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Stair calculator: risers, treads, stringer and skirt

Risers, treads, stringer and skirt cuts.

Open the Stairs tool

Free. Runs in your browser, no account, and keeps working offline.

What it does

Measure the total rise from finished floor to finished floor and enter it with the tread run you want. The calculator suggests the fewest risers that keep each one at or under the residential limit, then works out the riser height, total run, stair angle and the stringer length along its cut line.

It gives the framing square settings (riser on the tongue, run on the blade), a mark for each step along the stringer's edge, how much to cut off the first riser for the tread thickness, and the shortest common 2×12 that covers the stringer with room for the end cuts.

For finish work it gives the skirt board's plumb and level cuts as miter saw settings. Each stair is checked against IRC 2021 R311.7 numbers: riser no taller than 7 3/4″, tread at least 10″, headroom, and nosing. Check your local code — it may be amended.

Worked example

105″ total rise, floor to floor, with 10″ treads (run), 1″ thick treads and a 1″ nosing. The fewest risers that keep each under 7 3/4″ is 14.

Risers / treads
14 / 13
Riser height
7 1/2″
Total run
10′ 10″
Stair angle
36.9°
Stringer cut-line length
13′ 11 1/8″
First riser on the stringer (less tread thickness)
6 1/2″
2×12 to buy
16′

Set the framing square to the riser on the tongue and the run on the blade, then step it off.

Try it with your numbers

Questions

How do I calculate the number of stair risers?

Divide the total rise by the tallest riser you will allow and round up. With the IRC 2021 limit of 7 3/4″, a 105″ rise needs 14 risers of 7 1/2″. There is one fewer tread than risers, because the top riser lands on the floor above.

How long does my stair stringer need to be?

The cut line runs the hypotenuse of the total rise and total run. The calculator gives that length and the shortest common 2×12 that covers it plus a foot for the top and bottom cuts.

What do I set the framing square to?

The riser height on the tongue (the short leg) and the tread run on the blade (the long leg). Stair gauges clamped at those two marks keep every step the same.

Why is the first riser cut shorter?

The treads add thickness on top of every step, but the floor under the first step gets none, so the bottom of the stringer is cut down by one tread thickness to keep every finished riser equal.

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