Stair Calculator
Risers, treads, stringer length & angle
A floor-to-floor rise of 109 inches with 10 inch treads needs 15 risers of 7.27 inches and 14 treads, a total run of 140 inches and a stringer about 177 inches long. Enter your own total rise and tread depth for the steps, exact riser height, run, stringer length and stair angle.
Rise, run and why risers come first
Used as a stair rise and run calculator or a number of stairs calculator, everything below follows from one measurement.
Total rise is the vertical distance from one finished floor to the next — finished, because a floor covering added later changes the top and bottom steps and nothing else. Measure it accurately; every other number follows from it.
Divide the total rise by a comfortable riser height, around 7 to 7.5 inches, and round to a whole number of risers. Then divide the total rise by that whole number to get the exact height of each step. You cannot choose the riser height directly, because the risers must be identical and they must add up exactly to the floor-to-floor distance.
Treads are always one fewer than risers. The last step up is the upper floor itself, so a 15-riser stair has 14 treads.
| Total rise | Risers | Riser height | Treads | Total run | Stringer | Angle |
|---|---|---|---|---|---|---|
| 36 in (deck) | 5 | 7.20 in | 4 | 40 in | 53.8 in | 42.0° |
| 48 in | 6 | 8.00 in | 5 | 50 in | 69.3 in | 43.8° |
| 60 in | 8 | 7.50 in | 7 | 70 in | 92.2 in | 40.6° |
| 96 in | 13 | 7.38 in | 12 | 120 in | 153.7 in | 38.7° |
| 109 in (typical storey) | 15 | 7.27 in | 14 | 140 in | 177.4 in | 37.9° |
| 120 in | 16 | 7.50 in | 15 | 150 in | 192.1 in | 38.7° |
Look at the 48 inch row: six risers gives exactly 8.00 inches, which is over the usual code maximum. The calculator flags this, and the fix is to add a step — seven risers at 6.86 inches. That is the most common design problem on a short deck stair, and it is why you check the riser height rather than assuming the round number works.
Stair code requirements worth knowing before you cut
| Dimension | Limit |
|---|---|
| Maximum riser height | 7¾ in |
| Minimum tread depth | 10 in |
| Variation between risers | 3⁄8 in maximum |
| Variation between treads | 3⁄8 in maximum |
| Minimum headroom | 6 ft 8 in |
| Minimum stair width | 36 in |
| Handrail height | 34 to 38 in above the nosing |
| Maximum guard opening | 4 in sphere must not pass |
The 3⁄8 inch uniformity rule is the one that fails inspections. People trip on stairs because their foot expects the next step to be where the last one was, and a single odd riser at the top or bottom is a genuine hazard rather than a technicality. It is also the classic mistake of forgetting the finished floor thickness at one end.
Codes vary by jurisdiction and these are typical figures, not a substitute for your local code. Anything structural should be checked with your building department before you buy lumber.
The comfort rules
Three old carpentry formulas describe what feels right underfoot. They agree with each other more often than not, and a stair that satisfies all three will feel comfortable:
| Rule | Target | This stair | Verdict |
|---|---|---|---|
| 2 × riser + tread | 24 to 25 | 24.54 | Good |
| riser + tread | 17 to 18 | 17.27 | Good |
| riser × tread | 71 to 75 | 72.7 | Good |
The general principle behind all three: as risers get taller, treads should get shallower, and vice versa. A stair with both tall risers and deep treads feels like climbing a ladder; one with short risers and shallow treads makes you shuffle.
Total run, and whether it fits
Total run is treads × tread depth, and it is usually the constraint that decides the design. A typical storey stair needs about 11 to 12 feet of floor space, which is more than people expect when planning a renovation.
| Tread depth | Total run | In feet | Angle |
|---|---|---|---|
| 9 in (below code) | 126 in | 10.5 ft | 40.9° |
| 10 in (minimum) | 140 in | 11.7 ft | 37.9° |
| 11 in (comfortable) | 154 in | 12.8 ft | 35.3° |
| 12 in (generous) | 168 in | 14.0 ft | 33.0° |
Two inches of extra tread depth costs you more than two feet of floor. If the run does not fit, the answer is a landing and a turn, not shallower treads — the tread minimum exists because a foot needs somewhere to land.
