Rise and run
Stair calculator
Enter the total rise from finished floor to finished floor. This works out how many risers you need, how tall each one actually is, the run the flight will take up and how long the stringers have to be.
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The riser count rounds up, and then the height falls out of it
You cannot build a fraction of a step. Divide the total rise by the maximum riser your code allows and round up. Rounding down would put every riser in the flight over the limit. Then divide the rise by that count to get the actual riser height, which will always be a little under the maximum.
A 108 inch rise with a 7.75 inch maximum needs 14 risers, each 7.714 inches. Not 13 risers of 8.3 inches, which is over the limit, and not 14 risers of 7.75 with a short one at the end.
Treads are always one fewer than risers
The top of the flight lands on the floor above, and that floor is the last tread. So a 14 riser flight has 13 treads. This trips people up at the ordering stage more often than at the building stage.
The total run follows from the tread count: 13 treads at 10 inches is 130 inches, just under 11 feet of floor space. Check that against the room before you commit. A deeper tread walks better but eats floor, and this is usually where the compromise happens.
The 2R + T check
Twice the riser height plus the tread depth should land somewhere around 24 to 25 inches. It is a carpenter's rule of thumb rather than a code requirement, and it encodes something real: a stair with tall risers needs shallow treads and vice versa, or your stride does not match the flight.
The ticket runs this check and tells you when a flight will feel steep or stretched underfoot, even though it is buildable. If it fails, adjust the tread depth first; the riser height is already fixed by the rise and the code maximum.
What the code decides, not us
For houses, most of the US starts from the International Residential Code: a maximum riser of 7 3/4 inches, a minimum tread of 10 inches, and no riser in a flight more than 3/8 inch taller than the smallest. Commercial stairs under the International Building Code are stricter, at 7 inches and 11 inches. Those are the Residential and Commercial presets above.
States and cities amend them. Massachusetts, for one, allows an 8 1/4 inch riser and a 9 inch tread in homes, which is the third preset. The figure that counts is your building department's, so ask for it and enter it.
We quote the riser and tread because they are single figures in the national model codes and have not changed in more than twenty years. Headroom, handrail height, guard spacing and the rise allowed before a landing vary more, and we leave those to your building department.
Stairs are also the assembly where getting it wrong hurts someone. An unequal riser at the bottom or top of a flight is the single most common cause of a fall on domestic stairs.
Common questions
How do I calculate stair risers?
Divide the total rise by the maximum riser height your code allows and round up. Then divide the total rise by that riser count to get the actual height of each one, which will be equal all the way up.
What is the maximum riser height for stairs?
7 3/4 inches for a house under the International Residential Code, which most of the US builds to, with treads at least 10 inches deep. Commercial stairs under the International Building Code allow 7 inches and need 11 inch treads. Some states amend the residential figures, so confirm yours with your building department.
How many treads for 14 risers?
Thirteen. The floor at the top of the flight serves as the last tread, so there is always one fewer tread than riser.
How much space do stairs take up?
The total run is the tread count multiplied by the tread depth. Thirteen treads at 10 inches is 130 inches, or just under 11 feet, before any landing.
How long is a staircase for an 8 foot ceiling?
Just under 11 feet of floor, before any landing. An 8 foot ceiling over a floor about 10 inches deep, such as 2x10 joists and a subfloor, is roughly 106 inches of rise. At a 7 3/4 inch maximum that needs 14 risers of 7.57 inches and 13 treads, and 13 treads at 10 inches is 130 inches. A 9 foot ceiling takes 16 risers and 150 inches of run, a 10 foot ceiling 17 risers and 160. Measure your own rise, because floor depths vary.
How do I work out stringer length?
It is the hypotenuse of the total rise and the total run. For a 108 inch rise and a 130 inch run that is about 169 inches, or 14.1 feet, so you would buy 16 foot stock.
What is the 2R + T rule?
Two riser heights plus one tread depth should come to roughly 24 to 25 inches. It is a comfort check rather than a code rule, and it catches flights that will feel steep or stretched even though they are legal.
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