Stair Calculator
Every riser in a flight must be within about 3 mm of every other, which is why you divide the total height evenly rather than picking a riser and working up. This page does the division and checks the result against the limits.
The Staircase
Total rise is finished floor to finished floor, not floor to ceiling.
Code check
Why you divide rather than choose
The total rise is fixed by the building. Codes require all risers in a flight to be within 3/8 inch of each other — in practice within a few millimetres — so you pick the number of risers and let the height fall out of the division. Choosing a riser height first and adding them up leaves a short last step, which is the most common trip hazard on a staircase.
The comfort rules, and where they come from
Two old rules still work. Rise + run between 17 and 18 inches and Blondel's 2 × rise + run between 24 and 25 inches both encode the relationship between a comfortable stride and a climb. A staircase that satisfies both feels natural; one that fails both feels wrong to walk even when it is technically legal.
Nosing, run and stringer length
Total run is the number of treads times the tread depth — and there is always one fewer tread than riser, because the upper floor is the last step. Nosing overhang adds comfort and effective tread depth without adding run. The stringer is the hypotenuse of total rise and total run, so a 9-foot rise over a 12-foot run needs a stringer of about 15 feet.
The uniformity rule causes more failed inspections than the dimensions do
Residential codes in the US derived from the International Residential Code cap riser height at 7.75 inches and require a tread depth of at least 10 inches, but the requirement that actually fails inspections is uniformity: within a single flight, the largest and smallest riser may differ by no more than 3/8 of an inch, and the same limit applies to tread depth. This is a safety rule with real evidence behind it, because a stair users have learned by the third step is one they stop looking at, and a single odd riser near the bottom is where people fall.
The other dimensions come from the same family of requirements. Minimum headroom is 6 feet 8 inches measured vertically from the tread nosing, handrails sit between 34 and 38 inches above the nosing line and must be graspable rather than merely present, and nosing projection is regulated where treads are solid. Local amendments are common and take precedence, so the governing numbers are whatever the local building department has adopted.
Frequently Asked Questions
How do I calculate the number of stairs?
What is the maximum riser height?
Why is there one fewer tread than riser?
What is the 2R + T rule?
Sources
Official publications only. Links open the original document in a new tab.
- Occupational Safety and Health Administration 29 CFR 1910.25 — Stairways Federal stairway rise and run requirements
- National Institute of Standards and Technology Office of Weights and Measures Measurement standards used on site