Carpentry · Stair Stringer Calculator
IRC stair riser and tread code
Under IRC R311.7, residential stair risers may be no taller than 7 3/4 inches and treads no shallower than 10 inches, with at most 3/8 inch variation per flight. Stairs need 36 inches of width, 6 feet 8 inches of headroom, a 3/4 to 1 1/4 inch nosing unless treads reach 11 inches, and handrails at 34 to 38 inches.
Riser height: R311.7.5.1
The riser height shall be not more than 7 3/4 inches (196 mm). It is measured vertically between the leading edges of adjacent treads, which means from nosing to nosing, not from the top of one tread board to the top of the next at the back of the step. Dimensions are taken exclusive of carpets, rugs, or runners.
Uniformity is enforced separately. The greatest riser height within any flight shall not exceed the smallest by more than 3/8 inch (9.5 mm). This is the rule that catches most site built stairs: the bottom riser is often taller or shorter because the stringer was not dropped by the tread thickness, or the slab was not level.
Risers may be vertical or sloped from the underside of the nosing above at up to 30 degrees from vertical. Open risers are allowed, but where the opening is more than 30 inches above the floor or grade below, it may not allow a 4 inch diameter sphere to pass. Spiral stair riser height follows R311.7.10.1 instead.
Tread depth and winders: R311.7.5.2
The tread depth shall be not less than 10 inches (254 mm), measured horizontally between the vertical planes of the foremost projection of adjacent treads and at a right angle to the tread's leading edge. In plain terms, measure nosing to nosing horizontally. Because the nosing projects, a 10 inch code tread on a stringer with a 3/4 inch nosing is cut with a 9 1/4 inch run plus 3/4 inch of overhang.
As with risers, the greatest tread depth in a flight may not exceed the smallest by more than 3/8 inch. That tolerance applies to tread depth, riser height, and nosing projection each on their own, so a stair can fail on any one of the three.
Winder treads, the pie shaped steps that turn a corner, must be at least 10 inches deep at the walkline and at least 6 inches deep at any point within the clear width. Consistently shaped winders may share a flight with rectangular treads and are not required to match the rectangular tread depth within 3/8 inch, but winders must be within 3/8 inch of each other.
Nosing rules: R311.7.5.3
A nosing projection of not less than 3/4 inch (19 mm) and not more than 1 1/4 inches (32 mm) shall be provided on stairways. The nosing's curved edge may have a radius no greater than 9/16 inch (14 mm), or a bevel no greater than 1/2 inch (12.7 mm). The greatest nosing projection may not exceed the smallest by more than 3/8 inch within a stairway.
The exception is the one most deck builders use: a nosing projection is not required where the tread depth is not less than 11 inches (279 mm). Two 5 1/2 inch deck boards side by side give 11 inches and satisfy this exception without any overhang, which is why 11 inch runs are so common on outdoor stringers.
Landings, floors, and the top step of a stairway are subject to the same nosing limits. The nosing is also the reference point for measuring both riser height and tread depth, so a change in nosing projection changes both measured dimensions.
Width, headroom, and vertical rise: R311.7.1 through R311.7.3
Stairways shall be not less than 36 inches in clear width at all points above the permitted handrail height and below the required headroom height. Handrails may project into that width by up to 4 1/2 inches on either side, so the clear width between handrails can be 31 1/2 inches on a stair with rails on both sides, or 27 inches with rails on both walls and a 36 inch rough opening.
Headroom shall be not less than 6 feet 8 inches measured vertically from the sloped line adjoining the tread nosings, or from the floor surface of a landing or platform, to the ceiling or any obstruction above. Check this early: a stair that fits the floor opening can still fail headroom at the header of the opening above.
A flight of stairs shall not have a vertical rise larger than 151 inches (12 feet 7 inches) between floor levels or landings. Taller runs need an intermediate landing, and landings must be at least as wide as the stair they serve.
Handrails: R311.7.8
Handrails are required on at least one side of each flight with four or more risers. Height is measured vertically from the sloped line adjoining the tread nosings, and the top of the handrail must be not less than 34 inches and not more than 38 inches. The rail must be continuous for the full length of the flight from the top riser to the bottom riser, with ends returned to a wall or terminating in newel posts or safety terminals.
There must be a clearance of at least 1 1/2 inches between the wall and the handrail so a hand can wrap around it, and the handrail may project no more than 4 1/2 inches into the required stair width on either side.
Guards, the barriers on the open side of a stair, are covered by a separate section (R312) with their own height and opening limits, so a stair with an open side needs both a handrail check and a guard check.
A quick compliance check for a stringer
Suppose a deck surface sits 44 inches above grade. Six risers would be 7.33 inches each, which is under the 7 3/4 inch maximum. Five risers would be 8.8 inches and fail. With an 11 inch run, no nosing is needed and the total run is 5 treads times 11 inches, which is 55 inches. Width 36 inches, one handrail at 34 to 38 inches, and guards if the open side is more than 30 inches above grade.
The riser count comes from the total rise, not from a target step height. Dividing 44 by 7.75 gives 5.68, which rounds up to 6 risers; the actual riser is then 44 divided by 6. A stair calculator that draws the stringer from the total rise, such as the one at Benchlet, does exactly this arithmetic and flags any riser over 7 3/4 or tread under 10 before you cut lumber.
Always confirm the edition adopted locally. Most jurisdictions use the 2018 or 2021 IRC, and these dimensional limits have been stable across those editions, but some states and cities amend them with larger risers or smaller treads. The local building department's handout is the final word for a permit.
Frequently asked questions
What is the maximum stair riser height by code?
7 3/4 inches under IRC R311.7.5.1, measured nosing to nosing. Risers in one flight may not differ by more than 3/8 inch.
What is the minimum tread depth?
10 inches measured horizontally nosing to nosing, with a 3/4 to 1 1/4 inch nosing projection required. If the tread is 11 inches or deeper, no nosing is required.
How much variation is allowed between steps?
3/8 inch between the largest and smallest riser, and 3/8 inch between the largest and smallest tread, each measured within one flight.
How high does a stair handrail have to be?
Between 34 and 38 inches above the nosing line, required on at least one side of any flight with four or more risers, with 1 1/2 inches of clearance from the wall.
How wide does a residential stair have to be?
36 inches clear width above the handrail height. Handrails may project up to 4 1/2 inches into that width on each side.
