Quick answer: To calculate stair rise and run, divide the total height by about 7.5 inches to get the number of steps, then divide the height by that count for each riser; the run (tread depth) is typically 10 to 11 inches per step.
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Calculate exact rise, run, and stringer length. Includes IRC code checks and bottom drop warnings.
A stair stringer is the diagonal board that all your treads and risers attach to, and getting its length right starts with your total rise — the vertical distance from the lower floor to the upper floor. Divide that by a comfortable riser height (code caps it at 7.75") to get your number of steps, then multiply your tread depth by one fewer than that (since the top step is the landing itself) to get your total run. The stringer length is just the hypotenuse of those two numbers. The detail that trips up most first-timers is the "bottom drop": because the bottom riser sits on top of the floor, you have to shave the bottom of the stringer cut down by the thickness of your tread material, or every step ends up taller than the rest.
Indoor stairs are governed by the IRC residential stair code: max 7.75" riser, min 10" tread depth, and a minimum 36" headroom clearance. Exterior deck stairs follow the same riser/tread rules in most jurisdictions, but you'll also need to account for stringer rot protection (use pressure-treated lumber rated for ground contact if stringers touch concrete) and a wider tread depth (11"–12") since outdoor steps are walked more casually and often get wet or icy.
For long outdoor stair runs, most carpenters limit a single flight to 12 risers before requiring a landing — both for code compliance and so the stringers (typically 2x12) don't exceed safe unsupported spans.
For stair widths up to 36", two stringers (one on each side) is standard for indoor stairs with a center support added every 3 feet of run on wider stairs. For 36"–48" wide stairs, use three stringers; over 48" wide, use four. Outdoor deck stairs should always use a minimum of three stringers regardless of width, since they carry more dynamic load and weather exposure than interior stairs.
We keep four separate stair pages because stair questions split four ways. If this is not the one you wanted:
To calculate rise and run, measure the total vertical height, divide it by a comfortable riser height of about 7.5 inches to find the number of steps, then divide the total height by that number for the exact rise. The run is the horizontal tread depth, usually 10 to 11 inches per step.
Divide your total rise in inches by about 7.5 to estimate the number of steps. For example, a 108-inch (9 ft) floor-to-floor height needs about 14 to 15 steps.
The bottom drop is the thickness of your tread material. You must subtract this from the first riser cut on your stringer so that all steps remain perfectly level once the treads are installed.
The International Residential Code (IRC) states that the maximum riser height is 7.75 inches, and the minimum tread depth is 10 inches.
Divide your total rise by your total run and take the arctangent of that number. Most residential stairs land between 30° and 35° — anything steeper than 37° starts to feel more like a ladder than a staircase.
No. Building code requires all risers in a single flight to be within 3/8" of each other. Uneven risers are one of the most common causes of fall accidents on stairs.
Framing stairs, decks, or interior trim? These tools pair well with the stair calculator:
Last reviewed August 2026 · how we source these figures
Assumes a 10 in tread depth and the IRC maximum 7.75 in riser. Total rise is finished floor to finished floor.
| Total rise | Risers | Actual riser | Treads | Total run | Stringer length |
|---|---|---|---|---|---|
| 84 in (7.0 ft) | 11 | 7.64 in | 10 | 100 in (8.3 ft) | 130.6 in |
| 90 in (7.5 ft) | 12 | 7.50 in | 11 | 110 in (9.2 ft) | 142.1 in |
| 96 in (8.0 ft) | 13 | 7.38 in | 12 | 120 in (10.0 ft) | 153.7 in |
| 102 in (8.5 ft) | 14 | 7.29 in | 13 | 130 in (10.8 ft) | 165.2 in |
| 108 in (9.0 ft) | 14 | 7.71 in | 13 | 130 in (10.8 ft) | 169.0 in |
| 114 in (9.5 ft) | 15 | 7.60 in | 14 | 140 in (11.7 ft) | 180.5 in |
| 120 in (10.0 ft) | 16 | 7.50 in | 15 | 150 in (12.5 ft) | 192.1 in |
| 126 in (10.5 ft) | 17 | 7.41 in | 16 | 160 in (13.3 ft) | 203.7 in |
| 132 in (11.0 ft) | 18 | 7.33 in | 17 | 170 in (14.2 ft) | 215.2 in |
A 96 in rise (typical 8 ft ceiling with floor structure) needs 13 risers at 7.38 in. Riser count always rounds up — rounding down pushes the riser past the code maximum.
Every constant this calculator uses and where it comes from, so you can check it rather than take our word for it.
Local codes amend the model IRC and adopt different cycles — your building department governs. Manufacturers revise yields; where your product's data sheet disagrees with a default here, use the product's number. See our full methodology.