Stair Rise and Run Calculator
Risers, riser height, treads, total run and stringer length from your floor-to-floor measurement. Every riser in a flight has to be identical, so the height is derived rather than chosen.
Risers needed
15risers
- Risers
- 15
- Riser height
- 7.20 in
- Treads
- 14
- Total run
- 140.00 in
- Stringer
- 176.82 in
- Run in feet
- 11.67 ft
- Every riser in a flight must be the same height. That is why the riser height is worked out from the total rise rather than set directly — an odd step out is the classic trip hazard.
- There is always one fewer tread than riser, because the top riser lands on the floor above.
- Stair rules vary by jurisdiction and the figures cited here are the common ones. Check your local building department before cutting stringers.
About the stair calculator
One rule shapes every staircase: all the risers must be the same. A single step out by even a quarter inch is the most reliable way to trip someone, because people climb stairs by feel rather than by looking.
That means you do not pick the riser height — you pick how many risers there are, and the height is whatever the total rise divided by that number comes to. Ask for seven inches over a 108-inch rise and you get 15.43 risers, which is not a thing, so fifteen risers at 7.2 inches each is the answer.
Treads are always one fewer than risers, because the top riser lands on the floor above rather than on a tread. The stringer is then the hypotenuse of the whole flight. The page also runs the trade's comfort check — twice the riser plus one tread should land between 24 and 25 inches — and flags anything outside it.
What it works out
- Riser count and the exact riser height
- Tread count, total run and stringer length
- A comfort and code sanity check on the result
The formula
Risers = round(Total rise ÷ Preferred riser) Riser height = Total rise ÷ Risers
The one rule that shapes every stair is that all the risers must be identical. A single step out of line by even a quarter of an inch is the most reliable way to trip someone, because people climb stairs by feel rather than by looking.
That means you do not choose the riser height. You choose how many risers there are, and the height is whatever the total rise divided by that number happens to be. Ask for seven inches over a 108-inch rise and you get 15.43 risers, which is not a thing — so fifteen risers at 7.2 inches each is the answer.
Treads are always one fewer than risers, because the top riser lands on the floor above rather than on a tread. Fifteen risers means fourteen treads, and the total run is those fourteen treads multiplied by the tread depth. The stringer is then simply the hypotenuse of the whole flight.
The trade rule for whether a flight will feel right is that twice the riser plus one tread should land between 24 and 25 inches. It catches stairs that are technically legal but tiring to climb.
- Total rise
- Finished floor below to finished floor above. Measuring to the subfloor is the usual mistake.
- Preferred riser
- The height you are aiming for. The real height is derived from it.
- Tread depth
- The walking surface, not counting any nosing that overhangs the riser below.
- Stringer
- The diagonal timber the treads sit on — the hypotenuse of the rise and run.
A worked example
A flight rising 108 inches floor to floor, aiming for seven-inch risers and ten-inch treads.
That works out to 15 risers.
- Risers
- 15
- Riser height
- 7.20 in
- Treads
- 14
- Total run
- 140.00 in
- Stringer
- 176.82 in
- Run in feet
- 11.67 ft
Questions
How many stairs for 108 inches?
Fifteen risers at 7.2 inches each, giving fourteen treads. At a ten-inch tread that is 140 inches of total run, and a stringer about 177 inches long.
Why is there one fewer tread than riser?
Because the top riser brings you level with the floor above, and that floor is not a tread. A flight with fifteen risers has fourteen treads.
What is a comfortable stair riser height?
Around seven to seven and a half inches. Common building rules cap risers at 7¾ inches and require treads of at least ten, though the exact figures vary by jurisdiction, so check locally before cutting anything.
What is the 2R plus T rule?
Twice the riser height plus one tread depth should come to between 24 and 25 inches. It reflects how a stride shortens as it rises, and a flight outside that band tends to feel either cramped or tiring even when it meets code.
How do I measure total rise?
From the finished floor at the bottom to the finished floor at the top, not to the subfloor. If flooring is going down later, add its thickness — forgetting it is how one riser ends up different from the rest.
How long a board do I need for a stringer?
The stringer length shown here is the straight-line diagonal. Buy longer, since cutting the notches needs material beyond the bare diagonal and the ends have to be trimmed square to the floor and the header.