What are the IRC stair rules for risers, treads and headroom?

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In the IRC (2021 R311.7, 2024 R318.7), house stair risers are 7 3/4 in (196 mm) at most and treads 10 in (254 mm) at least, each within 3/8 in of the others in a flight. Headroom is 6 ft 8 in, clear width 36 in, and four or more risers need a handrail 34 to 38 in high.

The IRC stair rules for a house

They apply to every stair within or serving a house, porch or deck; stairs to a nonhabitable attic or a crawl space are left out (R311.7 in the 2021 IRC, R318.7 in the 2024 IRC). Sizes are measured without carpet, rugs or runners. The figures are the same in both editions; only the section numbers moved.

IRC stairway figures
RuleFigure2021 IRC2024 IRC
Riser height7 3/4 in (196 mm) at most, measured between the leading edges of neighbouring treadsR311.7.5.1R318.7.5.1
Tread depth10 in (254 mm) at least, measured nosing to nosingR311.7.5.2R318.7.5.2
Even stepsTallest and shortest riser in a flight within 3/8 in (9.5 mm); the same for treadsR311.7.5.1, R311.7.5.2R318.7.5.1, R318.7.5.2
Nosing3/4 to 1 1/4 in (19 to 32 mm) projection, not needed where treads are 11 in (279 mm) or deeperR311.7.5.3R318.7.5.3
Open risersNo 4 in (102 mm) sphere through, where the stair is more than 30 in (762 mm) above the floor or groundR311.7.5.1R318.7.5.1
Width36 in (914 mm) clear above the handrail; at and below it, 31 1/2 in (800 mm) with one handrail, 27 in (686 mm) with twoR311.7.1R318.7.1
Headroom6 ft 8 in (2032 mm), measured straight up from the sloped line of the nosingsR311.7.2R318.7.2
Rise of one flight12 ft 7 in (3835 mm) at most between floors or landingsR311.7.3R318.7.3
LandingsAt top and bottom, as wide as the stair, 36 in (914 mm) deep on a straight runR311.7.6R318.7.6
HandrailOn one side or more of a flight with four or more risers, 34 to 38 in (864 to 965 mm) above the nosingsR311.7.8, R311.7.8.1R318.7.8, R320.2

The steepest stair these limits allow, 7 3/4 in risers on 10 in treads, has a pitch of 37.8°.

Worked example: a 105 in floor-to-floor stair

A stair from a basement to the main floor: 105 in (8 ft 9 in, 2667 mm) from finished floor to finished floor, with 10 in treads.

  1. Fewest risers: 105 ÷ 7.75 = 13.55, rounded up to 14. Rounding down to 13 would give risers of 8.08 in, over the limit.
  2. Riser height: 105 ÷ 14 = 7.5 in (190.5 mm), under 7 3/4 in. ✓
  3. Treads: one fewer than the risers, 14 − 1 = 13, because the main floor is the top step.
  4. Total run: 13 × 10 = 130 in (3302 mm), or 10 ft 10 in, nosing to nosing.
  5. Pitch: 7.5 ÷ 10 gives 36.9°.
  6. Tread boards: a 10 in tread needs a 3/4 to 1 1/4 in nosing, so the board is 10 3/4 to 11 1/4 in deep. Two 2x6 boards, 5 1/2 in each, make 11 in: a 10 in tread with a 1 in nosing.
  7. Headroom: the opening in the main floor must keep 6 ft 8 in (80 in) above the nosing line. With a floor 10 in deep (a 9 1/4 in joist and 3/4 in subfloor), it runs (80 + 10) × 10 ÷ 7.5 = 120 in (3048 mm), 10 ft back from the top nosing.

For an easier climb, 15 risers of 105 ÷ 15 = 7 in (178 mm) and 14 treads take 14 × 10 = 140 in (11 ft 8 in) of run, 10 in more floor. Both pass. The IRC sets no comfort formula, only the limits in the table.

Why the 3/8 in rule fails stairs

Measure the total rise between finished floors. Say the stair above is built to the subfloor and 3/4 in of hardwood goes down upstairs later: the top riser becomes 7 1/2 + 3/4 = 8 1/4 in, over 7 3/4 in and 3/4 in taller than the rest. Tile laid at the bottom does the same in reverse. Some places relax the 3/8 in rule for the end risers: Oklahoma's 2024 code lets the top and bottom riser of a flight vary by 3/4 in (19 mm), though the 7 3/4 in maximum still holds. Check yours before you rely on it.

