Stair Riser Height and Tread Depth: Standard Dimensions

The 2021 IRC limits stair riser height to a maximum of 7 ¾ inches and a minimum of 4 inches, and requires tread depth to be at least 10 inches. Those are the hard numbers for a residential stair that passes inspection. If your design does not fit within those bounds, it fails, no matter how comfortable it feels. Use the code limits below, understand why some legal stairs feel terrible, and plan a stair that avoids the most common failure.

The International Residential Code (IRC) applies to one- and two-family dwellings and townhouses. The International Building Code (IBC) covers commercial buildings and multi-family structures above three stories. The limits differ. Using the wrong one is a permit failure.

IRC R311.7.5.1, R311.7.5.3 (Residential)

Maximum riser height: 7 ¾ inches. Minimum riser height: 4 inches. Minimum tread depth: 10 inches. Nosing projection: between ¾ and 1 ¼ inches, with a maximum nosing radius of 9/16 inch. Risers and treads must be uniform: the largest variation from the tallest to the shortest riser in a flight cannot exceed 3/8 inch. The same 3/8-inch limit applies to tread depth variation.

IBC 1011.5.2 (Commercial)

Maximum riser height: 7 inches. Minimum tread depth: 11 inches. Riser height variation within a flight: no more than 3/16 inch. Tread depth variation: no more than 3/8 inch. These are stricter than the IRC. If you are building a stair for a workplace, an apartment building with more than three stories, or any commercial space, the IBC controls. Use the IBC limits, not the IRC.

Dimension	IRC (Residential)	IBC (Commercial)
Maximum riser height	7 ¾ in (196.85 mm)	7 in (178 mm)
Minimum riser height	4 in (101.6 mm)	4 in (101.6 mm)
Minimum tread depth	10 in (254 mm)	11 in (279 mm)
Riser variation (max)	3/8 in (9.5 mm)	3/16 in (4.8 mm)
Tread variation (max)	3/8 in (9.5 mm)	3/8 in (9.5 mm)
Nosing projection	¾ to 1 ¼ in	Not specified in section

A stair built exactly to the IRC max riser (7 ¾ in) and min tread (10 in) is legal but uncomfortable. The stair angle hits about 38 degrees, which feels steep. Blondel's formula, 2R + T, gives a comfort target. For that combination, 2 × 7.75 + 10 = 25.5 inches, which is outside the ideal 24 to 25 inch range. Many users, especially older adults and those with mobility issues, find this stair hard to climb.

Comfortable Riser and Tread Pairs

The real-world comfort zone for most adults is a riser between 6 ½ and 7 ½ inches paired with a tread between 10 and 12 inches. Aim for a Blondel value near 25. A 7-inch riser with an 11-inch tread gives 2 × 7 + 11 = 25, which is ideal. A 6.5-inch riser with a 12-inch tread gives 2 × 6.5 + 12 = 25. A 7.5-inch riser with a 10.5-inch tread gives 2 × 7.5 + 10.5 = 25.5, which still feels fine for most people. Below 24 feels choppy; above 26 feels like a stretch.

Riser Height (in)	Tread Depth (in)	2R + T (in)	Usability
6.5	12.0	25.0	Ideal – feels natural for most users
7.0	11.0	25.0	Ideal – common in well-designed stairs
7.5	10.5	25.5	Good – acceptable for tight spaces
7.75	10.0	25.5	Legal but steep – many users complain

IRC R311.7.5.3 requires a nosing on every tread unless the tread itself has a built-in projection that meets the ¾ to 1 ¼ inch limit. The nosing is the projecting edge of the tread beyond the riser below. It is not optional. Omitting it reduces the effective tread depth, creating a code violation and a tripping hazard. Many builders assume any overhang is fine, but the IRC specifies the range. Use a nosing that projects ¾ to 1 ¼ inches beyond the riser. The nosing radius cannot exceed 9/16 inch. If you use a bullnose tread, measure from the leading edge to the riser face, not from the stringer.

Non-uniform risers are the most common stair inspection failure. The IRC R311.7.5.1 requires that no riser in a flight vary by more than 3/8 inch from the tallest to the shortest. The same 3/8-inch limit applies to tread depth in R311.7.5.2. This means you cannot split the difference in total rise unevenly. In practice, measure the finished floor to finished floor height precisely, then divide by the number of risers to get each riser height. If the remainder is more than 3/8 inch, add or subtract a riser.

