IRC Stair Requirements: Rise, Run, Headroom, and Handrails Explained

Code limits at a glance

These are the stair requirements that drive layout. Handrail and landing rules are included because they affect where the stair can start and stop.

Where a handrail is installed, the clear width at and below handrail height may drop to 31 1/2 in. with a rail on one side or 27 in. with rails on both sides. Commercial and most multifamily common stairs fall under the IBC, which generally sets a 7 in. maximum riser and an 11 in. minimum tread.

How rise and run set the layout

Start with total rise, the vertical distance between finished floor surfaces. Four steps follow.

1. Divide the total rise by 7 3/4 in. and round up. That is the minimum riser count.
2. Choose a riser count at or above the minimum, then divide the total rise by it. The result is the exact riser height for every riser.
3. Subtract one from the riser count to get the tread count. The upper floor is the last tread.
4. Multiply the tread count by the tread depth to get the total run, and check it against the floor space.

Use the exact quotient, not a rounded target. Fifteen risers cut at 7 1/4 in. instead of 7.283 in. leave the flight 1/2 in. short (15 × 0.0333 in.), and that whole error lands in one riser. A 1/2 in. difference already breaks the 3/8 in. variation limit.

The stair calculator rounds total rise divided by a target riser to the nearest whole riser count, then checks the result. A 48 in. rise at a 7.5 in. target rounds to 6 risers of 8.00 in., which fails, and the check tells you to add one. Seven risers give 6.857 in. Rounding to the nearest count can land on either side of the limit, so read the check result before you trust the count.

The angle is the same rise-over-run geometry that sets roof pitch. The roof pitch chart lists the same relationship for roofs.

The 2R + T comfort check

Blondel's rule, named for the 17th-century French architect François Blondel, says twice the riser plus the tread should fall between about 24 and 25 in. The calculator accepts 24 to 25.5 in. The rule describes proportions that feel natural underfoot. It is not in the code, and a stair can pass the code and miss the band, or sit in the band and fail the code.

A stair built right at both code limits (7 3/4 in. riser, 10 in. tread) sums to exactly 25.5 in., the top of the band. A sum above the band comes from tall risers, deep treads, or both, and it does not fail inspection by itself.

Deeper treads trade floor space for a flatter angle: 11 in. treads cut the angle from 38.9° to 36.3° and add 13 in. of run. A 7 in. riser with an 11 in. tread sums to 25 in. and satisfies both the code and the band.

What each dimension is measured from

The inspector measures riser height vertically between the leading edges of adjacent treads, and tread depth horizontally between the foremost projections of adjacent treads. Both are nosing-to-nosing dimensions. The nosing does not add to the 10 in. minimum. A stair cut with a 10 in. run needs boards about 1 in. deeper than the run when the nosing overhangs 1 in.

On solid-riser stairs, a tread under 11 in. deep needs a nosing of 3/4 to 1 1/4 in., and the nosing projection may vary by no more than 3/8 in. A tread of 11 in. or more needs no nosing. Open risers are permitted, but the gap between treads must not pass a 4 in. sphere. The code relaxes that for very low stairs, so check the adopted edition.

The calculator also flags risers under 4 in. The IRC sets no minimum riser. That check matches commercial practice and works as a sanity limit.

Worked example: 109 1/4 in. floor to floor

A measured rise of 109.25 in. (9 ft 1 1/4 in.) runs from the finished lower floor to the finished upper floor, with 10 in. treads.

1. 109.25 ÷ 7.75 = 14.10, so the minimum is 15 risers. Fourteen risers would each be 7.804 in., over the limit.
2. 109.25 ÷ 15 = 7.283 in. per riser, about 7 9/32 in.
3. Treads = 15 − 1 = 14.
4. Total run = 14 × 10 in. = 140 in. (11 ft 8 in.).
5. Stringer length = √(109.25² + 140²) = 177.58 in. (14 ft 9.6 in.), measured along the nosing line before the bottom and top cuts.
6. Angle = atan(109.25 ÷ 140) = 38.0°.
7. 2R + T = 2 × 7.283 + 10 = 24.57 in., inside the band.

With 11 in. treads the run grows to 154 in. (12 ft 10 in.), the angle drops to 35.4°, and 2R + T becomes 25.57 in., just above the 25.5 in. band. Both layouts meet the code. The choice depends on how much floor length the stair can take.

Headroom and the stairwell opening

Headroom is measured vertically from the sloped line that connects the tread nosings to the lowest thing overhead. The limit is 80 in. (6 ft 8 in.). The stairwell opening must be long enough that the nosing line has dropped 80 in. below the underside of the floor framing where the opening ends.

For an opening of length L measured from the top nosing, floor thickness T (finish floor, subfloor, joist, and ceiling finish), tread depth D, and riser height R:

L ≥ (80 + T) × D ÷ R

In the example, T = 9 1/4 in. joist + 3/4 in. subfloor + 3/4 in. finish floor + 1/2 in. drywall = 11.25 in. Then L ≥ 91.25 × 10 ÷ 7.283 = 125.3 in. (10 ft 5.3 in.). The 140 in. run continues past the header, where the nosing line is lower and the clearance is greater than 80 in.

Common layout mistakes

• Counting the top floor as a tread. Laying out 15 treads for 15 risers adds one tread depth to the run (10 in. in the worked example). The stair runs 10 in. longer than the floor plan allows, and the top riser lands in the wrong place.
• Measuring subfloor to subfloor when finishes differ. Effective rise equals the measured rise plus the upper finish minus the lower finish. A 108.5 in. subfloor rise with 3/4 in. hardwood above and 1/4 in. vinyl below is a 109.0 in. rise. The first gives 14 risers at exactly 7.75 in. The second needs 15 risers at 7.267 in. The 1/2 in. difference changes the count.
• Not trimming the bottom of the stringer. The tread sits on top of the first seat, so an untrimmed stringer makes the first riser one tread thickness taller than the rest. With 1 in. treads, that is 1 in. over the others and 5/8 in. past the 3/8 in. limit. Trim the bottom by the tread thickness, less any finish flooring still to be installed at the bottom. For 1 in. treads and 3/4 in. flooring added later, trim 1/4 in.
• Measuring headroom from the tread surface. The 80 in. is measured from the sloped line through the nosings. Between nosings the tread surface sits up to one riser height below that line, so measuring from it overstates headroom by up to one riser height, about 7 1/4 in. in the worked example.
• Forgetting the landing. A flight cannot rise more than 12 ft 7 in. between floors or landings, and a stairway needs a floor or landing 36 in. deep at the top and bottom. Check that the floor plan has room for the landing, not only for the run.

Stringers are commonly cut from 2x12 stock, and defects and mis-cuts cost material. The material waste guide covers allowances for dimensional lumber.

Handrails and guards

A handrail is required on at least one side of a flight with four or more risers. The top of the rail sits 34 to 38 in. above the sloped line through the nosings. The common Type I profile is round, 1 1/4 to 2 in. in diameter, with 1 1/2 in. clear between the rail and the wall.

Guards are required on open sides where the walking surface is more than 30 in. above the floor or grade below. Guards are 36 in. high in general. On the open side of a stair, the minimum is 34 in. measured vertically from the line connecting the tread nosings.

Frequently asked questions