Stair Calculator Widget
Embed a stair calculator for carpenters, joiners and loft-conversion sites. Visitors enter the floor-to-floor height and either a tread depth or the run they have room for, and get the number of risers, the riser height, total run, stringer length and angle, each checked against the US IRC or England's Approved Document K limits.
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Embed code
<iframe src="https://a2z.tools/embed/w/stair-calculator" title="Stair Calculator by A2Z Tools" width="100%" height="980" style="border:0;width:100%" loading="lazy" allow="clipboard-write"></iframe>
A plain iframe. Works everywhere, including site builders that strip scripts. Adjust height if your content needs more room.
<div data-a2z-widget="stair-calculator" data-height="980"></div> <script async src="https://a2z.tools/embed.js"></script>
Adds a small script (what it does) that sizes the widget to fit its content, loads it lazily and keeps it isolated from your page's CSS.
Works with
How it works
The total rise is divided by a target riser height and rounded up to a whole number of risers, so no riser is taller than the target; the actual riser is then the total rise divided by that count. A straight flight has one tread fewer than risers because the upper floor forms the last step. Tread depth is either your choice, or the available run divided by the number of treads. From these come the total run, the stair angle (the arctangent of riser over tread), the 2R + G comfort sum and an approximate stringer line length, the hypotenuse of the total rise and run. 108 in floor to floor with a 7.5 in target gives 15 risers of 7.2 in; with 10.5 in treads the run is 147 in at 34.4 degrees. The results are compared with IRC R311.7.5 (riser no more than 7-3/4 in, tread at least 10 in) or Approved Document K's private-stair limits (rise 150-220 mm, going 220-300 mm, 2R + G 550-700 mm, pitch at most 42 degrees), and each limit is labelled within or outside.
Calculation method
- Risers n = ceil(total rise / target riser); riser = total rise / n
- Treads = n - 1; tread = chosen depth, or available run / (n - 1)
- Total run = treads x tread; angle = atan(riser / tread); 2R + G = 2 x riser + tread
- Stringer line (approx.) = sqrt(total rise^2 + total run^2)
- IRC R311.7.5: riser <= 7-3/4 in (196.85 mm); tread >= 10 in (254 mm)
- Approved Document K, private stair: 150 <= rise <= 220 mm; 220 <= going <= 300 mm; 550 <= 2R + G <= 700 mm; pitch <= 42 degrees
Worked examples
Basement stair in inches
Inputs: Total rise 108 in; target riser 7.5 in; tread 10.5 in; IRC
Result: 15 risers of 7.2 in; run 147 in; angle 34.4 degrees; stringer about 182.4 in
108 / 7.5 = 14.4 -> 15 risers; 14 x 10.5 = 147; sqrt(108^2 + 147^2) = 182.4. Both IRC limits within.
UK loft stair
Inputs: Total rise 2,600 mm; target 190 mm; going 250 mm; Approved Document K
Result: 14 risers of 185.7 mm; 2R + G 621 mm; all four limits within
2,600 / 190 = 13.7 -> 14; 2 x 185.7 + 250 = 621.4.
A layout aid that checks published limits - not a statement of code compliance. Your building official or building control decides.
Limitations
- Checks dimensional limits only - not headroom, width, handrails, guards, nosings, winders or landings - and is not a statement of code compliance.
- The stringer figure is a line length; cutting the stringer also needs the seat cut, the top fixing and the thickness of finished treads and floors.
- Assumes a straight flight with equal risers; floor finishes added later change the top and bottom riser.
Where publishers use it
- Joinery and staircase manufacturers' quote pages
- Loft-conversion and basement-finishing guides
- Deck and porch builders laying out exterior steps
- Carpentry courses teaching stringer layout
- Self-build and renovation forums where people check a plan before a building-control visit
Questions
How many steps do I need for a 9 ft floor-to-floor height?
108 in divided by a 7.5 in target is 14.4, so 15 risers of 7.2 in and 14 treads. With 10.5 in treads the stair runs 147 in (12 ft 3 in) at about 34 degrees.
What are the IRC riser and tread limits?
IRC R311.7.5 sets a maximum riser height of 7-3/4 in and a minimum tread depth of 10 in for homes. The widget checks those two numbers only; headroom, width, handrails and nosings have their own sections.
What are the UK rules for a private stair?
Approved Document K allows a rise of 150-220 mm and a going of 220-300 mm, with twice the rise plus the going between 550 and 700 mm and a pitch no steeper than 42 degrees.
Why is there one tread fewer than risers?
The top riser lands on the upper floor, which acts as the last step. 15 risers need 14 treads, and the run is 14 x the tread depth.
What does 2R + G mean?
It is a comfort rule linking rise and going to a normal stride. 185 mm risers with 250 mm goings give 620 mm - inside the 550-700 mm range in Approved Document K.
Can I fit a stair into a fixed space?
Choose 'Fit into an available run'. For 2,600 mm of rise with 190 mm target risers you get 14 risers of 185.7 mm; a 3,250 mm run gives 13 treads of 250 mm.
How do I lay out a stringer with a framing square?
Clip stair gauges to the square at the riser height on the tongue and the tread depth on the blade - 7.2 in and 10.5 in in the basement example - then step the square along the board, tracing each notch. Drop the bottom of the stringer by one tread thickness so the first riser matches the others.
What about winders, spirals and landings?
This widget handles a straight flight only. Winders and spiral stairs are measured on a walkline with their own going rules, and a landing splits the rise into two flights - run each flight separately with its own share of the total rise.
Sources
- IRC Code Handout #437 Treads and Risers (quoting IRC R311.7.5.1 and R311.7.5.2) - City of Boise Building Division . Riser height not more than 7-3/4 in (196 mm); tread depth not less than 10 in (254 mm).
- Approved Document K: Protection from falling, collision and impact (2013 edition), Table 1.1 - HM Government (gov.uk) . Private stair: rise 150-220 mm, going 220-300 mm; 2R + G between 550 and 700 mm; maximum pitch 42 degrees.
Cite or recommend this tool
If you reference this tool in an article, course or documentation, these formats are ready to copy. They are optional - nothing is added to your site unless you paste it.
A2Z Tools Stair Calculator https://a2z.tools/embed/stair-calculator
<a href="https://a2z.tools/embed/stair-calculator">A2Z Tools Stair Calculator</a>
[A2Z Tools Stair Calculator](https://a2z.tools/embed/stair-calculator)
Stair Calculator by A2Z Tools - https://a2z.tools/embed/stair-calculator
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