Stair Calculator
Code & comfort checks
Estimates only. Verify against your locally adopted building code and have stairs inspected as required.
Most stairs need a riser height of about 7 inches and a tread depth of at least 10 inches, so a standard 108-inch floor-to-floor rise works out to 15 risers at 7.2 inches each and 14 treads — about 140 inches of total run. Enter your total rise below for exact riser height, tread depth, total run, stringer length, and angle, each checked live against the 2024 IRC residential stair code.
Stair code limits: IRC residential vs. commercial vs. comfort
| Dimension | IRC residential (2024) | ADA / commercial | Comfort-ideal |
|---|---|---|---|
| Riser height | 7.75" max | 7" max | 7"–7.5" |
| Tread depth | 10" min | 11" min | 10"–11" |
| Headroom | 6'8" (80") min | 6'8" (80") min | 80"+ |
| Total rise between landings | 151" max | 12'0" max (per flight) | under 151" |
| Stair width | 36" min | 36"+ (occupancy-based) | 36"+ |
| Riser variation within a flight | 3/8" max | 3/8" max | as uniform as possible |
| Comfort rule (2 × riser + tread) | not a code limit | not a code limit | 24"–25" |
How to calculate a staircase
Stairs are pure geometry driven by one number: the total rise, the finished floor-to-floor height. Divide it by a comfortable target riser height — about 7 inches — and round to the nearest whole number to get your riser count, then divide the total rise back by that count for the exact riser height. You can't simply pick 7 inches: the risers have to divide the rise evenly, and the 2024 IRC requires every riser in a flight to be within 3/8 inch of the others. The tread count is one less than the riser count on a standard mount (the top tread is the floor itself), and total run is tread depth times tread count. The stringer is the hypotenuse: √(total run² + total rise²) — once you have these numbers, the stair stringer calculator gives the stringer count, notch throat, and board length.
Riser height and tread depth are a trade-off
Taller risers mean fewer steps and less run, but steeper, less comfortable stairs; deeper treads eat floor space but ease the climb. The two classic comfort rules keep them balanced: 2 × riser + tread should land between 24 and 25 inches, and riser + tread between 17 and 18 inches. A 7-inch riser with an 11-inch tread hits both and is the widely cited "ideal" step. The calculator flags when your design drifts outside the comfort range even if it still passes code.
Standard vs. flush mount
Mount type decides whether the top tread sits flush with the upper floor (standard, one fewer tread than risers) or a full step below it (flush, equal treads and risers). It changes the tread count and total run but not the riser math. If you're applying finished tread boards, set the tread thickness so the calculator gives the first-step cut height — the bottom riser is cut shorter by one tread thickness so every finished step ends up equal.
Common mistakes
- Picking a riser height instead of solving for it. The rise must divide into equal risers. Choose a target and let the count round; forcing an exact height leaves an uneven top or bottom step, which fails code.
- Measuring rise before the floors are finished. Total rise is the finished floor-to-finished floor height. Flooring, underlayment, or a poured topping added later throws off every riser.
- Ignoring headroom and total-rise limits. The IRC caps a single flight at 151 inches of rise between landings and requires 6 feet 8 inches of headroom. A tall flight may need a landing you haven't planned for. The same rise-over-run triangle shows up in roof pitch — there it's the slope factor rather than the stringer length.
Handrails, guards, and baluster spacing
The calculator sizes the steps; the railing is separate code you lay out once the flight is set. The numbers below are the 2024 IRC residential requirements — always confirm the edition your local jurisdiction has adopted, since amendments vary.
Handrail height and when it's required
A handrail is required on at least one side of any flight with four or more risers (R311.7.8). Its top must sit 34 to 38 inches above the tread nosings, measured vertically, and run continuously the full length of the flight. That height is measured off the sloped line of the nosings, so the rail parallels the stair angle the calculator reports.
Guards and baluster spacing
Where a walking surface sits more than 30 inches above the floor or grade below, it needs a guard at least 36 inches tall on the open side. The infill — balusters or panels — has to be tight enough that a 4-inch sphere can't pass through, which is what keeps a child from slipping between them. There's one allowance specific to stairs: the triangular opening formed by the tread, riser, and the bottom rail may pass a slightly larger sphere (up to 6 inches) because of its shape. For evenly spaced round balusters, that 4-inch rule usually works out to about a 4-inch clear gap or a baluster roughly every 4 to 5 inches on center depending on baluster width.
Wood, concrete, and other stair types
The rise-and-run math above is the same whatever the stair is built from — the material changes how you build it, not the geometry.
Wood-framed stairs
The most common residential build: cut or solid stringers carry the treads and risers. Solve the layout here, then take the riser height, tread depth, and total run to the stair stringer calculator for the stringer count, notch throat, and board length.
Concrete stairs
Poured concrete stairs use the identical riser and tread dimensions, but you form the steps rather than notch a board, so there's no stringer. What you need instead is the concrete volume for the stepped form — a good use for the concrete calculators once you've fixed the step dimensions here. The same code riser, tread, and uniformity limits still apply.
Steel, floating, and spiral stairs
Steel-stringer and floating (open-riser) stairs still follow the same rise and run rules, though open risers have their own limit — the gap can't pass a 4-inch sphere. Spiral stairs are the exception this calculator doesn't cover: they're governed by a separate IRC section (R311.7.10.1) with their own tread-depth-at-a-walkline, minimum headroom, and riser rules, so a straight-flight rise-and-run layout won't size one correctly. Use dedicated spiral-stair geometry for those.
