A stair stringer calculator takes the total rise of a stair and works out how many steps it needs, how tall each riser is, how far the stair runs, and how long the stringer has to be. Contruva’s free stair stringer calculator also gives you the stair angle, the notch depth and the throat left on a 2x12, and checks your riser and tread against the US International Residential Code (IRC) and the National Building Code of Canada (NBC).
Open the free stair stringer calculator in your browser. No login, no email, nothing to pay.
Open the free toolIt works in inches or millimetres and needs no login. Below is the math, two worked examples you can check by hand, and the code limits it tests, with the sections to look up when your inspector asks.
How to use the stair stringer calculator
Finished floor to finished floor, at the stair location. If the flooring isn’t in yet, add its thickness at the top and bottom.
The tallest riser you’ll accept. Use the code maximum (7¾ in under the IRC, 200 mm under the NBC) or lower for an easier stair.
Run is the horizontal distance from one nosing to the next. Nosing is how far each tread overhangs the riser below.
2x12 (11¼ in or 286 mm actual), 2x10, or enter the depth of an LVL or other board.
The result card gives the riser count and exact riser height, the number of treads, total run, stair angle, stringer length, the board length to buy, the notch depth, the throat, and 2R + T. A second card shows pass or fail against each code rule.
How the stair math works
| Result | Formula |
|---|---|
| Number of risers | Total rise ÷ maximum riser, rounded up |
| Exact riser height | Total rise ÷ number of risers |
| Number of treads | Risers − 1 (the upper floor is the last step) |
| Total run | Treads × run per step |
| Step diagonal | √(riser² + run²) |
| Stringer length | Risers × step diagonal |
| Stair angle | arctan(riser ÷ run) |
| Notch depth | Riser × run ÷ step diagonal |
| Throat | Board depth − notch depth |
Rounding the riser count up is what keeps every riser at or under your maximum. The stringer length uses one step diagonal per riser rather than per tread, which leaves enough board for the plumb cut at the top and the level cut at the bottom. The calculator then rounds up to the next common lumber length: 8, 10, 12, 14, 16, 18 or 20 ft.
Two risers plus one run landing somewhere around 24 to 25 inches (about 610 to 635 mm) is an old carpenter’s check for a stair that feels natural to walk. The calculator shows it so you can compare options; the code limits are what an inspector checks.
Worked example: a 105 inch basement stair
Total rise 105 in (8 ft 9 in), maximum riser 7¾ in, run 10 in, 1 in nosing, 2x12 stringers.
105 ÷ 7.75 = 13.55, rounded up to 14. Then 105 ÷ 14 = 7.5 in exactly.
14 risers − 1.
13 × 10 in.
√(7.5² + 10²) = √156.25 = 12.5.
14 × 12.5 = 175 in.
arctan(7.5 ÷ 10).
Notch depth 7.5 × 10 ÷ 12.5 = 6 in, and 11.25 − 6 = 5.25 in.
2 × 7.5 + 10.
Against the IRC this stair passes: 7½ in risers are under 7¾ in, the 10 in run meets the 10 in minimum, and the 1 in nosing sits inside the ¾ to 1¼ in range. Against the NBC it fails on run, and this catches a lot of people: 10 in is 254 mm, and NBC 2020 raised the minimum run for private stairs to 255 mm. Bump the run to 10½ in and it passes both:
| Run | Total run | Stringer length | Angle | Throat on a 2x12 | IRC | NBC |
|---|---|---|---|---|---|---|
| 10 in | 130 in | 175 in | 36.87° | 5¼ in | Pass | Fail (run 254 mm) |
| 10½ in | 136½ in | 180.65 in | 35.54° | 5⅛ in | Pass | Pass |
The longer run costs you 6½ in of floor space at the bottom of the stair. Check that you still have the landing you need before you commit.
Worked example in millimetres
Total rise 2,670 mm, maximum riser 200 mm, run 255 mm, 25 mm nosing, 2x12 (286 mm) stringers.
- Risers: 2,670 ÷ 200 = 13.35, rounded up to 14 risers at 190.7 mm.