Handrails

  • Where: on at least one side of each flight with four or more risers (R311.7.8; R318.7.8).
  • Height: 34 to 38 in (864 to 965 mm), measured straight up from the sloped plane of the nosings. A volute, turnout or starting easing over the bottom tread is allowed, and fittings at a turn or a guard may go higher (R311.7.8.1; R320.2).
  • Space: it may project 4 1/2 in (114 mm) into the stair at most and needs 1 1/2 in (38 mm) clear of a wall (R311.7.8.2, R311.7.8.3; R320.3, R320.4).
  • Grip: a round rail 1 1/4 to 2 in (32 to 51 mm) across, or another shape with a perimeter of 4 to 6 1/4 in (102 to 160 mm) and no more than 2 1/4 in (57 mm) across. Bigger profiles need a finger recess on both sides (R311.7.8.5; R320.6).
  • Continuous: the whole flight, from above the top riser to above the lowest riser in 2021, and from above the nosing of the top landing to above the lowest nosing in 2024, which also limits the gap at a return to 1/4 in (6.4 mm). A newel post may break it at a landing, at winders or over the bottom tread (R311.7.8.4; R320.5).

Where the top of a guard on the open side doubles as the handrail it must be 34 to 38 in high; the guard rules are on the guards page.

Landings, winders and spiral stairs

Landings must be no steeper than 1 in 48 (2 percent) (R311.7.7; R318.7.7). The exceptions to the top and bottom landing differ by edition:

  • 2021: no landing is needed at the top of an interior flight, garage stairs included, if no door swings over the stairs.
  • 2024: the top landing of an interior stair may be on the far side of a door that doesn't swing over the stairs; in an enclosed garage it may sit up to 7 3/4 in below the threshold; an exterior stair of two risers or fewer needs no top landing if the door doesn't swing over it; and an exterior stair to grade of three risers or fewer from a deck, porch or patio may end on a bottom landing just 36 in wide, unless it is the required route from the egress door.
  • Winders: at least 10 in (254 mm) deep at the walkline, 12 in (305 mm) in from the inside of the turn, at least 6 in (152 mm) deep anywhere, and within 3/8 in of each other (R311.7.4, R311.7.5.2.1; R318.7.4, R318.7.5.2.1).
  • Spiral stairs: 26 in (660 mm) clear width, 6 3/4 in (171 mm) treads at the walkline, risers up to 9 1/2 in (241 mm) and 6 ft 6 in (1981 mm) headroom (R311.7.10.1; R318.7.11.1).

Working the stair in MeisterCalc

In the app, open Home ▸ Stairs, choose Rise Only and enter Total Rise 8' 9" and Going 10". Under Advanced, type 7 3/4" into Desired Riser Height and tick Riser Limited, which rounds the riser count up so no riser is taller than that. It shows Number of Risers 14, Actual Riser Height 7 1/2" and Number of Treads 13; with Floor / Joist Thickness 10" and Headroom Height 6' 8" it adds Stairwell Opening 10' 0". Its Total Run reads 11' 8", because Rise Only multiplies risers by the going: one tread more than the 130 in the treads cover. Without Riser Limited it aims for a comfortable riser instead and gives the gentler 15 risers of 7". The app's amber warnings use Australian limits, not the IRC, so read the riser and tread against the table yourself. Stairs is free: see Stairs in MeisterCalc. It doesn't do landings or winders.

Free stair calculator: choose Feet and inches and Rise only, enter 8 ft 9 in and a 10 in going, tick Riser Limited with a desired riser height of 7 3/4, and set Check against to United States (IRC).

Before you build

These are the model code's figures. States and cities adopt the IRC in different editions and amend it: North Carolina's 2018 Residential Code, still in force there, allows risers up to 8 1/4 in. Check with your local building department and build to your permit drawings; neither this page nor the app makes a stair pass inspection. To mark out the stringer, see how to lay out a stair stringer.

Sources

Checked on 2 November 2026. These pages explain the rules; the code itself, your plans and your council, building consent authority, building control body or certifier decide what a job needs.

Do the maths in MeisterCalc

MeisterCalc does the arithmetic behind these rules on an Android phone, with no signal needed: Stairs, Concrete and Check Square are free, and the other calculators come with Pro. It runs on Android 7.0 or later.

GET IT ONGoogle PlayJOIN WAITLISTiOS App

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