Stair Calculator
Divide a floor-to-floor height into risers, get the tread depth, total run, stringer length and cut layout — with the building-code limits and the comfort rules checked as you type.
Instant answers
Works offline once loaded
Nothing you type is sent anywhere
Every workable riser count
IRC / IBC limits checked
Stringer cut layout
The opening
Clear width between the handrails or walls.
How far each tread projects past the riser below.
Tread depth is measured nosing to nosing, not the board width.
The usual case: the upper floor is the last tread, so there is one fewer tread than riser.
Each riser
—
Risers × treads—
Tread depth (nosing to nosing)—
Total run — horizontal floor space—
Stair angle—
Stringer length (along the slope)—
Minimum stringer stock depth—
2R + T comfort rule—
Headroom needs an opening of—
Side elevation and stringer
The dashed line is the stringer's cut line. Mark the first triangle on your framing square at —, then step it down the board — every triangle is identical except the bottom riser, which is cut short by the tread thickness.
Choose the number of risers
Every whole-number division of your total rise. Click a row to use it — rows outside the code limits are greyed out.
| Risers | Riser height | Treads | Total run | Angle | 2R+T | Verdict |
|---|
Cut list and materials
Two at the sides plus one every 16 in of width is the usual rule.
Deducted from the bottom riser cut so the first step is not tall.
| Part | Qty | Size | Total length |
|---|
Instructions
How to use this calculator
Step by step
- Measure the total rise from the finished upper floor down to the finished lower floor. If the flooring is not down yet, add its thickness now — a 3/4 in oak floor added later makes the top riser 3/4 in short, which is exactly the kind of thing an inspector looks for.
- Enter the stair width and the nosing overhang. The nosing projects past the riser below and does not count towards the tread depth, which is always measured nosing to nosing.
- Pick the code you are building to. IRC residential allows a 7¾ in riser and needs a 10 in tread; IBC commercial is tighter at 7 in and 11 in.
- Look at the riser-count table. It lists every whole-number division of your rise, with the riser height, angle and comfort figure for each. Click the row you want — the drawing, cut list and checks all follow it.
- Leave the tread depth on Auto to let the 2R + T rule choose, or pin it to a stock tread size if you have already bought the material.
- Read the checks below the numbers. Green means the design meets that rule; red means it does not, and the text says by how much.
- Transfer the riser and tread figures to a framing square to lay out the stringer — the note under the drawing gives you the two numbers to set.
- Cut the bottom of the stringer down by one tread thickness before you install it. Skipping this is the single most common stair mistake: it leaves the first step taller than all the others.
Good to know
- Every riser in a flight must be within 3/8 in of every other. Cut them all from one stringer used as a template rather than measuring each one.
- A riser near 7 in with a tread near 11 in is the most comfortable staircase most people will ever climb. If your rise divides awkwardly, changing the tread depth is cheaper than changing the riser.
- The total run is floor space you have to find. A 9 ft rise at 7 in per riser needs roughly 13 ft of floor — check that before you frame the opening.
- Headroom is measured vertically from the nosing line, not from the tread. It is the constraint that most often forces the opening to be longer than people expect.
- Add a stringer whenever the span between them exceeds about 16 in. Three stringers under a 36 in stair is standard; a 48 in stair wants four.
- Winders and landings reset the count. Calculate each straight flight separately and treat the landing as a tread of its own.
The maths behind it
- Riser height R = total rise ÷ number of risers The number of risers is always a whole number, which is why the riser height is almost never round.
- Number of treads T = risers − 1 When the upper floor serves as the top tread — the usual arrangement.
- Total run Run = treads × tread depth The horizontal floor space the staircase consumes.
- Stair angle θ = arctan(R ÷ tread) Comfortable stairs sit between 30° and 37°.
- Stringer length L = √(run² + rise²) The straight-line length along the slope, before allowing for the cuts at each end.
- 2R + T rule 2 × riser + tread ≈ 25 in A carpenter's rule derived from the length of an average stride. It is not code, but stairs that break it feel wrong to walk.
- Headroom opening opening = (headroom + floor depth) × tread ÷ riser How long the hole in the upper floor has to be for the required clearance.
Why is there one fewer tread than riser?
Because the upper floor itself acts as the last tread. You step up onto it, so it is the top of the final riser but you never build a board for it. If your stair stops at a tread below the floor — a deck stair landing on a platform, say — switch the "Top step lands on" setting and the count changes to match.
What is the maximum riser height I am allowed?
Under the IRC, which most homes in the United States are built to, the maximum riser is 7¾ in and the minimum tread depth is 10 in. Commercial work under the IBC is tighter: 7 in and 11 in. Local amendments do exist, so treat these as the starting point and check with your building department before you cut.
Do I measure the tread depth including the nosing?
No. Tread depth is measured nosing to nosing — from the front edge of one tread to the front edge of the next. The nosing overhang is extra material hanging over the riser below, so a 10 in tread with a 1 in nosing is cut from an 11 in board.
Why do I have to cut the bottom of the stringer shorter?
The stringer notches are laid out to the finished step height. Once you lay a tread on top of each notch, every step gains the tread thickness — except the bottom one, which sits on the floor. Dropping the stringer by exactly one tread thickness puts all the risers back in line. Forget it and your first step is an inch taller than the rest, which is both a trip hazard and a code violation.
How much headroom do I need?
Six feet eight inches, measured vertically from the line joining the tread nosings to the lowest point above — usually the edge of the floor opening. It is the constraint that most often forces the stairwell opening to be longer than the run alone would suggest, which is why this calculator reports the opening length.
Can I use this for deck or outdoor stairs?
Yes. The geometry is identical. The two differences outdoors are that the bottom lands on a pad or footing rather than a finished floor — so measure the rise to the top of that pad — and that you may want a slightly shallower stair, since a 6 to 7 in riser is easier in wet or icy conditions.
What if my rise does not divide nicely?
It rarely does, and it does not need to. The riser height only has to be consistent, not round. Lay the risers out with a story pole or by stepping dividers down the board rather than trying to measure 7 and 3/32 in eleven times.