Stair stringer calculator: length and layout
The stringer is the diagonal board the steps are cut into, and its length is the hypotenuse of the total rise and total run. Our 109 inch example needs a stringer of 177.4 inches — nearly 15 feet — so you are buying 16 ft stock. Buy long: you cannot add to a stringer you cut short.
- Three stringers minimum for a 36 in wide stair, and four if the treads are thin or the stair is wider.
- Lay out with a framing square and stair gauges clamped at your rise and run figures, then step the square along the board.
- Drop the bottom stringer by the thickness of one tread, or the first step ends up taller than the rest. This is the classic stringer error.
- Account for finished floor at both top and bottom. Tile at the bottom and carpet at the top will change two risers if you ignore them.
- Cut one stringer, then test it in place before using it as a template for the others.
Building an exterior stair? The deck board calculator covers the treads and the concrete calculator handles the footing at the bottom.
What the stair layout assumes
The number of risers is total rise ÷ target riser height, rounded to the nearest whole number, then the actual riser height is total rise ÷ that count so every step is identical. Treads are risers minus one, total run is treads × tread depth, and the stringer is √(rise² + run²).
- Rounding to the nearest whole riser can push the height above code. The tool flags any riser over 7¾ in so you know to add a step: a 48 in rise gives exactly 8.00 in over six risers, and needs seven.
- Total rise means finished floor to finished floor. Flooring added afterwards at either end changes the top or bottom riser and breaks the uniformity rule.
- Code figures (7¾ in max riser, 10 in min tread, 3⁄8 in uniformity, 6 ft 8 in headroom, 36 in width) are typical IRC-based US residential values. Local codes vary and take precedence.
- Straight-flight geometry only. Landings, winders and turns are not modelled; calculate each straight flight separately.
- Not included: stringer count, tread and riser material, nosing overhang, handrails and guards.
Reviewed September 2026.
Both unit systems. Metric mode works entirely in millimetres, and checks the same code limit: the IRC's 7¾ in maximum riser is 197 mm.
Sources, checked
- International Residential Code 2021, Section R311.7 Stairways, International Code Council. Riser, tread, headroom and width limits.
Figures reviewed . Every worked example on this page is checked against the calculator above.
Stair Calculator: frequently asked questions
What is a comfortable stair riser height?
Around 7 to 7.5 inches, with most residential codes capping it at 7¾. You cannot pick the height directly: divide the total rise by a whole number of risers, since every riser must be identical and they must add up exactly to the floor-to-floor distance.
How many steps for an 8-foot floor height?
A floor-to-floor rise of about 109 inches needs 15 risers at 7.27 inches each, giving 14 treads. At 10 inch treads that is 140 inches of total run, or 11.7 feet of floor space.
How long should a stair stringer be?
It is the hypotenuse of total rise and total run. A 109 inch rise with a 140 inch run needs a 177.4 inch stringer, nearly 15 feet, so you would buy 16 ft stock. Always buy long.
Why are there one fewer treads than risers?
Because the last step up is the upper floor itself. A 15-riser stair has 14 treads, and forgetting this is a common way to end up one tread short of the material you need.
What is the maximum riser height by code?
Typically 7¾ inches in US residential code, with a minimum 10 inch tread depth. Risers must also be uniform within 3/8 inch of each other, which is the rule that most often fails inspection.
Why must stair risers be uniform?
Because people climb stairs by feel, not by looking. Your foot expects the next step where the last one was, so a single odd riser is a real trip hazard. It is usually caused by forgetting the finished floor thickness at one end.
How much floor space does a staircase need?
About 11 to 12 feet for a typical storey, and more with deeper treads. Two extra inches of tread depth adds over two feet of run. If it does not fit, add a landing and a turn rather than shortening the treads.
What is the 2R plus T rule for stairs?
Twice the riser height plus the tread depth should fall between 24 and 25 inches. A 7.27 inch riser with a 10 inch tread gives 24.54, which is comfortable. The principle is that taller risers want shallower treads.