- Treads and run: 13 treads × 255 mm = 3,315 mm total run.
- Angle: arctan(190.7 ÷ 255) = 36.79°.
- Throat: about 133 mm left on the 2x12.
- Stringer: about 4,458 mm, so a 16 ft board.
- Code check: passes the NBC rise (125 to 200 mm) and run (255 to 355 mm) limits, and the IRC limits once converted to inches.
How much wood is left on a 2x12?
The throat is the narrowest part of a cut stringer: the distance from the inside corner of each notch, square to the bottom edge of the board. It’s what actually carries the load, so a deep notch in a narrow board makes a bouncy stair.
The notch depth is riser × run ÷ step diagonal. On a 2x12 (11¼ in actual), a 7½ in rise and 10 in run notch 6 in deep, leaving 5¼ in. Steeper, taller steps cut deeper: a 7¾ in rise on a 2x10 (9¼ in actual) leaves well under 4 in.
The calculator flags a throat under 5 in, the minimum for cut stringers in deck codes based on the American Wood Council’s DCA 6 guide (Wisconsin’s deck rules, for example, checked November 2026). Interior stair stringers are covered by your building code’s span and support rules; if the throat is thin, use deeper stock, add a centre stringer, or support the stringer along its length.
Stair code limits: IRC vs National Building Code of Canada
These are the limits the calculator checks, for stairs serving a single dwelling, checked November 2026. Provinces, states and municipalities adopt these model codes with their own amendments and on their own schedule, so confirm with your building department before you cut.
| Rule | IRC (US), R311.7.5 | NBC 2020 (Canada), private stairs |
|---|---|---|
| Maximum riser | 7¾ in (196 mm) | 200 mm |
| Minimum riser | Not checked | 125 mm |
| Minimum run | 10 in (254 mm), nosing to nosing | 255 mm |
| Maximum run | Not checked | 355 mm |
| Nosing | ¾ to 1¼ in; not required where the tread is 11 in or deeper | Not checked |
| Riser and tread variation in a flight | 3/8 in max | Check your provincial code |
The NBC run minimum went from 210 mm to 255 mm in the 2020 edition, and provinces have been adopting it at different times. Ontario’s builder bulletins, for example, describe the same change. If your province is still on an older edition, the calculator’s NBC run check will be stricter than what you need, which is not a bad thing. Headroom (6 ft 8 in under the IRC; see NBC 9.8.2 in Canada), stair width, handrails and guards also apply, and the calculator doesn’t check them.
Laying out and cutting the stringer
Clamp stair gauges to a framing square at the riser height and run. Line up the gauges on the top edge of the 2x12 and trace the first notch.
Slide the square along until the gauge meets the last mark and trace again, one notch per tread.
Take one tread thickness off the bottom of the stringer so the first riser finishes the same height as the rest once treads are on.
Stop the circular saw at the line and finish the inside corner with a hand saw or jigsaw so you don’t overcut into the throat.
Test-fit the first stringer, then trace it for the others.
Stair mistakes that fail inspection
- Measuring the rise before the flooring. A ¾ in hardwood floor at the top makes the top riser short and the flight non-uniform.
- Forgetting the tread drop. Skip it and the bottom riser is a full tread thickness taller than the rest.
- Measuring the rise in the middle of the room. Floors and slabs aren’t level. Measure at the landing spot.
- Using a 10 in run in Canada. 254 mm is 1 mm under NBC 2020’s 255 mm minimum.
- Overcutting the notches. Every saw kerf past the line comes out of the throat.
Pricing the stair
Stringers are the cheap part. Price the treads, risers, railing, guards and finish, plus the hours for layout, cutting and fitting. For an outdoor stair, the deck calculator covers the framing and boards around it, and the board foot calculator helps when you’re buying hardwood treads. Then put a margin on it with the contractor markup calculator.
If you build stairs as part of deck work or renovations, Contruva puts the stringers, treads, railing and labour on a quote your customer can sign and pay a deposit on from their phone, and job costing shows what the stair really cost when it’s done. One plan, Contruva Pro, at $249.99 CAD a month. Try it free for 7 days, no credit card needed.