Stair Calculator
This stair calculator lays out a single straight flight. Enter the total rise — the finished floor-to-floor height — and it divides that into a whole number of equal risers, then reads back the going, total run, pitch angle and stringer length. Every step in the flight gets the same rise and the same going, which is what both the IRC and the UK Building Regulations require.
It also checks your design against code. Pick IRC 2021 (US) or UK Approved Document K, and each limit — maximum riser, minimum going, maximum pitch, headroom and more — is listed with your actual value and a pass or fail, so you can see exactly which rule a design breaks instead of a bare yes-or-no. Two comfort rules, Blondel's 2R + G and the rise-plus-going rule, show whether the flight will also feel right underfoot.
Everything runs in your browser: no sign-up and nothing uploaded. Switch between metric and imperial at any time, export a CSV cut list or print a set-out sheet for the workshop. Building limits vary by jurisdiction and are amended locally, so always confirm the final design with your building authority.
How to use the stair calculator
Start in Rise-driven mode: enter the total rise, a target riser height and the going, then choose your code. The calculator rounds to the nearest whole number of equal risers and shows the resulting layout, pitch, stringer length and a full compliance report live as you type.
Switch to Fit-to-run when the horizontal space is fixed — enter the rise and the run you have, and it solves for a going that fits, or tells you the flight will not fit within code so you can add a riser, add a landing, or lengthen the run. Use Code check to test a stair you have already measured against the limits.
How to lay out a stair in 5 steps
- Choose unitsPick metric (mm) or imperial (inches) at the top of the page. Every input and result converts as you switch.
- Pick an input modeChoose Rise-driven to lay a flight out from the total rise, Fit-to-run to fit a flight into a fixed horizontal run, or Code check to test dimensions you have already measured.
- Enter your dimensionsType the total rise and a target riser height (around 178 mm / 7 in is comfortable), plus the going — the step depth from nosing to nosing.
- Choose a code standardSelect IRC 2021 for the US or UK Approved Document K (private, utility or general access) for Britain, or enter custom limits for another jurisdiction.
- Read the resultsRead the riser count and height, going, total run, pitch and stringer length, then check the per-rule compliance and comfort badges. Print or export the cut list.
The parts of a stair
A straight flight is built from a few repeating parts. The riser is the vertical face of each step, and the going is the horizontal depth of the tread measured from one nosing to the next — the going, not the wider tread board, is the dimension the code checks. The tread is the board you walk on; the nosing is the small overhang at its front edge.
The stringer, or string, is the raking board on each side that the treads and risers fix to; its length is the straight-line diagonal from the bottom of the flight to the top. Total rise is the sum of every riser and total run is the sum of every going. Because the flight terminates at the upper floor, a flight of N risers has N − 1 treads — the classic off-by-one this tool always gets right. Headroom is the clear vertical height measured above the pitch line to any ceiling or opening over the stair.
The comfortable angle
Pitch is the angle the flight climbs, set by the ratio of riser to going. A comfortable private stair sits between roughly 30° and 37°: the 7 in × 11 in (178 × 280 mm) default works out at about 32°, right in the middle of that band. Shallower than 30° wastes floor space; steeper than about 42° starts to feel like a ladder.
UK Approved Document K caps a private stair at 42°. The IRC sets no explicit pitch limit but reaches one indirectly through its 7¾ in (196 mm) maximum riser and 10 in (254 mm) minimum tread, which together hold a compliant IRC stair near 37–38°. The comfort band shown here is tighter than either code, because a legal stair is not always a comfortable one.
IRC and UK code limits
The two standards this tool ships take different routes to a safe stair. IRC 2021 (Section R311.7) is imperial-native: a maximum riser of 7¾ in (196 mm), a minimum tread depth of 10 in (254 mm), minimum headroom of 6 ft 8 in (2032 mm), a minimum clear width of 36 in (914 mm), and a nosing of ¾–1¼ in (19–32 mm) required whenever the tread is under 11 in (279 mm). No single flight may rise more than 12 ft 7 in (3835 mm) without a landing.
