Work out riser height, tread depth, stringer length, and total run with this free stair calculator. Checks IRC, IBC, and OSHA compliance plus costs.
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Building a staircase without checking the numbers first is a recipe for sore knees and a failed inspection. This stair calculator takes the one measurement you can’t change, floor-to-floor rise, and works out everything else from there. Feed it your tread depth and stair width, and it returns the actual riser height, the number of steps, the total run, the stringer length, and the angle of the flight. It flags any dimension that fails your chosen building code and shows exactly which limit was crossed. For a typical 8-foot ceiling with a 10-inch tread, the tool lands you at 14 or 15 risers within a minute. Contractors use it for framing takeoffs. Homeowners use it to check a design before ordering lumber. Cost fields turn the geometry into a full materials estimate.
Pick your unit system. Imperial uses inches and feet. Metric uses millimeters and meters.
Enter the total rise. Measure from finished floor to finished floor, not to the subfloor.
Choose a layout method. Auto picks the number of risers for you. Manual lets you set the step count yourself.
Set the tread depth and stair width. Ten inches of tread and 36 inches of width are common baselines.
Choose a code standard. IRC, IBC, OSHA, UK, or NCC. The tool loads that code’s riser, tread, width, and ratio limits.
Confirm the max riser, min tread, min width, and max ratio. Override them for a custom job.
Add a landing if the stair turns. Enter the length and width in the same unit.
Set stringer count, stock length, riser type, and baluster spacing. Then check the summary for compliance and material counts.
The math starts with rise and works backward to riser count. In auto mode, the tool targets an ideal riser height of about 7 inches (0.178 m) and rounds the total rise into whole risers. That means a 108-inch rise becomes 15 risers at 7.2 inches each, not 14 at 7.71.
Formula: Number of Risers = Round(Total Rise ÷ Ideal Riser)
Formula: Actual Riser Height = Total Rise ÷ Number of Risers
Formula: Number of Treads = Number of Risers − 1
Formula: Total Run = Number of Treads × Tread Depth
Formula: Stringer Length = √(Total Rise² + Total Run²)
Formula: Stair Angle = atan2(Total Rise, Total Run) × 180 ÷ π
The 17–18 rule (riser plus tread should land between 17 and 18 inches) is why auto mode aims for a 7-inch riser. On a 10-inch tread, that puts you at 17 inches exactly. The tool doesn’t enforce this rule directly, but it gives you the ratio output so you can check it.
Compliance uses the code you pick. IRC caps the riser at 7.75 inches, sets the minimum tread at 10 inches, the minimum width at 36 inches, and the max riser-to-tread ratio at 1.0. IBC is stricter on commercial stairs. Headroom is checked against 80 inches (2032 mm) regardless of code.
Stringer length is the diagonal across the whole flight. It’s the hypotenuse of rise and run. A carpenter cutting a stringer lays that number out on a 2×12 and traces the steps. Stock length matters: a 16-foot board covers a stringer up to 16 feet, and anything longer needs a second piece.
Material counts come from the geometry. Stringer total length is one stringer length times the count (two for most residential stairs). Tread and riser lengths multiply step count by stair width. Handrail length equals stringer length on each side.
A main staircase in a two-story house. Total rise is 112 inches (a 9-foot ceiling with standard floor thickness). Tread depth is 10.5 inches. Stair width is 38 inches. IRC code. Closed risers.
Step 1 – Riser count. Auto mode: 112 ÷ 7.008 = 15.98, rounded to 16 risers.
Step 2 – Actual riser height. 112 ÷ 16 = 7.0 inches.
Step 3 – Tread count. 16 risers means 15 treads.
Step 4 – Total run. 15 × 10.5 = 157.5 inches, or 13.13 feet.
Step 5 – Stringer length. √(112² + 157.5²) = √(12,544 + 24,806.25) = √37,350.25 = 193.26 inches, or 16.11 feet.
