Stair Calculator

Plan the risers, the run, and the stairwell opening before you build.

Total rise

6 to 240 inches, finished floor to finished floor.

Maximum riser

4 to 9 inches. IRC residential stairs stay at or under 7 3/4.

Tread depth

9 to 14 inches, used unless you enter an available run.

Target riser

Optional. Leave blank to use the fewest risers.

Top of the stair

The risers, run, and stairwell opening appear when the total rise is in range.

Available run

Optional, 12 to 480 inches. The page divides this by the tread count.

Floor thickness

Optional, 4 to 24 inches. Leave blank to skip the opening.

Existing opening

Optional, 12 to 240 inches. The page checks headroom under it.

Nosing

0 to 1 1/2 inches. A tread under 11 inches needs 3/4 to 1 1/4.

Stair width

24 to 120 inches. Code minimum is 36 inches.

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Code reference

Riser height

7 3/4 in maximum

IRC R311.7.5.1 (2024: R318.7.5.1)

Residential stair riser. The IRC does not set a minimum.
Tread depth

10 in minimum

IRC R311.7.5.2

Measured horizontally from nosing to nosing.
Nosing

3/4 to 1 1/4 in

IRC R311.7.5.3 (2024: R318.7.5.3)

Required when the tread is under 11 inches.
Headroom

6 ft 8 in minimum

IRC R311.7.2

The stairwell opening is sized so the headroom stays at this line.
Stair width

36 in minimum

IRC R311.7.1

Clear width above the handrail height.
Flight rise

151 in maximum

IRC R311.7.3

A taller flight needs a landing before it continues.

Code limits cited on this calculator
Check	Limit	Reference	Note
Riser height	7 3/4 in maximum	IRC R311.7.5.1 (2024: R318.7.5.1)	Residential stair riser. The IRC does not set a minimum.
Tread depth	10 in minimum	IRC R311.7.5.2	Measured horizontally from nosing to nosing.
Nosing	3/4 to 1 1/4 in	IRC R311.7.5.3 (2024: R318.7.5.3)	Required when the tread is under 11 inches.
Headroom	6 ft 8 in minimum	IRC R311.7.2	The stairwell opening is sized so the headroom stays at this line.
Stair width	36 in minimum	IRC R311.7.1	Clear width above the handrail height.
Flight rise	151 in maximum	IRC R311.7.3	A taller flight needs a landing before it continues.

Code checks reference the 2021 International Residential Code (section numbers for the 2024 edition shown in parentheses). States and cities adopt different editions and often amend them, so limits in your area may differ.

Based on the IRC model code. Your local jurisdiction may adopt a different edition or amendments — confirm with your building department.

How to measure

Total rise is the vertical distance from the finished floor you leave to the finished floor you arrive on. Set a level on the upper floor and drop a tape to the lower floor, and read the inches. Measure both sides of the stairwell. If the two sides differ, enter the taller one so the risers still fit the high side. Do not measure from a subfloor on one end and a finished floor on the other, and do not stop the tape on a threshold or a mat. Available run is the horizontal length you can give the stair, from the face of the first riser to the face of the upper floor. Measure it along the floor, not along the slope. Floor thickness is the upper floor from the finished surface down to the underside of the joists, including the subfloor and the finish. A 2x10 joist at 9 1/4 inches plus a 3/4 inch subfloor plus a 1/2 inch finish is 10 1/2 inches. The opening length uses that thickness. Write all three numbers before you pick a riser count, because the run and the opening both change when the count changes.

Formula

The riser count is the total rise divided by the maximum riser, rounded up. Riser height is the total rise divided by that count. With the top tread below the upper floor, the tread count is one less than the riser count, and the run is that count times the tread depth. If you enter an available run, the tread depth becomes that run divided by the tread count. The stair length is the straight line from the bottom of the first riser to the upper floor. The opening length is 80 inches plus the floor thickness, times the tread depth, divided by the riser height. That length is how far the well must reach back from the top of the stair so the headroom stays at 6 feet 8 inches.

A 108 inch rise with a 10 inch tread and a 10.5 inch floor is 14 risers at 7.714 inches, 13 treads, a 130 inch run, and a 117.3 inch opening.

Common mistakes

Measuring to the subfloor at one end and the finished floor at the other.
Leaving the target riser filled when you wanted the fewest risers.
Entering the sloped length as the available run.
Using joist depth alone and forgetting the subfloor and the finish.
Treating the opening length as the length of the whole upper room.

FAQ

How do I measure total rise?

Measure from the finished floor you step off to the finished floor you step onto, and record inches. If the two sides of the opening differ, use the taller measurement so every riser still clears the high side. Do not mix a subfloor at one end with a finished floor at the other. The number is the whole flight, not the height of one step. A landing in the middle starts a new rise.

What happens when I enter an available run?

The page divides that horizontal length by the number of treads and uses the result as the tread depth. The riser count and the riser height stay the same. A result under 10 inches fails the tread check, and a result over 14 inches is a warning that the step will feel long. Leave the field blank to keep the tread depth you typed.

How is the stairwell opening length found?

The opening has to reach back from the top of the stair until the headroom is 6 feet 8 inches. The page takes 80 inches plus the floor thickness, multiplies by the tread depth, and divides by the riser height. Floor thickness is the finished surface down to the underside of the joists. Leave that field blank and the opening is not shown.

Why does the page show two riser counts?

The first row is the fewest risers that stay at or under the maximum, so the stair uses less floor. The second row adds one riser, which shortens each riser and lengthens the run. Choose the second row when the comfort number or the angle is better and you have the extra length. Reset to fewest steps clears that choice.

What changes when the top tread is flush with the floor?

Flush adds one tread, so the tread count equals the riser count and the run grows by one tread depth. The riser count and the riser height do not change. The default leaves the top tread below the upper floor, and the floor itself is the last step. Pick the setting that matches how the stair lands on the upper floor.

Does a passing check mean the stair meets my local code?

A passing row means the number meets the model-code limit printed beside it. Riser, tread, nosing, headroom, width, and flight rise follow the 2021 IRC, with the 2024 section in parentheses. Your city may adopt another edition or amend a number. A pass is not a permit or a substitute for the local inspector. Confirm the adopted code before you frame the stair.

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By Lumber Calc Editorial Team · Verified against 2021/2024 IRC & AWC DCA 6 · Last updated Oct 24, 2026