Stair Calculator

Calculate the number of risers and treads, the exact riser height, total run and stringer length for a staircase from the floor-to-floor rise, with IRC limits flagged.

By CalcBuilt Editorial TeamUpdated September 9, 2026Formulas checked against manufacturer specs, see how we calculate.

Quick answer: To lay out stairs, divide the total rise by a target riser height and round to a whole number of risers, then divide the rise by that number for the exact riser height; treads are one fewer than risers. A 108 in rise with a 7.25 in target gives 15 risers of 7.2 in, 14 treads and a 147 in total run.

Results

Risers: 15 risers; Each 7.2 in, within IRC limits
Treads: 14 treads; 10.5 in deep, 3 ft wide
Total run: 147 in; 12.25 ft of floor space
Stringer length: 15.2 ft; Buy 3 x 18 ft 2x12s to allow for the top and bottom cuts
Stair angle: 36.3 degrees; 30 to 37 degrees is the comfortable range

Stair formula

Risers = Total rise / target riser height, rounded to the nearest whole number
Actual riser height = Total rise / Risers
Treads = Risers - 1;  Total run = Treads x tread depth
Stringer length = sqrt(Total rise² + Total run²)

Worked example

A basement stair with a 114 in floor-to-floor rise, a 7.5 in target riser and 11 in treads: 114 / 7.5 = 15.2, so 15 risers of 7.6 in. That gives 14 treads and a total run of 14 x 11 = 154 in. Stringer length is sqrt(114² + 154²) = 191.6 in, or 16 ft, so buy 18 ft 2x12s to leave room for the top and bottom cuts.

Quick reference

Stair layout by total rise (7.25 in target riser, 10.5 in treads)
Total rise	Risers	Riser height	Treads	Total run
96 in (8 ft)	13	7.38 in	12	126 in
102 in	14	7.29 in	13	136.5 in
108 in (9 ft)	15	7.20 in	14	147 in
114 in	16	7.13 in	15	157.5 in
120 in (10 ft)	17	7.06 in	16	168 in

How to calculate stair rise and run

Start with the total rise, measured from finished floor to finished floor. Divide by the riser height you would like, round to a whole number, and divide again to get the exact riser. Every riser must be the same height: uneven steps are the most common cause of stair falls and a code violation.

Multiply the number of treads (one fewer than risers) by the tread depth to get the total run, and check that the run fits the space with the required headroom above. If it does not, a steeper riser within the code limit, a landing, or a winder may be needed.

Cutting stringers

Lay out the rise and run on a 2x12 with a framing square and stair gauges. The bottom riser is cut shorter by the tread thickness so the first step is not taller than the rest once the treads are installed. After cutting, at least 5 in of solid wood must remain at the throat of each notch; if not, use a deeper stringer or add a center stringer.

This calculator is for planning. A licensed contractor, local code and the inspector govern riser limits, headroom, guard height and stringer support.

Key facts

The IRC limits residential risers to 7.75 in maximum and treads to 10 in minimum, with no more than 3/8 in variation between steps.
A comfortable stair follows the rule that 2 x riser + tread is between 24 and 25 in.
Treads are always one fewer than risers because the top floor acts as the last tread.
Stringers are cut from 2x12s; a 36 in wide stair needs three stringers when treads are 1 in thick.
Minimum headroom over the stair is 6 ft 8 in measured plumb from the tread nosing.

Frequently asked questions

How many steps for a 9 foot ceiling?

With a typical 108 to 110 in total rise you need 15 risers of about 7.2 in and 14 treads. The exact count depends on the floor-to-floor height including joists and flooring.

What is the standard rise and run for stairs?

A 7 to 7.5 in riser with a 10 to 11 in tread. The IRC allows up to 7.75 in risers and requires at least 10 in treads for residential stairs.

How long of a 2x12 do I need for stair stringers?

Compute the diagonal of total rise and total run, then add about a foot for the plumb and level cuts. A 108 in rise with a 147 in run needs a 15.2 ft stringer, so buy 18 ft 2x12s to leave room for the cuts.

How many stringers do stairs need?

Three for a 36 in wide stair with 1 in thick treads, or two if the treads are 1.5 in (2x) lumber. Add a stringer every 16 in for wider stairs.

Sources and references

Section R311.7 Stairways, 2021 IRC (ICC)
Stair Geometry Standards policy (NAHB)
2021 Visual Interpretations of IRC Stair Codes (Stairbuilders and Manufacturers Association)

Next steps for this project

The usual order for a framing, decks & fences project. See all framing, decks & fences calculators.

Results are estimates based on standard formulas and typical product specifications. Confirm quantities with your supplier and local code before ordering.