What is the maximum stair rise and minimum run allowed by code?

Under the IRC for homes, the maximum riser height is 7.75 inches and the minimum tread depth is 10 inches. Commercial stairs under the IBC are stricter, usually a 7-inch maximum riser and 11-inch minimum tread. Just as important, within one flight the tallest and shortest risers cannot differ by more than 3/8 inch, and the same tolerance applies to tread depth.

How do I calculate the number of risers and treads?

Divide the total finished-floor-to-finished-floor rise by a target riser height (about 7 to 7.5 inches) and round to a whole number of risers. Then divide the total rise back by that riser count to get the exact equal riser height. The tread count is always one less than the riser count, because the top riser lands on the upper floor rather than on a cut tread.

Why are stair stringers cut from 2x12 lumber instead of 2x10?

Code requires at least 5 inches of solid, uncut throat behind the notches. Once you cut the rise and run into the board, a 2x10 usually does not leave 5 inches of remaining depth, while a 2x12 does. That throat keeps the stringer strong enough to carry the load, which is why 2x12 is the default for cut stringers.

What is the dropped bottom step and why does it matter?

When you frame stringers, the first riser must be cut shorter by the thickness of one tread. If you skip this, the bottom step ends up taller than the rest once treads are installed, and the uneven riser fails the 3/8-inch uniformity rule. Subtracting the tread thickness from the bottom riser keeps every finished step equal.

How to Lay Out Stairs: Rise, Run, Stringers, and Code Limits

Stairs are unforgiving. A riser that varies by half an inch is a trip hazard, a failed inspection, and a callback. Here is the field-tested method for laying out rise and run, cutting stringers, and staying inside code on the first try.

Start With Total Rise, Then Divide for Riser Count

Everything begins with total rise: the exact vertical distance from the lower finished floor to the upper finished floor. Measure it with a level and tape, accounting for finished floor thickness on both ends. A common error is measuring subfloor to subfloor and forgetting the tile, hardwood, or topping that goes down later. Get this number wrong and every riser is wrong.

Divide total rise by your target riser height to get the number of risers. Most residential work targets a comfortable riser around 7 to 7.5 inches. Example: a total rise of 109 inches divided by 7.5 equals 14.5, so round to 15 risers. Then divide back: 109 divided by 15 equals 7.27 inches per riser. Risers must come out equal, so always divide the total rise by a whole number of risers rather than picking a riser height and hoping it lands evenly.

Remember the count rule: a flight always has one more riser than it has treads. A run with 15 risers has 14 treads, because the top riser lands you on the finished upper floor, which is not a tread you cut into the stringer.

Set the Run and Check It Against the Rise

Tread run is the horizontal depth of each step, measured nosing to nosing, not including the nosing overhang. With your 7.27-inch riser, a 10- to 11-inch run is typical and code-compliant. Total run equals tread run multiplied by the number of treads: 14 treads at 10.5 inches gives 147 inches of horizontal travel, which is what you need to confirm there is floor space for the stair to land.

Use a proportion check so the stair feels right underfoot. The two long-standing rules of thumb are: rise plus run should total roughly 17 to 18 inches, and two times the rise plus the run should total roughly 24 to 25 inches. A 7.27 riser with a 10.5 run gives 17.77 and 25.04, both inside the comfort window. Steep, choppy stairs and long, shallow stairs both come from ignoring these ratios.

If the total run does not fit the available floor, you have three levers: add a landing to split the flight, adjust the riser height within code, or relocate the bottom of the stair. Do not solve a space problem by shrinking the tread below the code minimum.

Know the IRC Code Limits Cold

For one- and two-family dwellings, the International Residential Code sets hard limits you cannot exceed. Maximum riser height is 7.75 inches. Minimum tread depth is 10 inches. Minimum headroom is 6 feet 8 inches, measured vertically from the sloped plane along the nosings. Commercial stairs under the IBC are more restrictive, typically a 7-inch max riser and 11-inch min tread, so confirm which code governs your job.

