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 10, 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.

Your measurements

Stair profile: total rise divided by the riser height gives the number of steps; treads set the total run.

Total rise (floor to floor)inSubfloor to subfloor plus both floor finishes. 3/4 in hardwood upstairs over tile below shifts the top riser, so measure finished floor to finished floor.Target riser heightin7 to 7.5 in is comfortable. IRC maximum is 7.75 in (R311.7.5.1).Tread depth (run per step)inHorizontal depth of each step, nosing excluded. IRC minimum is 10 in, and treads under 11 in need a 3/4 to 1-1/4 in nosing (R311.7.5.3).Stair widthftIRC minimum is 36 in clear above the handrail and 31-1/2 in at handrail height with one rail (R311.7.1).StringersThree for a 36 in stair with 1 in treads, two with 1-1/2 in (2x) treads, one more every 16 in of extra width. Cut from SYP or Douglas fir #1 2x12, or 1-3/4 x 11-7/8 in LSL.

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Risers15 risersAll results

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. Stairs are framed before the flooring goes down, so measure subfloor to subfloor and add the thickness of both finishes. Divide by the riser height you would like, round to a whole number, and divide again to get the exact riser. IRC R311.7.5.1 allows 3/8 in between the tallest and shortest riser in a flight; a 3/4 in surprise at the top fails inspection and trips people.

Multiply the treads (one fewer than risers) by the tread depth for the total run. Then check headroom: 6 ft 8 in measured plumb from the nosing line to the ceiling or floor framing above (R311.7.2). If the run does not fit, a landing at least 36 in in the direction of travel turns the stair (R311.7.6). No flight may rise more than 151 in between landings (R311.7.3).

Cutting and hanging stringers

Lay out the rise and run on a 2x12 with a framing square and stair gauges. Drop the bottom of the stringer by the tread thickness so the first step is not taller than the rest once the treads go on. At least 5 in of solid wood must remain at the throat of each notch. A 7-3/4 in rise with an 11 in run on a 2x12 leaves under 5 in, so switch to 1-3/4 x 11-7/8 in LSL or add a center stringer.

Hang the top on a hanger board or stringer hangers, never on toe-nails, and anchor the bottom to a 2x4 kicker, treated on a slab. Any flight with four or more risers needs a handrail 34 to 38 in above the nosings, continuous, 1-1/2 in off the wall, ends returned (R311.7.8). A graspable rail is 1-1/4 to 2 in round or a Type II profile with a finger recess; a 2x4 does not qualify. Open sides more than 30 in above the floor need a guard, 34 in minimum on the stair and 36 in on landings (R312.1). Guard openings must stop a 4 in sphere.

What this calculator leaves out

Treads and risers (1 in hardwood or 5/4 stock inside, 2x12 for a deck), skirt boards and the landing frame. Also the hanger board and stringer hangers, kicker and anchors, handrail with brackets and wall blocking, guards and balusters, and the permit. It does not check headroom, which depends on where the floor opening ends.

Code limits that apply

IRC R311.7.5.1checked
Riser 7.75 in max; no more than 3/8 in variation between risers in a flight
If you miss it: One tall closer at the top after flooring goes down; the inspector measures every riser
IRC R311.7.5.2checked
Tread depth 10 in min, nosing to nosing; no more than 3/8 in variation in a flight
If you miss it: Short treads land the heel off the step on the way down; the fix is recutting every stringer
IRC R311.7.3checked
No flight rises more than 151 in (12 ft 7 in) between floors or landings
If you miss it: A tall basement run needs a landing framed in after the stringers are cut, or the flight is torn out
IRC R311.7.1checked
36 in min clear width above the handrail; 31-1/2 in at handrail height with one rail
If you miss it: A 36 in rough opening finishes under 36 in once drywall and skirts go on; the wall or the stair moves
IRC R311.7.2
6 ft 8 in headroom measured plumb from the nosing line to the ceiling or framing above
If you miss it: The floor opening above ends too soon and the header clips headroom; the fix is reframing the floor
IRC R311.7.8
Handrail on 4 or more risers, 34 to 38 in above nosings, continuous, graspable, ends returned
If you miss it: A 2x4 on brackets or an open rail end gets tagged; no blocking in the wall means opening finished drywall
IRC R312.1
Guard on open sides over 30 in drop, 34 in min on stairs, 36 in on landings; 4 in sphere rule
If you miss it: Balusters laid out at a full 4 in pass a sphere once the wood shrinks; every one gets pulled and respaced

Provision	Limit	If you miss it
IRC R311.7.5.1checked	Riser 7.75 in max; no more than 3/8 in variation between risers in a flight	One tall closer at the top after flooring goes down; the inspector measures every riser
IRC R311.7.5.2checked	Tread depth 10 in min, nosing to nosing; no more than 3/8 in variation in a flight	Short treads land the heel off the step on the way down; the fix is recutting every stringer
IRC R311.7.3checked	No flight rises more than 151 in (12 ft 7 in) between floors or landings	A tall basement run needs a landing framed in after the stringers are cut, or the flight is torn out
IRC R311.7.1checked	36 in min clear width above the handrail; 31-1/2 in at handrail height with one rail	A 36 in rough opening finishes under 36 in once drywall and skirts go on; the wall or the stair moves
IRC R311.7.2	6 ft 8 in headroom measured plumb from the nosing line to the ceiling or framing above	The floor opening above ends too soon and the header clips headroom; the fix is reframing the floor
IRC R311.7.8	Handrail on 4 or more risers, 34 to 38 in above nosings, continuous, graspable, ends returned	A 2x4 on brackets or an open rail end gets tagged; no blocking in the wall means opening finished drywall
IRC R312.1	Guard on open sides over 30 in drop, 34 in min on stairs, 36 in on landings; 4 in sphere rule	Balusters laid out at a full 4 in pass a sphere once the wood shrinks; every one gets pulled and respaced

Rows marked checked produce a warning in the results when an entry crosses the limit. The others are on the plans or on site and this calculator does not test them. Local amendments can be stricter; the adopted edition and the inspector govern.

Mistakes that cost money

Measuring rise subfloor to subfloor: 3/4 in of flooring on one floor only puts the top riser 3/4 in off, twice the 3/8 in allowed.
Not dropping the stringer by the tread thickness: the bottom step comes out 1 in taller than the rest and fails the 3/8 in rule.
Toe-nailing stringers to the rim joist: hang the top on a hanger board or stringer hangers and anchor the bottom to a kicker.
Under 5 in of throat on a 2x12: a 7-3/4 in rise with an 11 in run gets there; use LSL or add a stringer.
A flat 2x4 or 2x6 handrail fails IRC R311.7.8.5; use a 1-1/4 to 2 in round or a stock graspable profile, returned to the wall.
Stringers sitting on a bare basement slab: the bottoms wick water and rot; set them on a treated 2x4 kicker anchored to the slab.

Key facts

The IRC limits residential risers to 7.75 in maximum and treads to 10 in minimum (R311.7.5), 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 (IRC R311.7.2).

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.

Do I need a handrail on my stairs?

Yes on any flight with four or more risers (IRC R311.7.8). Set it 34 to 38 in above the nosings, run it the full flight, keep 1-1/2 in to the wall and return the ends into the wall or a newel. It has to be graspable: 1-1/4 to 2 in round, or a stock profile with a finger groove. A 2x4 on brackets fails. Any open side more than 30 in above the floor also needs a guard with balusters under 4 in apart (R312.1).

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.