Carpentry tool
Stair Stringer Calculator
Lay out and cut stair stringers with confidence: equal risers, tread run, notch geometry, throat depth, stringer count and the exact board length to buy.
Finished floor to finished floor or deck surface.
7-7.5in is typical; the tool rounds to equal risers.
Horizontal cut, nosing excluded.
Use 0 for open-riser deck stairs.
Extra length for the top and bottom cuts.
Stringers to cut
4 × 8.8ft
8 risers at 7.13in / 18.1cm · 7 treads · 28 notches · 35.0 linear ft of stock
- Riser height (equal)
- 7.13in / 18.1cm
- Total run
- 73.50in / 186.7cm
- Cut line (diagonal)
- 93.01in / 236.3cm
- Board length to buy
- 105.01in / 266.7cm (8.8ft)
- Throat behind notch
- 5.35in / 13.6cm
- Spacing on centre
- 12.00in / 30.5cm
- Stair angle
- 37.8 degrees
Code & comfort checks
IRC allows a 7 3/4in maximum riser; UK Part K allows 220mm on private stairs.
IRC requires a 10in minimum run; UK Part K wants a 220mm going.
Two risers plus one tread should land between 24in and 25in for a comfortable pace.
Keep at least 3 1/2in of solid timber behind every notch, or move up a board size.
30-37 degrees is the comfortable range; past 42 degrees it walks like a ladder.
Guidance only — always confirm against your local building code before cutting.
Adding a runner to this flight?
These stairs would take roughly 12.0ft / 3.64m of runner including a 10% waste margin. Refine it in the stair runner length calculator.
Quick answer
How do you calculate stair stringers?
Divide the total rise by your target riser height and round to a whole number to get the riser count, then divide the total rise by that count for an equal riser height. Multiply the tread run by the number of treads for total run, and the stringer board length is the diagonal: the square root of rise squared plus run squared, plus about 12in for the top and bottom cuts. Most stairs need one stringer every 16in of width, so a 36in stair takes three.
- Risers = total rise divided by target riser height, rounded to a whole number.
- Riser height = total rise divided by that riser count, so every riser is equal.
- Treads = risers minus one when the top step is flush with the deck or floor.
- Board length = sqrt(rise squared + run squared) + 12in of waste.
- Stringer count = stair width divided by spacing (16in typical) plus one.
- Keep at least 3.5in of throat behind every notch, or move up to a 2x12.
The four numbers behind every stringer
Every cut stringer comes down to four figures: total rise, riser count, tread run and board diagonal. Get those right and the rest of the layout is mechanical. Measure the total rise from finished floor to finished floor, not from subfloor, because a 3/4in flooring build-up at the top turns an equal flight into an uneven one.
- Measure total rise from finished surface to finished surface.
- Divide by your target riser (7 to 7.5in is comfortable) and round to a whole number of risers.
- Divide the total rise by that riser count so every riser is exactly equal.
- Multiply the tread run by the tread count for total run, then take the diagonal for board length.
| Total rise | Risers | Riser height | Treads (flush top) | Total run at 10.5in |
|---|---|---|---|---|
| 36in / 914mm | 5 | 7.20in | 4 | 42in |
| 45in / 1143mm | 6 | 7.50in | 5 | 52.5in |
| 57in / 1448mm | 8 | 7.13in | 7 | 73.5in |
| 72in / 1829mm | 10 | 7.20in | 9 | 94.5in |
| 104in / 2642mm | 14 | 7.43in | 13 | 136.5in |
Throat depth: the check most DIY layouts skip
The throat is the narrowest strip of timber left behind a notch, measured square to the back edge of the board. It carries the whole load of the stair. Deep treads eat into it fast: a 2x12 with a 7.25in riser and a 10.5in run leaves about 5.5in of throat, but push the run to 12in and it drops under 5in. Keep 3 1/2in as an absolute floor, and prefer 5in on anything carrying regular traffic.