The uniformity rule causes more failed inspections than the maximums do: within a single flight, the greatest riser height may not exceed the smallest by more than 3/8 inch, and the same 3/8-inch tolerance applies to tread depth. This is why the bottom riser is the one that bites you. When the bottom tread sits on a finished floor that is thicker or thinner than planned, that riser changes height and can blow the 3/8-inch tolerance.

Nosings get checked too. Where treads are less than 11 inches deep, a nosing of 3/4 inch to 1.25 inches is required, and the nosing profile must be consistent. Open risers, where used, generally cannot allow a 4-inch sphere to pass through. Pull the adopted code edition for your jurisdiction before you cut, since amendments are common.

Lay Out and Cut the Stringers

Stringers are almost always cut from 2x12 stock because of one number: the throat. After you cut the rise and run notches, the remaining uncut depth of the board, measured perpendicular from the back edge to the inside corner of the notch, must be at least 5 inches under the IRC. A 2x10 rarely leaves enough throat once notched, which is why 2x12 is the standard.

Mark the notches with a framing square fitted with stair gauges (the little brass buttons). Clamp the gauges at your rise on the tongue and your run on the body, then step the square down the board, scribing each tread and riser in sequence. The most common mistake is the dropped bottom step: you must shorten the first riser by the thickness of one tread so that every finished step ends up equal once treads are installed. Subtract the tread material thickness from the bottom riser cut.

Cut the notches with a circular saw and finish each inside corner with a handsaw or jigsaw; overcutting with the circular saw weakens the stringer and is visible on inspection. For typical residential widths plan on three stringers (one each side and one center), tightening to 12 inches on center under heavy or commercial loads. Always cut one stringer, test-fit it against the actual opening, and only then use it as a template for the rest.

Material Takeoff and Waste

For the stringers themselves, figure stringer length with the Pythagorean theorem: the diagonal equals the square root of total rise squared plus total run squared, then add length for the top connection and bottom cut. Buy one length up from the calculated dimension so you have room for the bottom-step adjustment and a clean top cut. For a typical interior flight, three 2x12s at 12 or 16 feet is a common buy.

Treads and risers are counted directly off your layout: one tread per step (14 in the example) and one riser board per riser if you are closing them in (15). Add roughly 10 percent waste on tread and riser stock for end trimming, defects, and the occasional miscut, and a bit more if you are working in hardwood where grain and color matching cost you boards. Account for nosing return pieces on any open ends.

Capturing the total rise and the surrounding framing accurately is where layout errors are born, so many crews now verify the opening with a LiDAR scan. Apps like ProBuildCalc let you scan the stairwell, pull exact rise and run dimensions, and generate a stringer and tread takeoff before anyone climbs a ladder with a tape. Whatever method you use, confirm field dimensions against the plan before cutting, because lumber spent on a miscut stringer is gone.

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FAQ

What is the maximum stair rise and minimum run allowed by code?: Under the IRC for homes, the maximum riser height is 7.75 inches and the minimum tread depth is 10 inches. Commercial stairs under the IBC are stricter, usually a 7-inch maximum riser and 11-inch minimum tread. Just as important, within one flight the tallest and shortest risers cannot differ by more than 3/8 inch, and the same tolerance applies to tread depth.
How do I calculate the number of risers and treads?: Divide the total finished-floor-to-finished-floor rise by a target riser height (about 7 to 7.5 inches) and round to a whole number of risers. Then divide the total rise back by that riser count to get the exact equal riser height. The tread count is always one less than the riser count, because the top riser lands on the upper floor rather than on a cut tread.
Why are stair stringers cut from 2x12 lumber instead of 2x10?: Code requires at least 5 inches of solid, uncut throat behind the notches. Once you cut the rise and run into the board, a 2x10 usually does not leave 5 inches of remaining depth, while a 2x12 does. That throat keeps the stringer strong enough to carry the load, which is why 2x12 is the default for cut stringers.
What is the dropped bottom step and why does it matter?: When you frame stringers, the first riser must be cut shorter by the thickness of one tread. If you skip this, the bottom step ends up taller than the rest once treads are installed, and the uneven riser fails the 3/8-inch uniformity rule. Subtracting the tread thickness from the bottom riser keeps every finished step equal.