Conversions & Core
Stair Calculator
Riser count, equal riser height, tread count, total run and stringer length for a straight stair.
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Adjust the inputs to see your result.
About this calculator
A safe stair has identical risers, and that means rounding the riser count up and dividing the rise evenly. This calculator lays out a straight flight: risers, actual riser height, treads, total run and the stringer diagonal to cut.
What this number means
A stair is two numbers that must agree: the risers that climb the height and the treads that cover the run. The classic mistake is dividing the rise by a comfortable riser height and getting a fraction — a stair with one odd riser is a trip hazard, because feet learn the rhythm of the first steps and expect it to continue.
The fix is to round the riser count up and then divide the rise evenly. Every riser ends up slightly shorter than the maximum, all of them identical, and the stair is both code-friendlier and safer to walk. The tread count is always one less than the riser count, because the upper floor itself is the last step.
The formula
- Risers = ceil(Total rise ÷ maximum riser height)
- Actual riser height = Total rise ÷ Risers (all risers equal)
- Treads = Risers − 1
- Total run = Treads × tread depth
- Stringer length = √(Total rise² + Total run²)
Worked example
An 8 ft (96 in) floor-to-floor rise, 7.5 in maximum riser, 10 in treads.
- Risers: 96 ÷ 7.5 = 12.8 → 13 risers
- Actual riser height: 96 ÷ 13 = 7.38 in on every step
- Treads: 13 − 1 = 12 treads
- Total run: 12 × 10 = 120 in (10 ft)
- Stringer length: √(96² + 120²) = 153.67 in (12.81 ft) — order 14 ft stringer stock
Assumptions and limitations
- One straight flight, no landing and no winder. For an L- or U-shaped stair, split the rise across flights and calculate each flight separately.
- Riser heights must be equal within a flight — codes typically allow no more than 3/8 in variation between the tallest and shortest riser.
- The stringer length is the straight diagonal; the cut stringer board needs extra length for the top and bottom connections, so round up to the next stock length.
- Headroom, handrails and landing dimensions are code items this calculator does not cover — check local code before cutting.
Frequently asked questions
Because the upper floor is the final step. Each riser lifts you to a tread, and the last riser lifts you onto the landing — so a 13-riser stair has 12 treads plus the landing.
Around 7 to 7.5 in for a main stair. Below 6 in feels like a ramp and wastes run; above 8 in feels steep and tiring. Most US codes cap risers at 7.75 in.
From the finished surface of the lower floor to the finished surface of the upper floor — not subfloor to subfloor. Flooring thickness at either end changes the rise, and a quarter inch of error spread across 13 risers is small, but a full inch is not.
Split the total rise into two flights with a landing between them, and run each flight through the calculator separately. Each flight gets its own equal risers, and the landing counts as a tread for neither flight.
At least the diagonal length the calculator gives — 12.81 ft in the example, so 14 ft stock. The diagonal is the cut length; the board also needs material for the top connection and the bottom cut.
Because walking a stair is muscle memory: after two steps your feet expect the same lift on the next. A single riser even half an inch off is the most common cause of stair falls, which is why codes limit variation to 3/8 in.