JB PARCELStair Tread Calculator
Enter the finished floor-to-floor height and get a stair that passes code: how many risers, how tall each one is, how many treads, how much floor the stair takes, and how long a stringer board to buy.
How this calculator works
Start with total rise: finished floor to finished floor, measured where the stair will actually land. Include the flooring on both levels. A quarter inch of tile or hardwood matters here, because both the IRC and IBC allow only 3/8 inch of difference between the tallest and shortest riser in a flight, and a floor added after the stair is built usually lands entirely on the top or bottom step.
Divide the rise by the maximum riser height the code allows and round up. That is your riser count. Divide the rise by the riser count and you get the actual height of each riser. Under the IRC the maximum is 7-3/4 inches; under the IBC it is 7 inches, with a 4 inch minimum. There is always one fewer tread than risers, because the upper floor is the last step.
Total run is the tread count times the tread depth. It is the floor space the stair consumes, and it is usually the number that decides whether a layout works. Both codes also require a landing at the top and bottom at least as wide as the stair, and the IRC requires it to be at least 36 inches deep in the direction of travel.
The stringer is the diagonal board the treads sit on. Its length is the hypotenuse of the total rise and total run, before you cut the top and bottom, so buy the next standard length up. A code-minimum IRC stair runs close to 38 degrees. Most people find 30 to 35 degrees comfortable, which is why a slightly shorter riser and deeper tread is worth the extra floor space where you have it.
Worked example: a 9 foot floor-to-floor basement stair
108 inches of total rise in a house, built to the IRC with 10-1/2 inch treads.
| Risers | 14 |
|---|---|
| Riser height | 7 11/16 in |
| Treads | 13 at 10 1/2 in |
| Total run | 11 ft 4 1/2 in |
| Stringer length | 14 ft 6 1/16 in |
| Stair angle | 36.3 degrees |
Thirteen risers would come out at 8.3 inches each, over the 7-3/4 inch limit, so fourteen is the minimum. A 16 foot 2x12 covers the stringer with room for the cuts. Want a gentler climb? Set the preferred riser to 7 inches and you get 15 risers at 7 3/16 inches and 14 treads, and the run grows to 12 ft 3 in. That extra ten and a half inches of floor is the whole trade.
Where the numbers come from
Limits are from the International Residential Code, section R311.7, and the International Building Code, section 1011. States and cities adopt these codes by edition and often amend them, so confirm the edition and any local changes with the building department that will inspect the work.
Questions investors ask
How many stairs do I need for a 9 foot ceiling?
Measure floor to floor, not the ceiling. A 9 foot ceiling usually means about 118 inches floor to floor once you add the joists and subfloor above it. At 118 inches the IRC limit gives 16 risers at about 7-3/8 inches and 15 treads. A true 108 inch floor-to-floor gives 14 risers and 13 treads.
What is the maximum riser height?
7-3/4 inches under the International Residential Code, which covers houses and townhouses. 7 inches under the International Building Code, with a 4 inch minimum. Inside individual dwelling units the IBC generally allows the residential 7-3/4 and 10 inch figures. Your jurisdiction may amend either code, so check the edition it has adopted.
What is the minimum tread depth?
10 inches under the IRC and 11 inches under the IBC, measured horizontally from nosing to nosing. The IRC also requires a nosing of 3/4 to 1-1/4 inches when a tread is less than 11 inches deep, so a 10 inch tread board really means a board about 11 inches wide.
Why is there one fewer tread than risers?
Because the top of the stair is the upper floor itself. You step up onto the floor from the last riser, so a 14 riser stair has 13 treads you cut into the stringer. Getting this wrong is the most common reason a stair built from a guess ends up one step short or with an extra tall top step.
How wide does a stair have to be, and how much headroom?
The IRC requires 36 inches of clear width and 6 feet 8 inches of headroom. The IBC requires 44 inches, or 36 inches where the stair serves fewer than 50 occupants, and 80 inches of headroom. Headroom is measured straight up from the line connecting the tread nosings, which is where basement stairs under a beam often fail. Budget the fix in the rehab budget calculator.
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