UK Approved Document K is metric-native and splits by use. A private stair allows a riser up to 220 mm with a going of 220–300 mm and a pitch up to 42°; utility stairs tighten to a 190 mm riser and a 250 mm going, and general-access stairs to 170 mm and 250 mm. Across all categories, twice the rise plus the going (2R + G) should fall between 550 and 700 mm, and headroom should be at least 2000 mm. A frequent trap: a tidy 250 mm going is legal in the UK but fails the IRC's 254 mm minimum tread — the calculator flags exactly that.
Laying out the stringer
The stringer length is the diagonal of the rise-and-run triangle — the square root of the total rise squared plus the total run squared — so a 2600 mm rise over a 4200 mm run needs a board a little over 4.9 m long before cutting. Order stringer stock longer than this figure to leave room for the plumb and seat cuts at each end.
To mark the steps, set a framing square to the riser on the tongue and the going on the blade, and step it along the board once per tread, scribing each notch. The cut list below the calculator gives the cumulative rise and run to every step, so you can measure from a single datum instead of stacking small errors step by step. Cut one stringer, then use it as a template for the rest.
Frequently asked questions
What is the difference between rise and run?
The total rise is the vertical height the stair climbs, floor to floor. The total run is the horizontal distance it covers. Each individual step has a riser (its vertical height) and a going (its horizontal depth). Divide the rise by a comfortable riser height to get the number of steps, and the run follows from the going multiplied by the number of treads.
How many steps do I need?
Divide the total rise by a comfortable target riser height — around 178 mm (7 in) — and round to the nearest whole number. That whole number is the riser count, and dividing the total rise by it gives the exact equal riser height. A flight of N risers has N − 1 treads, because the top riser lands on the upper floor. This calculator does the rounding for you.
What is the difference between the going and the tread?
The going is the horizontal depth of a step measured from the nosing of one tread to the nosing of the next — this is the dimension building codes check. The tread is the physical board you stand on, which is the going plus the nosing overhang. Because the nosing is shared between steps, it is the going, not the wider tread, that sets the total run.
What is a comfortable stair angle?
Between about 30° and 37°. A 178 mm riser with a 280 mm going gives roughly 32°, near the ideal. UK Approved Document K allows a private stair up to 42°, and anything much steeper becomes ladder-like. Two comfort rules back this up: twice the rise plus the going (2R + G) should be about 600–640 mm, and the rise plus going should be 432–457 mm (17–18 in).
What are the maximum riser and minimum tread?
Under the 2021 IRC the maximum riser is 7¾ in (196 mm) and the minimum tread depth is 10 in (254 mm). UK Approved Document K allows a taller 220 mm riser on a private stair with a going of at least 220 mm, tightening to 190 mm and 250 mm for utility stairs. Watch the trap: a 250 mm going meets UK rules but fails the IRC's 254 mm minimum tread.
What is the difference between IRC and UK Building Regulations for stairs?
The IRC (US) is imperial-native and sets one set of residential limits: a 7¾ in riser, a 10 in tread, a 36 in width and 6 ft 8 in of headroom. UK Approved Document K is metric and splits stairs into private, utility and general-access categories with different riser and going limits, adds a maximum 42° pitch and a 2R + G rule of 550–700 mm, and requires 2000 mm of headroom. The calculator applies whichever you select and lists every rule with a pass or fail.
How do I work out the stringer length?
The stringer length is the diagonal of the rise-and-run triangle: the square root of (total rise² + total run²). For a 2600 mm rise over a 4200 mm run that is about 4940 mm. Buy the board longer to allow for the plumb cut at the top and the seat cut at the bottom, then transfer the step marks with a framing square set to the riser and going.
Does the calculator work in metric and imperial?
Both. Switch units at the top and every dimension converts — rise, going, riser height, run, nosing, headroom and width. The engine works in SI internally and only converts for display, so switching units never changes the underlying layout or the compliance result.
Is the stair calculator private?
Yes. Every calculation runs entirely in your browser. Nothing you enter is sent to a server, and your inputs are saved only in your own browser's local storage.