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Stair Calculator

Enter the floor-to-floor height to get the number of risers and treads, the exact riser height and the stringer length, checked against the IRC.

  • Feet + inches
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Calculator
ft
in

Finished floor to finished floor, measured plumb

in

Code minimum 10". 10 1/2"–11" feels better. Fractions like 10 1/2 work.

Riser height

7 11/16"

14 risers and 13 treads

Total run
11' 4 1/2"
Stringer length (approx.)
14' 6 1/16"
Stair angle
36.3°
Comfort rule: 2 × riser + tread
25.9"

Max riser (IRC)

7 3/4"

Passes

Min tread (IRC)

10"

Passes

Comfort guideline (not code): 2 × riser + tread is 25.9", above the classic 24"–25" target. Adding one more step lowers each riser to 7 3/16" and makes the stair easier to climb, at the cost of 10 1/2" more run.

Keep at least 6' 8" of headroom over the whole stair (IRC R311.7.2). Treads under 11" also need a 3/4"–1 1/4" nosing. Local codes can differ, so check with your building department.
How it works Formula, reference tables and pro tips

IRC stair requirements

ItemRequirementSection
Maximum riser height7 3/4"R311.7.5.1
Minimum tread depth10"R311.7.5.2
Riser / tread variation in a flight3/8" maxR311.7.5
Nosing (treads under 11")3/4" to 1 1/4"R311.7.5.3
Minimum headroom6' 8"R311.7.2
Minimum width36"R311.7.1

Many cities amend the IRC or use an older edition, so confirm with your building department before you cut stringers.

Examples (10 1/2" treads)

Total riseRisersRiser heightTotal run
2' 0"46"2' 7 1/2"
3' 0"57 3/16"3' 6"
4' 0"76 7/8"5' 3"
8' 0"137 3/8"10' 6"
8' 6"147 5/16"11' 4 1/2"
9' 0"147 11/16"11' 4 1/2"
9' 6"157 5/8"12' 3"

How to measure total rise

Measure plumb from the finished surface of the lower floor to the finished surface of the upper floor. If the flooring isn't installed yet, add its thickness. For deck stairs, measure from the deck surface down to where the bottom step will land, which may not be level.

Quick answers

What is the maximum stair riser height?

Under the International Residential Code (R311.7.5.1), riser height can be no more than 7 3/4 inches, and within a flight the tallest and shortest risers can differ by no more than 3/8 inch.

What is the minimum tread depth?

The IRC (R311.7.5.2) requires treads at least 10 inches deep, measured nosing to nosing. Treads shallower than 11 inches also need a nosing of 3/4 to 1 1/4 inches.

How do I calculate the number of stairs?

Divide the total rise in inches by 7.75 and round up. That is the number of risers. Divide the total rise by that number to get the exact riser height. There is one fewer tread than risers, because the top floor acts as the last step.

How many steps do I need for a 9 ft rise?

A 9 ft (108 inch) total rise needs 14 risers of about 7 11/16 inches (108 ÷ 14 = 7.71) and 13 treads. With 10 1/2 inch treads the stair runs 11 ft 4 1/2 in horizontally. Remember that rise is measured floor to floor, so with 9 ft ceilings it is usually more than 9 ft once the floor framing is added.

How long is a stair stringer?

It is the hypotenuse of the total rise and total run: √(rise² + run²). Buy lumber at least a foot longer than that to allow for the top and bottom cuts. Stringers are usually cut from 2×12s.

Sources

Last reviewed November 2, 2026.

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