Stair Calculator
Work out a straight stair from the floor-to-floor rise: enter the total rise and you get evenly divided risers, the number of treads, the total run, the stringer and handrail length, the stair angle and a check against the IRC limits.
Divide the total run and the stringer length by 12 for feet. Cut the stringer with the treads level and the risers plumb, and check the first riser against the finished floor — a thick floor covering adds to it.
Formula and calculation
Worked example
A 108 in rise with a 7.5 in target works out at 14 risers of 7.71 in each — divide 108 by 14 and every step is the same. That gives 13 treads, a total run of 130 in at a 10 in tread, and a stringer and rail length of 169.01 in at 39.72 degrees. The code check says: Riser 7.71 in and tread 10 in are both within the IRC limits (riser max 7.75 in, tread min 10 in).
Measure the rise before you cut anything
The total rise is from the finished floor at the bottom to the finished floor at the top, and both numbers matter: a 3/4 in hardwood floor plus underlayment can add an inch and a half to the bottom of the flight, which is enough to throw every riser out. Measure at the stair location, not from the drawings. Then check the headroom over the first few treads and the floor opening at the top — a stair that fits the numbers but not the hole is still a stair that does not fit. Mark the riser height on a story pole and check it at the top before cutting stringers.
Comfort rules that are not in the code
The code gives a maximum riser and a minimum tread; it says nothing about whether a stair is pleasant to climb. The old rule of thumb is that one riser plus one tread should come to about 17 to 18 in, so a 7.5 in riser pairs with a 10 in tread and a 7 in riser with an 11 in tread. Wide treads and shallow risers are gentler on knees going up and safer coming down, which is why outdoor stairs to a deck often use a 6 in riser even where 7.75 in would pass. Consistency matters more than either number: a single odd riser at the top is the stair everyone trips on.
What the results mean
- Risers
- How many risers (steps up) the flight has, counted as whole steps so that every riser is the same height.
- Riser height
- The height of each riser in inches: the total rise divided by the number of risers. Every riser in a flight must be within 3/8 in of the others, which is why the count is a whole number.
- Treads to cut
- How many treads to cut. It is one fewer than the risers, because the top riser climbs onto the floor rather than onto a tread.
- Total run
- How far the stair reaches horizontally, in inches: the number of treads times the tread depth. Divide by 12 for feet and compare it with the floor opening you have.
- Stringer and rail length
- The length of the sloped side members in inches, measured along the slope. It is also the minimum length of a handrail at the same angle, and it is usually cut from 2 x 12 lumber sold in 12, 14 and 16 ft lengths.
- Angle
- The angle of the stair from horizontal, in degrees — the number to set on a miter saw for the stringer and rail cuts. A comfortable home stair sits around 30 to 38 degrees.
- Code check
- A plain-language check against the IRC limits for a home stair: riser no more than 7.75 in and tread no less than 10 in. Local codes can be stricter, so confirm with your building department.
Frequently asked questions
How many stairs do I need for a 9 foot rise?
A 108 in rise over a 7.5 in target gives 14 risers at 7.71 in each, with 13 treads. That is the sweet spot for a 9 ft floor-to-floor in a house. Punch your own total rise into the calculator above and the riser height and step count come out together.
What is the maximum riser height for stairs?
The IRC caps a riser at 7-3/4 in for a dwelling, and every riser in the flight has to be within 3/8 in of the others. Some local codes are stricter — 7-1/4 in appears in a few jurisdictions. Commercial stairs are limited to 7 in. Always check the code your building department enforces.
How deep should stair treads be?
Ten inches is the IRC minimum and 10 to 11 in is the comfortable range; a tread that is 12 in deep starts to feel like a shallow ramp and lengthens the stair. If the stair is short on space, do not shrink the tread below 10 in — increase the riser height, up to the code maximum, and check the headroom.
How long should the stringer be?
There is no fixed length: it is the diagonal of the rise and the run. A 108 in rise with a 130 in run needs a stringer about 169 in (14 ft) long, which is why 2 x 12 lumber comes in 16 ft lengths. Enter your run and the calculator gives the length before you buy.
How many treads does a staircase with 15 steps have?
Fourteen. The top of the stair is the floor, so a 15 riser stair has 14 treads and 15 risers. On an open-riser deck stair the same rule holds, and the top landing is just the deck surface.