Step safely.
Stair stringer length, rise, run, and tread count for any height. Meets IRC code limits on rise and tread dimensions.
How we calculated this
The calculator divides total vertical rise by your target riser height and rounds to the nearest whole number — because stair risers must all be equal. Once you know the riser count, actual riser height is total rise divided by that count. If your total rise is 108 inches and you target 7.5-inch risers, you get 14 risers at 7.71 inches each (close to target but equal across all risers).
The number of treads is always one less than the number of risers because the last 'step' at the top is the upper floor itself, not an additional tread. This catches people off by one — if you have 14 risers, you need 13 treads.
Total run is treads times tread depth. A 14-riser / 13-tread staircase at 11-inch treads needs 143 inches (11 feet 11 inches) of horizontal space. This is the most commonly underestimated dimension when planning a staircase inside a house — measure your available horizontal space before committing to a stair layout.
Stringer length is the hypotenuse of the rise and run — Pythagoras. For ordering: round up to the next standard lumber length. 2×12 stringers come in 8, 10, 12, 14, 16, 20 foot lengths. The calculated stringer length is the actual path along the stair; you need some extra for the top and bottom mounting, so order one standard length above the calculated number.
IRC 2021 compliance: riser height 4"-7.75" (no more than 3/8" variation between any two risers), tread depth minimum 10", stair width minimum 36". The calculator checks these automatically. Some municipalities have stricter local codes — always verify with your building department before cutting stringers.
The 'rule of 25' is a carpenter's comfort guideline: 2 × riser + tread ≈ 24-25 inches. Too low (aggressive rise, shallow tread) feels steep. Too high (shallow rise, deep tread) feels awkward and stretched. 7.5" rise + 10" tread = 25. 7" rise + 11" tread = 25. Most comfortable stairs land near 25.
Sources
- IRC 2021 Chapter 3 — Stairways — Riser, tread, and width code requirements
- This Old House — Stair Design — Practical stair design and the rule of 25
Frequently asked
How many stairs do I need for an 8-foot ceiling?
For a floor-to-floor rise of about 8.75 feet (105 inches — 8 ft ceiling plus floor assembly), you need 14 risers and 13 treads with a 7.5" target. For a 9-foot ceiling (about 117 inches total rise), 16 risers and 15 treads. The calculator above handles any rise.
What's the IRC code for stair dimensions?
IRC 2021 residential: risers 4"-7.75" (variation ≤ 3/8" between any two risers), treads minimum 10", stair width minimum 36". Handrails required if more than 4 risers. Some local codes are stricter — always check before building.
How long should my stair stringer be?
The calculated stringer length is the actual slope length. For lumber ordering, round up to the next standard length (2×12 stringers come in 2-foot increments from 8 to 20 ft). A 108-inch rise at 7.5" risers needs a stringer about 155 inches long — order a 14-foot stringer (168 inches) with extra for cutting.
How many stringers do I need?
Minimum 2 (one each side) for stairs up to 36" wide. Add a middle stringer for stairs 36"+ wide or when treads are 1" or thinner. Always add a middle stringer for commercial or exterior stairs, and for any stair carrying heavy loads.
What's the rule of 25?
A comfort test: 2 × riser height + tread depth should be around 24-25 inches. Below 23 feels steep and unsafe; above 26 feels awkward. Most comfortable stairs: 7" rise + 11" tread = 25. Code allows stairs outside this range but experienced carpenters use it as a design target.
Can I adjust tread depth if my space is tight?
Yes, down to 10" (IRC minimum). Below that is not code-compliant. If you're truly tight on horizontal space, consider a winder stair (wedge-shaped treads turning at a corner) or a spiral stair — both consume less horizontal space than a straight run but are harder to navigate with furniture.
How do I measure total rise if the floor isn't finished?
Measure from the top of the lower finished floor surface to the top of the upper finished floor surface. If either floor is unfinished, add the finish thickness: typical hardwood adds 3/4", tile adds 1" with underlayment, carpet with pad adds 1/2". Miss this step and your top or bottom riser will be uneven.
What about landings?
Required by IRC for any stair with more than 12 risers in a single run, or at any turn. Landings must be at least as deep as the stair is wide (so a 36" wide stair needs a 36" deep landing). The calculator treats the stair as a single straight run — for L-shaped or U-shaped stairs, calculate each run separately.