Stair stringer calculator
Risers and treads from the total rise, checked against IRC R311.7 (and AWC DCA 6 for deck stairs) — stringer layout with the throat check, headroom and stairwell opening, side elevation, plan and isometric drawings, cut and shopping lists, build steps, and a 1:1 printable step template.
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How to use it
Enter the total rise — finished lower floor to finished upper floor, measured where the stair will land — the largest riser you accept, the tread run (nosing to nosing), and the width. The tool divides the rise into the fewest equal risers that stay under your maximum, so every step is the same height; there is one tread fewer than risers because the top step is the upper floor.
How to use it
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Drawings — the side elevation with the stringer, treads, risers, the nosing line and the 6′ 8″ headroom line, the stringer laid out on its board, an isometric view and a plan. ⛶ Full screen (or
f) makes the drawing fill the screen; Escape leaves. -
Cut list and Shopping — stringers (the board length the cut stringer fits on), treads and risers, packed into standard lengths with the cut list optimiser’s engine, plus hangers, screws and the handrail. Type your prices to total the job; they stay in your browser.
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Steps — measure, lay out with a framing square and stair gauges, drop the stringer by one tread thickness, cut and test-fit, fix, then risers, treads, handrail — each with a small diagram.
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Template — one notch at full size. Print view prints it at 1:1 with a check square; trace it onto the stringer, or use it to check the stair gauges on your square.
The geometry
Each riser is R = total rise ÷ n, with n = ⌈total rise ÷ maximum riser⌉. The notches cut R·T/√(R²+T²) deep, measured square to the board’s edge, so the throat — the wood left under the notches — is the board width minus that. A 2x12 (11-1/4″) with 7-5/8″ risers and 10″ treads keeps about 5.2″.
The stringer is dropped: treads sit on it, so the first riser on the stringer is one tread thickness shorter than the others. Headroom is measured vertically from the line through the nosings; with an upper floor F deep, the stairwell opening must run at least (80″ + F) × T ÷ R from the top riser.
The rules it checks
From the 2021 International Residential Code, R311.7: risers at most 7-3/4″ (R311.7.5.1) and treads at least 10″ (R311.7.5.2), each varying by no more than 3/8″ in a flight; nosings 3/4″–1-1/4″ where risers are solid and treads are under 11″ (R311.7.5.3); open risers must not pass a 4″ sphere where the stair rises more than 30″; 36″ minimum width (R311.7.1); 6′ 8″ headroom (R311.7.2); at most 151″ of rise between floors or landings (R311.7.3); a handrail 34″–38″ above the nosings on stairs with four or more risers (R311.7.8). Guards are required on open sides more than 30″ above the floor or grade (R312.1).
Deck stairs add the American Wood Council’s Prescriptive Residential Wood Deck Construction Guide (DCA 6-15): cut stringers at least 2x12 with a 5″ minimum throat and no more than 6′ 0″ of horizontal span (Figure 28), at least three of them and at most 18″ apart (Figure 29), sloped hangers rated at least 625 lb at the top (Figure 31), and a landing on stairs more than 12′ high.
The comfort check — 2 × riser + tread between about 24″ and 25″ — is a rule of thumb (Blondel’s formula), not code.
Limits
Straight flights only: no winders, landings, or spiral stairs. The code values are the model IRC as published; your jurisdiction may amend them (many do), and the building official has the final say — treat this as a planning aid, not an approval. Lumber sizes are North American nominal sizes. It does not check stringer or tread strength for interior stairs.
Related: deck planner, angles and slopes, cut list optimiser, board feet, inch fractions.
Linking to an example
Share link copies a URL with every setting. Lengths in links are millimetres; units is in or
mm (display only). Add #template or #steps to open a tab.
| Parameter | Meaning |
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A preset (below) |
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Total rise, maximum riser, tread run, stair width (mm) |
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Tread thickness, nosing, |
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Stringer stock |
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Upper floor depth and stairwell opening length (mm) for the headroom check |
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Saw kerf (mm) |
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