This stair calculator estimates rise, run, and related staircase dimensions from the inputs on the page. Building codes vary by location, so treat results as planning figures and confirm with a local code reference.
For floor area before laying finishes, use the square footage calculator. For covering materials, open the tile calculator or the roofing calculator.
IRC stair dimensions to check after you calculate
- Maximum riser height: 7¾ inches (R311.7.5.1), measured vertically between the leading edges of adjacent treads.
- Riser uniformity: the tallest riser in a flight may not exceed the shortest by more than ⅜ inch.
- Minimum tread depth: 10 inches (R311.7.5.2), with the same ⅜-inch uniformity limit.
- Minimum headroom: 6 feet 8 inches (R311.7.2), measured vertically from the sloped line along the tread nosings.
- Minimum clear width: 36 inches above handrail height (R311.7.1). At and below the handrail, the minimum is 31½ inches with one handrail or 27 inches with handrails on both sides.
Commercial buildings follow the International Building Code, which is stricter. Local amendments can also change these numbers, so check with your building department before you cut stringers.
From riser count to total run and tread count
A straight flight always has one fewer tread than risers, because the top “step” is the upper floor. So:
- Treads = risers − 1
- Total run = treads × tread depth
- Minimum risers = total rise ÷ 7.75, rounded up
Round up, never down. Rounding down makes each riser taller than the code maximum.
Stair calculator worked example: 105-inch floor-to-floor rise
You measure 105 inches from finished lower floor to finished upper floor.
- Minimum risers: 105 ÷ 7.75 = 13.55, so round up to 14 risers.
- Actual riser height: 105 ÷ 14 = 7.5 inches, which is under the 7¾-inch limit.
- Treads: 14 − 1 = 13 treads.
- Total run at the 10-inch minimum: 13 × 10 = 130 inches (10 ft 10 in). With 10½-inch treads it becomes 136.5 inches (11 ft 4½ in).
If the floor plan only allows 120 inches of run, 13 treads at 10 inches will not fit. You would need a landing, a turn, or a different layout. You cannot fix it by making the treads shallower.
Measuring rise when floor finishes are not installed yet
The code measures the finished stair. If the lower slab will get ¾-inch tile and the upper subfloor ¾-inch hardwood, the finished rise equals the framed rise. If only one level gets a finish, the top or bottom riser will be off by that thickness. Add the planned finish thickness at each level before you divide, and keep the ⅜-inch uniformity limit in mind.
Exterior steps and decks
The IRC stairway section exempts stairways “not within or serving a building, porch or deck”, such as landscape steps. Deck stairs that serve the deck are covered. Exterior stairs on soil often need a concrete landing pad at the bottom, so size it before ordering materials.
Before you build, mark the rise and run on a long board and test them against the space. Walk the layout with a tape measure to confirm headroom at the top, where the ceiling or a beam is most likely to cut in.
Write down every measurement, including the finish materials you plan to add. The stair calculator is only as accurate as the rise you enter, so measure the floor-to-floor height twice and use the average.
Finally, ask your building department which edition of the code applies and whether an inspection is needed before the stairs are enclosed.
When you use the stair calculator, keep the same tread depth on every step and keep the riser heights even, since uneven steps are a common cause of trips and falls.
Last reviewed: October 2026