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.
Building code sets the numbers. Comfort decides where inside those numbers you land.
Stair design is one of the few areas where building code dictates the exact dimensions of what you build. Maximum riser height, minimum tread depth, minimum width, baluster spacing, headroom, handrail height. These are not suggestions. An inspector will measure them, and a staircase that fails code cannot be signed off. The calculator above computes rise and run from your total height using these IRC requirements.
How we calculated these numbers▾
Rise and run calculations follow IRC 2021 R311.7. Stringer length uses the Pythagorean theorem: √(total_rise² + total_run²). Stringer spacing per AWC span tables for 2×12 southern pine or Douglas fir. All dimensions assume standard residential interior or deck stairs.
Rise and run from your total height
The formula is: divide your total rise (in inches) by the target riser height (7.5 inches is the sweet spot). Round up to get the step count. Then divide total rise by step count for the actual rise per step. A 48-inch total rise divided by 7.5 gives 6.4, rounded up to 7 steps, with an actual rise of 6.86 inches each. Every step must be the same height. Variation of more than 3/8 inch between any two risers fails inspection and is a trip hazard.
Stringers: how many and how long
Composite illustration based on typical project dimensions, regional contractor pricing, and 2026 material costs. Not a specific real project.
Interior stairs | Deck/exterior stairs | |
|---|---|---|
| Stringer material | 2×12 SPF or Douglas fir | 2×12 pressure-treated |
| Tread material | Hardwood, LVP on plywood | PT, composite, or cedar |
| Railing required? | Yes if 4+ risers | Yes if 30"+ above grade |
| Open or closed risers? | Closed (finished look) | Open allowed by most codes |
| Landing required? | At top and bottom, 36" min | At top, 36" min |
Exterior stairs use pressure-treated lumber throughout. Interior stairs use dimensional lumber for stringers and finished wood or LVP for treads.
For deck projects where stairs are part of a larger build, the deck calculator and the deck planner chain the stair calculation with decking, framing, and footing quantities automatically.
How many stair stringers do I need?
Stringer count depends on stair width and tread material. For 36-inch-wide stairs (code minimum) with standard 2× treads: 3 stringers (one on each side plus one center). For stairs wider than 36 inches, add a stringer every 16 to 18 inches of additional width. A 48-inch-wide staircase needs 4 stringers. Stairs using composite or thin decking boards as treads need stringers every 12 to 16 inches because those materials flex more than solid lumber under foot traffic.
Stringer count | Max span between | |
|---|---|---|
| Stair width 36" | 3 stringers | 18" between |
| Stair width 48" | 4 stringers | 16" between |
| Stair width 60" | 4–5 stringers | 15–16" between |
| Composite treads (any width) | Add 1 extra | 12–16" max span |
Center stringers prevent tread bounce. If your treads flex when you step in the middle, you need another stringer.
Standard stair rise and run dimensions (IRC R311.7)
The IRC sets exact limits that every residential staircase must meet. These are not recommendations. Stairs that fall outside these ranges fail inspection and must be rebuilt.
IRC requirement | What it means | |
|---|---|---|
| Max riser height | 7.75 inches | Each step can be no taller than this |
| Min tread depth | 10 inches | Each step must be at least this deep |
| Min width | 36 inches | Clear width between walls or railings |
| Max variation | 3/8 inch | Largest riser minus smallest riser |
| Headroom | 80 inches minimum | Measured from nosing to ceiling above |
| Landing | 36" deep minimum | Required at top; at bottom if total rise > 12 ft |
The 3/8-inch variation rule is the one most DIYers fail. If your first riser is 7.5 inches and your last is 7.0 inches, that 1/2-inch difference fails code. The calculator keeps all risers uniform to prevent this.
Common rise and run combinations that pass code: 7.0-inch rise with 10.5-inch run (comfortable, gentle slope), 7.5-inch rise with 10-inch run (standard, the most common residential stair), 7.75-inch rise with 10-inch run (steepest allowed, saves horizontal space). The calculator finds the optimal combination from your total height.
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.
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