Rise, run, riser height, cut angle and notch marks — layout with just a tape measure.
Hook your tape at the bottom of the stringer and mark each step at these cumulative distances — no protractor needed. Rise is measured up the back edge, run along the bottom, and along board follows the sloped edge.
| Step | Cumulative rise | Cumulative run | Along board |
|---|
This stair stringer calculator turns a total rise and available run into a complete cutting plan: the number of steps, riser height, going, pitch angle, board length, and the notch marks for laying out a sawn stringer with a tape measure. It checks the layout against Blondel's rule and carpentry limits, and warns when a staircase would be too steep or a stringer too weak. Everything runs in your browser.
Steps are sized to Blondel's rule and checked for ergonomics:
Risers = round(Rise ÷ 175) Riser = Rise ÷ Risers 2·Riser + Going ≈ 630
It warns against extremely steep or shallow stairs.
The sloped stringer edge is a right-triangle hypotenuse:
Run = Going × Treads Board = √(Rise² + Run²) Angle = atan(Rise ÷ Run)
Connect the front and back marks to get the exact cut line.
Two cuts every carpenter forgets:
Drop = Tread thickness Throat = Board − (Riser·Going ÷ Step)
Dropping the base keeps steps equal; the throat check keeps the stringer strong.
Building stairs follows strict rules based on years of carpentry experience and human ergonomics. Here is a quick overview of the terms worth knowing before you cut.
The basic rule for comfortable stairs is 2× riser + 1× tread = 630 mm (25"). The calculator uses this formula to judge the layout.
For comfortable interior stairs, 170–180 mm (about 7") is recommended. Above 200 mm the stairs become steep and less safe.
The net horizontal area for the foot. Aim for 270–290 mm (about 11") to safely hold a whole foot.
The narrowest strip of wood left after cutting notches. Keep at least 115–120 mm (4.5") of solid wood for standard loads.
The bottom cut is shortened by exactly the tread thickness, or the first step would end up higher than the rest.
Overhang that increases the stepping area, usually 20–30 mm. It gives extra heel space when walking down.
These dimensions are universal. Use them for cutting a sawn (open) stringer, or as router-template marks for a housed one.
Don't forget your head. Vertical clearance above the stairs should be at least 2000 mm along the whole flight.
A stringer can end "dropped" (the floor above acts as the top step) or "flush" (the last tread sits level with the finished floor).
Divide the total rise by an ideal riser height (about 175 mm) and round to whole steps for the number of risers. Riser height is the rise divided by risers, and the number of treads is risers minus one. Divide the available run by the treads to get the going, then the stringer board length is the diagonal, √(rise² + run²). The calculator does all of this live as you type.
Blondel's rule says two risers plus one going should total about 630 mm (25"): 2R + G ≈ 630. The status pill turns green when the result sits between roughly 600 and 660 mm, amber when it drifts outside that comfortable band, and red when the stairs would be genuinely steep or awkward.
The bottom cut is dropped by the tread thickness. The top of the first tread adds that thickness back, so dropping the stringer keeps every finished step the same height. Skip it and the first step ends up one tread thickness taller than the others — the classic beginner mistake. The calculator shows the exact drop in the results.
The throat is the narrowest solid strip left behind the notches. Keep at least 115–120 mm (about 4.5") for standard residential loads. The calculator works the throat out from your board width and the notch geometry, and warns you to use a wider board if it falls short.
The inputs are in millimetres, but every rule scales: 175 mm ≈ 7" risers, 280 mm ≈ 11" treads, 630 mm ≈ 25" for Blondel's stride, and a 115 mm throat ≈ 4.5". Enter your imperial figures converted to mm and the layout marks come straight off the tape.
“Cut a full flight of 13 for a loft conversion straight off the running dimensions — didn't touch a protractor once. The first-step drop reminder saved me a re-cut.”
“The throat warning flagged my board as too narrow before I'd cut anything. Bumped up to a wider stringer and the stairs feel rock solid.”
“Great for a quick sanity check on site. I'd love an imperial toggle, but converting to mm takes a second and the Blondel check is spot on.”
“Built my first set of garden steps with this and they came out perfect. The side diagram made it click for me as a total beginner.”
Results are estimates for general guidance only and are not professional advice. Please double-check anything that affects pay, safety, or spending — see our full disclaimer.