Know your numbers in 30 seconds
Enter your step count, tread depth and riser height — the stair runner calculator gives you the exact length to order, in metres and feet, with waste included.
Open the stair runner calculatorSpacing, width and tread material
Stringer spacing is set by what the treads are made of, not by the stair width. Solid 2x timber can span 24in, 5/4 decking wants 16in, and most composite decking manufacturers specify 12in on stairs because composite loses stiffness on a diagonal span. Always check the tread maker's stair span table, since a warranty claim will be refused on over-spanned stringers.
| Tread material | Max spacing on centre | 36in wide stair | 48in wide stair |
|---|---|---|---|
| 2x solid timber | 24in | 3 stringers | 3 stringers |
| 5/4 decking | 16in | 3 stringers | 4 stringers |
| Composite decking | 12in | 4 stringers | 5 stringers |
Cutting the stringers cleanly
- Mark every notch with a framing square and stair gauges before you cut anything.
- Cut the first stringer, test-fit it in place, then use it as the template for the rest.
- Stop the circular saw short of each inside corner and finish with a handsaw or jigsaw.
- Drop the bottom of the stringer by one tread thickness so the first riser matches the others.
- Deduct the landing or rim joist thickness from the top cut if the stringer hangs off a deck.
- Seal every cut face on exterior stairs; raw end grain is where deck stringers rot first.
Equal risers are a code requirement, not a preference
The IRC allows no more than 3/8in variation between the tallest and shortest riser in a flight. Rounding the riser count and recalculating the height, rather than forcing a round 7in riser and fudging the last step, is what keeps you inside that tolerance.
From stringers to a finished staircase
Once the carcass is built, the same rise, run and nosing figures drive the soft finish. If the flight is going indoors and taking carpet, feed those numbers straight into the stair runner calculator to get the runner length, or use the stair runner length calculator on the home page for a full flight with landings and winders. The stringer tool shows an approximate runner length underneath the cut list so you can budget both trades at once.
Whatever you decide, get the numbers first: the stair runner calculator doubles as a stair runner length calculator, adding nosing wrap, landings, winders, pattern repeat and waste so you order the right length the first time.
Frequently asked questions
How many stair stringers do I need?
Take the stair width, divide by the maximum spacing your tread material allows, and add one. A 36in stair at 16in on centre needs three stringers; the same stair with composite treads at 12in on centre needs four. Anything wider than 48in should get a stringer every 12in to stop treads flexing underfoot.
What size board do I need for a stair stringer?
A 2x12 (11 1/4in actual) is the standard because it leaves enough timber behind each notch. Check the throat figure the calculator gives you: below 3 1/2in the stringer is structurally weak and you should move to a 2x12 or 2x14, or reduce the tread run.
How do you lay out stair stringers?
Set a framing square to your riser height on the tongue and your tread run on the blade, then step it along the board marking each notch in turn. Cut the notches with a circular saw stopping short of the inside corner, and finish every corner with a handsaw so you do not overcut and weaken the stringer.
How do you cut stair stringers so the bottom step is not too tall?
Drop the bottom of the stringer by the thickness of one tread. That cut is called the dropping of the stringer, and without it the first step ends up one tread-thickness taller than the rest, which fails inspection and is a genuine trip hazard.
What is the maximum riser height for stairs?
The IRC allows a maximum 7 3/4in riser and a minimum 10in tread run for residential stairs. UK Approved Document K allows up to 220mm rise with a 220mm minimum going on private stairs. The calculator flags anything outside those limits.
How long a board do I need for a deck stair stringer?
Take the diagonal length the calculator returns and round up to the next stock length. A 57in rise with a 10.5in run over seven treads gives a 94in diagonal, so a 10ft board covers it with cutting waste; buy 12ft if you want the offcut for a landing framing piece.
Do I need an account to use the stair stringer calculator?
No. The stringer calculator is free and open, the same as the stair runner length calculator on the home page. Creating a free account only adds saving, project attachment and client-ready exports.