Stair Calculator
Enter the total rise from floor to floor, the riser height you want and the tread depth, in inches or millimeters. The calculator returns the number of risers, the actual riser height, the total run and the stringer length for a straight flight. It also checks the riser height and the tread depth against the 2021 International Residential Code.
How to use the Stair Calculator
- Enter your numbers in the Stair Calculator above.
- Press the Calculate button to see the result.
- To try other numbers, change any value and press the button again.
The stair geometry
Risers = total rise ÷ desired riser, rounded up · · · Actual riser = total rise ÷ risers · · · Total run = (risers − 1) × tread depth · · · Stringer = √(total rise² + total run²)
Rounding the number of risers up means no riser is taller than the height you asked for, and all of them come out the same. The run counts one tread fewer than there are risers, because the top riser lands on the upper floor. The stringer length is the straight line from the bottom of the first riser to the top of the last one. The code check follows sections R311.7.5.1 and R311.7.5.2 of the 2021 IRC. A riser may be no taller than 7¾ inches (196 mm), and a tread no shallower than 10 inches (254 mm). In metric mode the calculator converts millimeters to inches for the check.
Worked example
A total rise of 108 inches with a desired riser of 7 inches: 108 ÷ 7 = 15.43, rounded up to 16 risers of 6.75 inches each. With 11-inch treads the total run is 15 × 11 = 165 inches, and the stringer length is √(108² + 165²) ≈ 197.2 inches. Both the riser and the tread pass the IRC check.
Frequently asked questions
What does the IRC require for residential stairs?
The 2021 International Residential Code sets a riser height of not more than 7¾ inches, a tread depth of at least 10 inches, headroom of at least 6 feet 8 inches and a handrail height from 34 to 38 inches. The calculator checks the riser and the tread. Your state or city may use another edition or local amendments, so check with your local building department.
Why are all risers the same height?
The calculator divides the total rise evenly among the risers. The IRC also limits the difference. Within one flight, the tallest riser may be no more than 3/8 inch taller than the shortest, and the same limit applies to tread depths.
Does the IRC set a minimum riser height?
No. Section R311.7.5.1 sets only a maximum of 7¾ inches, so the calculator flags a riser that is too tall and accepts any shorter one. A lower riser does mean more risers and a longer total run for the same rise.