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Stair Calculator

Calculate rise, run, stringer length and headroom to code from total rise.

Stair opening
in

Finished floor to finished floor, not subfloor to subfloor.

in

Nosing to nosing. The IRC minimum is 10 in for a dwelling.

Riser layout

Treads always come out one fewer than risers.

Headroom

Headroom is measured vertically from the tread nosing to the underside of the floor structure above, so it depends on how far back the stairwell is framed.

in

Joist depth plus subfloor and finish floor at the upper level.

in

Rough opening, measured back from the upper floor edge.

Riser height

7-1/4 in× 15 risers

14 treads at 10-1/2 in over a total run of 12 ft 3 in.
TreadsRisers minus one
14
Total run
12 ft 3 in
Stringer lengthDiagonal on the nosing line; add for the top and bottom cuts
15 ft 3 in
Stair angleComfortable stairs land between 30° and 37°
36.6°
HeadroomMinimum 80 in required
87-11/16 in

Riser height 7-1/4 in — within limit

IRC R311.7.5.1 caps the riser at 7-3/4 in. Adding a riser would shorten the step further if you want a gentler climb.

Tread depth 10-1/2 in — within limit

IRC R311.7.5.2 requires at least 10 in of tread depth measured nosing to nosing. Nosing projection does not count toward it.

Headroom 87-11/16 in — clears

A 144 in opening clears the 80 in minimum with 7-11/16 in to spare.

Riser variation 1/4 in — within limit

Laid out at 7-1/4 in per riser, the closing riser works out to 7-1/2 in. The IRC allows no more than 3/8 in between the tallest and shortest riser in a flight, so split the leftover across the flight rather than dumping it on one step.
Riser count options for this total rise
RisersRiser heightTreadsTotal runAngleStatus
147-13/16 in1311 ft 4-1/2 in38.6°Riser too tall
157-1/4 in1412 ft 3 in36.6°Selected
166-13/16 in1513 ft 1-1/2 in34.7°Compliant

Measure the finished floor, then measure it again

Every number here follows from the total rise. If the upper level gets tile and the lower level gets carpet after the stringers are cut, the first and last risers change and the flight can fail the 3/8 in variation rule on its own. Commercial work follows the IBC, which uses a 7 in riser and an 11 in tread — do not carry these dwelling limits onto a commercial job.

What this assumes

  • Checks against IRC R311.7: maximum 7¾ inch riser, minimum 10 inch tread depth for dwellings.
  • Riser heights within a flight must not vary by more than 3/8 inch.
  • Minimum headroom of 6 feet 8 inches is measured vertically from the tread nosing.

Important

Stair geometry is among the most commonly failed residential inspection items and among the most common causes of injury. Commercial stairs follow the IBC, which uses different limits than the IRC values applied here.

Understanding the result

Total rise governs everything

Start from finished floor to finished floor, not subfloor to subfloor. Flooring thickness that differs between levels changes the first or last riser, and an inconsistent riser at the top or bottom of a flight is both a code violation and the single most common trip hazard.

The 3/8 inch rule

The IRC allows no more than 3/8 inch variation between the largest and smallest riser in a flight. People climb stairs from muscle memory after the first two steps, so a single odd riser is what catches a foot.

Headroom is measured at the nosing

Minimum headroom of 6 feet 8 inches is measured vertically from the tread nosing, not from the tread surface. A sloped ceiling or duct run above the stair must clear this line at every step. Basement stairs under floor joists are the most common headroom failure.

The rise-plus-run comfort formula

A comfortable stair has 2R + T between 24 and 25 inches, where R is riser height and T is tread depth. A 7-inch riser with a 10-inch tread gives 24 inches — within the comfort range. A 7-3/4 inch riser needs at least a 10-inch tread to stay comfortable.

Common questions

What is the standard stair rise and run?
Residential stairs typically use a 7-inch riser and 10-inch tread depth. The IRC allows a maximum 7-3/4 inch riser and minimum 10-inch tread. A 9-foot floor-to-floor height with 7-inch risers needs 15 steps (9 × 12 ÷ 7 = 15.4, round to 15) with an actual riser of 7.2 inches.
How many steps for 8 foot ceiling?
An 8-foot floor-to-floor height (96 inches) divided by a 7-inch target riser gives 13.7 steps — use 14 steps with a riser height of 6.86 inches. With a 7-3/4 inch maximum riser, you need at least 13 steps. Always divide total rise by the number of steps to get the actual riser height.
What is the maximum stair rise?
The IRC maximum riser height for residential stairs is 7-3/4 inches. The IBC allows 7 inches for commercial stairs. Risers must be uniform within 3/8 inch across the flight. Open risers are allowed only if the opening is less than 4 inches.
How do you calculate stair stringer length?
Stringer length equals the square root of (total rise squared plus total run squared). A stair with 96 inches of rise and 140 inches of run (14 treads × 10 inches): √(96² + 140²) = √(9,216 + 19,600) = √28,816 = 170 inches, or 14.2 feet per stringer.
What is the minimum stair width?
The IRC requires a minimum 36-inch clear width between handrails for residential stairs. Three feet is the practical minimum for moving furniture. Main stairs are often 42 to 48 inches wide. Basement and utility stairs may be 36 inches.
How much headroom do stairs need?
Minimum headroom is 6 feet 8 inches (80 inches) measured vertically from the tread nosing. At the top landing, measure from the nosing of the top step. Sloped ceilings must clear this dimension at every point along the stair run.
How many stairs for 10 foot ceiling?
A 10-foot floor-to-floor height (120 inches) with 7-inch risers needs 18 steps (120 ÷ 7 = 17.1, round up to 18) with an actual riser of 6.67 inches. With the maximum 7-3/4 inch riser, you need 16 steps at 7.5 inches each. More steps means a longer horizontal run.
What is the formula for stair layout?
Divide total rise by desired riser height to get the number of steps. Round to a whole number and divide total rise by that count to get the actual riser. Multiply the number of treads (steps minus one) by tread depth for total run. Check 2R + T = 24 to 25 inches for comfort.
Do outdoor deck stairs have different code?
Deck stairs follow the same IRC riser and tread limits as interior stairs: maximum 7-3/4 inch riser, minimum 10-inch tread, minimum 36-inch width. Outdoor stairs also need a graspable handrail on at least one side if there are four or more risers.
How wide should stair treads be?
Minimum tread depth is 10 inches measured nose-to-nose. Standard residential treads are 10 to 11 inches. Wider treads (12 inches or more) feel more comfortable and are common on main stairs. The nosing projection (up to 1-1/4 inches) is included in the tread depth measurement.

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