Stair rise and run calculator

Unit rise is the height of one step. Unit run is the depth of one tread. Total rise is the full vertical distance from finished floor to finished floor, and total run is the full horizontal length the stair eats up on the floor plan.

Almost every rise-and-run mistake comes from someone using a unit number where a total number belongs, or vice versa. When a plan says "rise 7¼, run 10½" it means unit numbers. When it says "total rise 105 in" it means the whole flight.

You never get to pick a riser height freely and expect it to land on a whole number of steps. Instead you divide your total rise by a target riser height, round to the nearest whole number of risers, then divide the total rise by that whole number to get the real, even riser height every step will use.

Example: total rise 108 in, target riser 7.25 in. 108 ÷ 7.25 = 14.9, which rounds to 15 risers. 108 ÷ 15 = 7.2 in per riser. That 7.2 in, not the 7.25 in you started with, is the number you cut every riser to.

1. Measure total rise, finished floor to finished floor.
2. Pick a target riser height between about 7 and 7¾ in.
3. Divide total rise by the target and round to the nearest whole number of risers.
4. Divide total rise by that whole number to get the actual, even riser height.
5. Pick a tread depth using the ratio guidance below, then multiply treads (risers minus one) by that depth for total run.

You do not have to invent a pair from scratch every time. Three combinations show up again and again in framing books because they sit inside commonly cited residential limits and feel comfortable underfoot: 7 in riser with an 11 in tread, 7¼ in riser with a 10½ in tread, and 7½ in riser with a 10 in tread.

All three land close to the old rule of thumb that riser plus tread should equal about 17 to 18 in, and that riser times tread should land somewhere around 70 to 75 sq in. Neither rule is code, both are just carpenter shorthand for "this won't feel awkward."

The rise:run ratio is just the riser height divided by the tread depth, and it is the same number you'd use to find the stair's angle: angle = arctan(riser ÷ tread). A 7.2 in riser over a 10.5 in tread gives 7.2 ÷ 10.5 = 0.686, or about 34.4 degrees.

A higher ratio (tall riser, shallow tread) makes a steeper, more compact stair. A lower ratio (short riser, deep tread) makes a gentler, longer stair. Outdoor and accessible stairs usually aim lower on that ratio than tight interior stairs.

Total run is treads times tread depth, and the tread count is always one less than the riser count because the top riser lands you on the upper floor rather than on another tread. Fifteen risers means fourteen treads.

14 treads at 10.5 in each gives 147 in of total run, just over 12 feet of floor space. If your stairwell opening is shorter than that, you need a shallower tread, a steeper riser, or a different stair shape entirely.

• Rounding the target riser height instead of recalculating it from the rounded riser count.
• Forgetting the top tread is the finished floor itself, not an extra board.
• Measuring total rise to the subfloor instead of the finished floor when flooring hasn't gone down yet.
• Mixing a unit run number into a total run field, which throws off every board length that follows.

Commonly cited IRC reference values for residential stairs put the maximum riser at 7¾ in, the minimum tread at 10 in, and no more than ⅜ in variation between the tallest and shortest riser in a flight. IBC reference values for most commercial stairs are tighter: a 7 in maximum riser and an 11 in minimum tread.

Codes vary by jurisdiction, and your local building department has final say. This calculator gives you the math; it does not check your stair against any specific code.

If you only need one number, the riser height, the stair riser calculator strips this down further. If you're solving for tread board width including nosing overhang, use the stair tread calculator. And once your rise and run are set, the stair stringer calculator turns them into stringer layout cuts.