hand saw — overcutting into the inside corner removes material that carries the load and weakens the stringer significantly.
The bottom stringer must be dropped by one tread thickness so the first step ends up the same height as the rest once flooring is on. Forgetting this is the second most common stair mistake after uneven risers.
Three stringers is standard for a 36-inch stair; add a fourth for wider flights or thin treads. Cut and test-fit one stringer before cutting the others, so a measurement error costs one board rather than three.
Frequently asked questions
What is the standard stair riser height?
Between 7 and 7.75 inches for residential stairs, with 7 to 7.5 inches generally considered most comfortable. The International Residential Code caps risers at 7.75 inches and requires the tallest and shortest in a flight to be within 3/8 of an inch of each other.
Why is there one fewer tread than riser?
Because the upper floor acts as the final tread. A stair with 15 risers has 14 treads plus the landing at the top. Cutting 15 treads is a classic error that leaves the last step an inch or two proud of the floor.
How much horizontal space do stairs need?
Multiply the tread count by the tread depth. Fifteen risers at a 10.5-inch tread needs 147 inches — over 12 feet — plus landing space at both ends. This is why basement stairs are so often the tightest part of a renovation.
What is the maximum stair angle?
The code limits work out to roughly 37 to 38 degrees for residential stairs. Anything steeper either exceeds the maximum riser or falls below the minimum tread. Comfortable stairs land between 30 and 37 degrees; ships' ladders and alternating-tread stairs are separate categories with their own rules.