Failure: Mismeasuring Total Rise

The most common cause of non-uniform risers is measuring from subfloor to subfloor instead of from finished floor to finished floor. If you cut stringers based on subfloor measurements, the riser at the bottom will be short by the thickness of the flooring, and the top riser will be tall by the same amount. That single error produces a variation of half an inch or more, which fails inspection. Measure from the top of the finished lower floor to the top of the finished upper floor. For a deck stair, that is from the top of the decking to the top of the landing surface.

Outdoor stairs, such as those on a deck, and basement stairs follow the same IRC limits as interior stairs: max riser 7 ¾ inches, min tread 10 inches, nosing required, uniformity rule applies. The only difference is the structural material. For a wood deck stair, use a 2x12 stringer. The American Wood Council DCA 6 provides stringer span tables; a 2x12 notched for treads and risers can span up to about 6 feet without intermediate support, but check the table for your specific load. A common failure is notching the stringer too deeply, cutting more than one-third of the stringer depth, which weakens it. The notch depth should leave at least 4 inches of solid wood below the notch for a 2x12.

Basement stairs often have headroom issues. The IRC requires a minimum of 6 feet 8 inches of headroom measured from the nosing of the tread to the nearest overhead obstruction. If the basement ceiling is low, measure before you build. A stair under a sloped ceiling is a frequent violation. Measure from the finished nosing, not from the subfloor.

The minimum tread depth under the IRC is 10 inches. The practical comfort range is 10 to 12 inches. A 10-inch tread paired with a 7 ¾-inch riser is legal but feels short, especially for men's shoe sizes. Many users' heels overhang the tread. An 11-inch tread is the real-world minimum for comfortable foot placement. A 12-inch tread paired with a 6.5-inch riser gives the best comfort but increases the total run. If space is tight, you cannot shorten the tread below 10 inches without a code violation. The only way to reduce total run is to increase the riser height, up to the 7 ¾-inch max, or use a landing or winder to change direction.

Tread depth variation within a flight cannot exceed 3/8 inch. That means if one tread is 10 inches, another cannot be 10.5 inches. The inspector measures every tread. Cut all treads to the same dimension.

Rise is the vertical height of a single step. Run is the horizontal depth. The relationship between them determines the stair angle. The IRC max riser and min tread produce an angle of about 38 degrees. Older homes often have stairs at 35 degrees or less, which feel more comfortable. The OSHA standard for workplace stairs (29 CFR 1910.25) allows risers from 6.5 to 9.5 inches and treads from 9.5 to 14 inches, with a max angle of 50 degrees, but those are for industrial settings, not homes.

To calculate the number of risers: divide the total rise by a target riser height (e.g., 7 inches). Round up to the nearest whole number, then divide the total rise by that number to get the actual riser height. If the result is over 7 ¾ inches, add one more riser. If it is under 4 inches, remove one riser. This is the step most DIYers get wrong: they forget to account for the remainder and end up with a riser height that exceeds the max or violates the uniformity rule.

The maximum riser height for a residential stair is 7 ¾ inches, per IRC R311.7.5.1. For a commercial stair, it is 7 inches, per IBC 1011.5.2. A riser above these limits is a code violation. Do not build a stair with risers taller than 7 ¾ inches, even if the total run is too short. The correct response to a short total run is to add a landing or a winder, not to increase the riser height. If the run does not fit, redesign the floor plan or use an L-shaped stair with a landing.

Comfortable stair dimensions are not the same as legal ones. The IRC allows a stair that feels steep and cramped. A stair that passes inspection but has a Blondel value above 25.5 will draw complaints from users. For a stair used daily, aim for a riser between 6.5 and 7.5 inches and a tread between 10.5 and 11.5 inches. That keeps the Blondel value between 24 and 25.5. The angle will be between 30 and 35 degrees, which feels natural.

If you have a fixed total rise and limited floor space, use the comfort table above to find a combination that fits. If no combination fits the space, the stair needs a landing or a turn. Do not try to squeeze a comfortable stair into a footprint that is too short. The result will be a stair that fails inspection or is uncomfortable to use.