Stair Calculator
Even risers, the right tread count, and a warning when you're outside code.
Getting your stairs right
Small arithmetic errors on stairs become permanent. Divide the rise evenly, and count one fewer tread than risers.
Why there is always one fewer tread than riser
Count the risers in a flight and you count every vertical step from the lower floor to the upper one. Count the treads and you count every horizontal surface you stand on — except the last, because the last one is the floor you have arrived at.
A flight with fourteen risers has thirteen treads. Build fourteen and the top step lands a riser’s height above the floor.
It sounds obvious written down. It is still one of the most common stair mistakes there is, because the two numbers feel like they should match and nothing about the drawing says otherwise until the stringer is cut.
Every riser has to be the same height
It is not enough for each riser to be under the maximum. The difference between the tallest riser in a flight and the shortest is itself limited — commonly to three-eighths of an inch.
The reason is that people climb stairs without looking. The body learns the rhythm of a flight in the first two steps and stops checking. One riser a half-inch out of pattern is the one that catches a toe, and it is a common cause of falls on stairs that otherwise look fine.
In practice this means the total rise gets divided evenly across whatever number of risers you have settled on, rather than cutting a stack of equal steps and leaving the remainder at the top or bottom. The remainder step is exactly the one that fails inspection.
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FAQFrequently asked questions
Divide your total rise by the maximum riser height your code allows and round up — that is your riser count. Then divide the total rise by that number to get an even rise per step. Enter the floor-to-floor height above and the calculator does both.
Residential code commonly caps it at just under eight inches, but the figure varies by jurisdiction and local amendments are common. The calculator flags when your rise per step exceeds the limit it is set to, though your local building department is the authority.
Always one fewer. The top riser brings you level with the upper floor, and that floor is the last tread. A flight with fourteen risers has thirteen treads, and building fourteen leaves the top step standing proud of the floor.
Yes, within a small tolerance — commonly three-eighths of an inch between the tallest and shortest in a flight. People climb stairs on rhythm rather than by looking, so a single odd step is a genuine trip hazard. It is also one of the most common reasons a flight fails inspection.
Two long-standing rules of thumb are that rise plus run lands between seventeen and eighteen inches, and that twice the rise plus the run lands between twenty-four and twenty-five. Both describe a flight that feels comfortable to climb. Code minimums and maximums take precedence over either.
The stringer spans the diagonal across the total rise and the total run, so its length comes from those two figures. Cut depth matters as well: the notches reduce the remaining timber, and too little material left behind the cut weakens the stringer. The calculator gives the length; the stock size is a structural question.
Commonly six feet eight inches measured vertically from the tread nosing. It is the constraint people discover last, usually after framing the opening, and it is expensive to correct. Enter your available headroom above and the calculator will flag a shortfall.
Finished floor to finished floor. Measuring to the subfloor at one or both ends gives a flight that is out by the thickness of your flooring, which is usually enough to push the top or bottom riser outside the variation limit.