Stair Code Requirements

Stairs are the most heavily regulated part of most residential projects, and the one where a small arithmetic error becomes a permanent trip hazard. This is a reference for the figures the International Residential Code sets, where the commercial code differs, and which rule actually causes the most failed inspections.

Everything here follows the model code. Your jurisdiction can and often does amend it. Verify with your building department before cutting stringers.

The core dimensional limits

Section numbers below are from the 2021 IRC. They have been stable across recent editions, but they do change, so check the edition your jurisdiction has adopted. Many are still on the 2018 or 2015 IRC.

Requirement	IRC (residential)	IBC (commercial)	IRC Section
Maximum riser height	7 3/4 in	7 in	R311.7.5.1
Minimum tread depth	10 in	11 in	R311.7.5.2
Maximum riser variation in a flight	3/8 in	3/8 in	R311.7.5.1
Maximum tread variation in a flight	3/8 in	3/8 in	R311.7.5.2
Minimum headroom	6 ft 8 in	6 ft 8 in	R311.7.2
Minimum stair width	36 in	Varies by occupancy	R311.7.1
Handrail height above nosing	34–38 in	34–38 in	R311.7.8
Handrail required at	4+ risers	4+ risers	R311.7.8
Guard required above	30 in drop	30 in drop	R312.1
Guard height (open sides)	36 in	42 in	R312.1

One detail the “36 inches” row hides: R311.7.1 requires that 36 in of clear width above the handrail height and below the required headroom. At and below handrail height the clear width may reduce to 31½ in with a handrail on one side, or 27 in with handrails on both sides. A 36-inch-wide stairwell with handrails on both sides is legal but leaves a genuinely tight passage.

Riser height is measured vertically from the top of one tread to the top of the next. Tread depth is measured horizontally from the leading edge of one tread to the leading edge of the next. The nosing overhang does not count toward it, which is a common source of a tread that measures 10 inches but does not comply.

The stair calculator checks a proposed layout against the IRC riser and tread limits directly, and the stair stringer calculator handles the cutting dimensions once the layout is settled.

The rule that fails inspections: uniformity

The 3/8 inch limit on variation between the tallest and shortest riser in a flight (R311.7.5.1, and R311.7.5.2 for treads) causes more rejections than the maximum riser height does. Every riser in a flight must be within 3/8 inch of every other riser, not 3/8 inch of the average, but 3/8 inch across the full spread.

The usual cause is not sloppy cutting. It is finished flooring.

Stringers are typically cut to a subfloor-to-subfloor dimension. If 3/4 inch of hardwood then goes down at the top but the bottom stays bare concrete, the top riser becomes 3/4 inch shorter than every other riser in the flight, nearly double the allowable variation, produced entirely after the stairs were built correctly.

The fix is to account for both finished floor levels when calculating total rise, before cutting anything. Two adjustments handle it:

• Bottom riser: subtract the tread material thickness from the bottom stringer cut, since the bottom tread sits on the lower floor but has material on top of it.
• Top riser: account for the finished floor thickness at the landing.

Getting these backwards is common enough that it is worth measuring the finished floor at both ends before the first cut, not after.

Total rise and the arithmetic that follows

Total rise is measured from finished floor to finished floor, not subfloor to subfloor. Everything else follows from it:

1. Number of risers = total rise ÷ target riser height, rounded to a whole number. There is always one more riser than tread in a straight flight.
2. Actual riser height = total rise ÷ number of risers. This must be at or under 7 3/4 inches.
3. Total run = number of treads × tread depth, where treads = risers − 1.

The step people skip is (2). Rounding the riser count changes the riser height, and a layout that was 7.6 inches per riser before rounding can land above 7.75 after. Always recompute the actual height from the rounded count rather than assuming the target held.

The comfort formula

Beyond code compliance, there is a long-standing rule of thumb for how a stair actually feels underfoot:

2 × riser + tread = 24 to 25 inches

A 7 inch riser with an 11 inch tread gives 25, which is comfortable. A 7 3/4 inch riser with a 10 inch tread gives 25.5, which is legal but noticeably steep. The formula reflects the mechanics of an average stride: as risers get taller, treads need to get shorter to keep the walking rhythm natural, but only within a narrow band.

Code sets the outer boundary. The comfort formula tells you where inside that boundary the stair will feel right, which matters on a staircase people use every day.

Headroom

Minimum headroom under R311.7.2 is 6 feet 8 inches, measured vertically from a line connecting the tread nosings up to the ceiling, finished surface to finished surface. It applies along the entire flight and continues to the landing.

Headroom is the constraint that most often forces a stair redesign in a basement or loft conversion, because it interacts with the floor structure above. A stairwell opening that is too short leaves the lowest point of headroom at the top of the flight, where someone descending meets the header. Lengthening the opening is usually the fix, and doing so may require the header to be resized, which is structural work, not a trim adjustment.

Handrails and guards

These are two different things and the codes treat them separately.

Handrails (R311.7.8) are for grasping while using the stair. Required once a flight has four or more risers, mounted 34 to 38 inches above the nosing line, continuous for the full flight, and returned to the wall or terminated in a newel at the ends. “Graspable” is a defined cross-section: a round rail of 1 1/4 to 2 inches diameter, or a non-circular profile within specified perimeter and dimension limits. A wide, flat board at the right height is a common and non-compliant substitute.

Guards (R312.1) prevent falls from open sides. Required where the walking surface is more than 30 inches above the surrounding floor or grade, at 36 inches minimum height in residential and 42 inches in commercial. The IRC also limits openings so that a 4-inch sphere cannot pass through, with a 6-inch allowance on the triangular opening formed by a stair’s tread, riser, and bottom rail.

A stair can need both: a handrail for grasping and a guard for the open side.

Winders and landings

Winder treads, the wedge-shaped treads on a turn, must under R311.7.5.2.1 provide at least 10 inches of depth measured 12 inches in from the narrow end, and at least 6 inches of depth at any point. This effectively rules out a true pie-slice tread narrowing to a point.

Landings must be at least as deep as the stair is wide, with a 36 inch minimum, and a door opening onto a landing must not reduce that dimension by more than half when it swings.

Deck stairs

Exterior stairs follow the same rise, run, uniformity, and handrail rules. Two differences matter in practice:

• The bottom typically lands on a footing or pad rather than a finished floor, and that pad’s final elevation must be settled before the total rise is measured. A pad poured after the stringers are cut is a reliable way to break the uniformity rule.
• Exterior stringers, treads, and fasteners need to be rated for exterior exposure and, in contact with the ground or treated lumber, for corrosion resistance.

The deck calculator handles the framing side; the stair dimensions themselves follow this same table.

Where this stops

These figures are the model code, and jurisdictions amend them. Some adopt an older IRC edition, some tighten the riser maximum, and some add local requirements for guard infill or exterior stair construction. Anything load-bearing is structural work: stringer sizing, header resizing for a stairwell opening, guard post attachment. A stair that meets every dimension in this table can still fail if the stringers are undersized for the span.

Use this to arrive at your building department knowing which questions to ask, and confirm the final numbers with a licensed professional before you build.