Step 6 – Angle. atan2(112, 157.5) × 180 ÷ π = 35.43°.
Step 7 – Riser/tread ratio. 7.0 ÷ 10.5 = 0.667. IRC max is 1.0, so it passes.
Step 8 – Code checks. Riser 7.0 in (max 7.75) passes. Tread 10.5 in (min 10) passes. Width 38 in (min 36) passes. Headroom 80 in passes. Overall: compliant.
Step 9 – Materials. Stringers: 2 × 16.11 = 32.22 ft total. At 16-ft stock, that’s 2 pieces per stringer, 4 total. Treads: 15 × 38 in = 570 in, or 47.5 ft. Risers: 16 × 38 in = 608 in, or 50.67 ft. Handrails: 16.11 ft per side, 32.22 ft total. Balusters at 4 in spacing: 49 per side, 98 total. Newel posts: 4.
Step 10 – Cost at typical rates. Stringers at $42/ft = $1,353. Treads at $38/ft = $1,805. Risers at $12/ft = $608. Handrails at $28/ft = $902. Balusters at $8 each = $784. Newels at $65 each = $260. Material subtotal around $5,712.
Interpretation: The flight fits IRC without changes. You need four 16-foot stringer boards, about 48 feet of tread material, 51 feet of riser material, 98 balusters, and 4 newel posts. Tread and handrail material dominate the cost, so upgrading those first makes the biggest difference.
Divide total rise by your target riser to get the step count, then divide again for the actual riser height. Run equals tread depth times (risers minus one). A 112-inch rise with a 7-inch riser gives 16 risers and 15 treads. At a 10.5-inch tread, total run is 157.5 inches.
7.75 inches under IRC for residential stairs. IBC drops it to 7 inches for commercial. OSHA allows 9.5 inches in industrial settings, and UK Part K caps it near 7.5 inches. The calculator loads these limits when you pick a code.
About 14 or 15 risers, depending on ceiling thickness and flooring. An 8-foot ceiling with standard framing usually gives 104 to 108 inches of total rise. At a 7.5-inch riser, that’s 14 risers. At 7 inches, it’s 15.
The stringer length equals the square root of rise squared plus run squared. For a 112-inch rise and a 157.5-inch run, that’s 193.26 inches, or 16.11 feet. You’ll need 16-foot stock for that flight. Longer flights need a second piece or a landing.
Between 0.6 and 0.8. A 7-inch riser on a 10-inch tread gives 0.7, which walks comfortably. IRC allows up to 1.0, but anything above 0.85 starts to feel steep. Lower is gentler, but it uses more floor space.
Divide the handrail length by baluster spacing, then round up, then multiply by two sides. A 16.11-foot handrail at 4-inch spacing needs 49 balusters per side, so 98 total. Add extras for the landing if you have one. Building code caps the gap at 4 inches for safety.
A closed riser has a board filling the vertical space between treads. An open riser leaves that space empty. Open risers look modern and let light through, but they usually need a solid substrate behind them for structural reasons. The calculator includes riser material only when closed risers are selected.
Yes. The geometry and code checks work the same for exterior stairs. IRC limits apply if the deck serves a single-family home. Use pressure-treated or composite materials and check local code for soil contact rules.
Usually yes, for any permanent staircase inside a home. Most jurisdictions treat stairs as structural work. Minor repairs and cosmetic updates sometimes skip the permit, but full replacements and new builds almost always need one. Check with your local building department before framing.
Take the arctangent of rise divided by run. A 112-inch rise over a 157.5-inch run gives atan2(112, 157.5) = 35.43°. Angles between 30° and 37° feel natural for residential stairs. Below 30° the stair feels shallow. Above 40° it feels like a ladder.
Yes. The headroom check compares your input against the 80-inch minimum (2032 mm). Below that, the tool flags it as non-compliant. Headroom is measured vertically from the nosing line to the ceiling or any obstruction above.
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