Frequently asked questions
How many stairs do I need to go up 8 feet?
For a total rise of 8 feet (96 inches) you need about 14 risers at roughly 6.9 inches each, which gives 13 treads. Divide the total rise by a comfortable target riser height near 7 inches, then round to a whole number of risers: 96 ÷ 7 ≈ 13.7, so 14 risers at 96 ÷ 14 = 6.86 inches. The riser height has to divide the total rise evenly, so you rarely land on exactly 7 inches — enter your real floor-to-floor height above for the exact count.
What is the formula for stair stringers?
The stringer length is the hypotenuse of a right triangle, so it follows the Pythagorean theorem: stringer length squared equals total run squared plus total rise squared. In other words, stringer length = √(total run² + total rise²). The total run is the tread depth times the number of treads, and the total rise is your floor-to-floor height. The calculator above works this out for you along with the stair angle.
What is the maximum riser height and minimum tread depth for stairs?
Under the 2024 International Residential Code (R311.7), the maximum riser height is 7¾ inches and the minimum tread depth is 10 inches. Riser heights within a single flight can't vary by more than ⅜ inch, headroom must be at least 6 feet 8 inches, and the total rise between landings can't exceed 151 inches. The calculator checks your riser height, tread depth, and total rise against these limits live, but always confirm the code edition your local jurisdiction has adopted.
What is the ideal rise and run for stairs?
A 7-inch rise paired with an 11-inch run is widely cited as the most comfortable combination for stairs. Comfort is usually judged by the rule that two risers plus one tread should total about 24 to 25 inches, which matches a natural stride. Codes allow steeper stairs — up to a 7¾-inch riser and down to a 10-inch tread — but staying near 7 and 11 makes a stair that feels easy to climb.
What is the rule of 27 for stairs?
The rule of 27 refers to stairway width: a stairway should be at least 27 inches wide measured below the handrail when there are rails on both sides. It's a quick clearance check, not a rise-and-run formula. Note that the general code minimum for stair width is 36 inches above the handrails, so the 27-inch figure is specifically about the clear space left once handrails intrude.
How do you count the number of steps on a stair?
Count risers, not treads — every time you lift your leg to go up, that's one step. A flight always has one more riser than it has treads, because the top tread is the floor you're arriving at. That's why a standard-mount stair with 14 risers has 13 treads. When people miscount, it's usually because they counted the treads and forgot the final rise onto the upper floor.
What is the code height for a stair handrail?
Under the IRC, the top of a stair handrail must be 34 to 38 inches above the tread nosings, measured vertically along the slope of the stair. A handrail is required on at least one side of any flight with four or more risers, and it has to run continuously the full length of the flight. Where the walking surface is more than 30 inches above the floor below, the open side also needs a guard at least 36 inches tall. Confirm the code edition your local jurisdiction has adopted, since amendments vary.
How far apart can stair balusters be spaced?
Guard infill on a stair has to be tight enough that a 4-inch sphere can't pass between the balusters, which is what stops a child from slipping through. For evenly spaced round balusters, that usually works out to about a 4-inch clear gap, or a baluster roughly every 4 to 5 inches on center depending on the baluster width. Stairs get one allowance: the triangular opening formed by the tread, riser, and bottom rail may pass a slightly larger sphere (up to 6 inches). This calculator sizes the steps, not the railing, so lay out the balusters against your local code once the flight is set.
Can I use this calculator for spiral stairs?
No — this calculator is for straight flights, where every tread has the same depth. Spiral stairs are governed by a separate IRC section (R311.7.10.1) with their own rules: tread depth is measured at a set walkline distance from the narrow edge, and the minimum headroom and riser limits differ from a standard flight. A straight rise-and-run layout won't size a spiral stair correctly, so use dedicated spiral-stair geometry for those. The same applies to winder treads on a turn, which taper across their width.
Related calculators
Sources
- 2024 International Residential Code — Section R311.7 (Stairways)International Code Council (ICC)The residential stair limits the calculator checks against: max 7.75" riser, min 10" tread depth, min 6'8" headroom, 151" max total rise between landings, 36" min width, and the 3/8" within-flight riser/tread variation.
- Stair Calculator — code limits, formulas, and cut detailsmycarpentry.comCorroborates the 2024 IRC figures (151" max total rise, 3/8" step variation, 5" min uncut stringer) and the first-step-height cut value (riser height minus tread thickness).
- Stairs Calculator — formulas and comfort ruleOmni CalculatorSource for the stringer Pythagorean formula, the standard-vs-flush mount tread-count rule, and the comfort formula (2 × rise + run ≈ 24–25"); also the origin of the verified on-page FAQ questions.
- 2010 ADA Standards — Section 504 (Stairways)U.S. Access BoardSource for the commercial/accessible comparison column: 7" max riser and 11" min tread depth, which are stricter than the IRC residential limits.
- 2024 International Residential Code — Sections R311.7.8 (Handrails), R312 (Guards), and R311.7.10.1 (Spiral Stairways)International Code Council (ICC)Source for the railing figures: handrail required at 4+ risers and set 34–38" above the nosings, 36" min guard height above a 30" drop, the 4" sphere baluster rule and 6" stair-triangle allowance, and the separate spiral-stair provisions.