How to use the stair calculator
- Measure the total rise. This is the vertical height from the finished surface at the bottom to the finished surface at the top. If the floors are not finished yet, add the thickness of the flooring that will go on.
- Enter a comfortable riser height. 7 to 7 1/2 inches suits most people. The calculator rounds to a whole number of equal risers.
- Enter the tread depth you want. 10 to 11 inches is common.
- Check the limits under Adjust assumptions. They start at the common US residential limits, but your city may use different ones.
The stair formulas
Risers = total rise ÷ ideal riser, rounded to a whole number
Riser height = total rise ÷ risers
Treads = risers − 1
Total run = treads × tread depth
Stringer length = √(total rise² + total run²)
If rounding gives risers taller than your maximum, the calculator adds one more riser so every step stays inside the limit.
Worked example
Total rise 108 in (9 ft), ideal riser 7.5 in, tread 10.5 in
- 108 ÷ 7.5 = 14.4, which rounds to 14 risers.
- Riser height = 108 ÷ 14 = 7.71 in, about 7 11/16 in. That is under 7 3/4 in.
- Treads = 14 − 1 = 13.
- Total run = 13 × 10.5 = 136.5 in (11 ft 4 1/2 in).
- Stringer = √(108² + 136.5²) = 174.1 in (14 ft 6 in), so buy 16 ft boards.
- Angle = arctan(108 ÷ 136.5) = 38.4°.
Common US residential stair limits
Most states base their residential code on the International Residential Code. These are its usual figures. Your local code may differ, so treat this as a starting point and confirm locally.
| Item | Common limit |
|---|---|
| Riser height | 7 3/4 in maximum |
| Tread depth | 10 in minimum (measured nosing to nosing) |
| Difference between risers or treads | 3/8 in maximum |
| Nosing | 3/4 in to 1 1/4 in where treads are under 11 in |
| Stair width | 36 in minimum |
| Headroom | 6 ft 8 in minimum |
| Handrail | Required with four or more risers |
Laying out a stringer
With a framing square, set the riser height on one arm and the tread depth on the other, using stair gauges to lock them. Step the square along the board once for every tread, marking each notch. At the bottom, cut off one tread thickness so the first riser comes out the same height as the rest once the tread is on. That last step, called dropping the stringer, is the one people most often forget.
Common mistakes to avoid
- Measuring to the subfloor. Measure to the finished floor at top and bottom, or add the flooring thickness.
- Counting treads and risers as equal. There is always one more riser than treads.
- Not dropping the stringer. Without trimming the bottom by one tread thickness, the first step ends up too tall and the top step too short.
- Measuring the rise at one spot on uneven ground. For outdoor stairs, measure where the bottom step will actually land.
- Forgetting the landing. Long runs often need a landing partway; check your code.
Frequently asked questions
How do I calculate the number of stairs?
Divide the total rise by your ideal riser height (about 7 to 7.5 inches) and round to a whole number. That gives the number of risers. Divide the total rise by that whole number to get the exact riser height, and check it is under your code maximum. The number of treads is one less than the number of risers.
Why is there one less tread than risers?
The top step is the upper floor or deck itself. So 14 risers need only 13 treads.
What is the maximum riser height in the US?
The model residential code most states adopt sets a maximum riser of 7 3/4 inches and a minimum tread depth of 10 inches, but states and cities can change these. Always confirm with your local building department.
How long should a stair stringer be?
The diagonal from bottom to top is √(rise² + run²). For a 108 in rise and 136.5 in run that is about 174 in, or 14 ft 6 in. Buy a board about a foot longer for the cuts at each end, so a 16 ft board in that case.
What is the 2R + T rule?
Two risers plus one tread should come to about 24 to 25 inches. It matches a normal walking stride, so stairs that follow it feel comfortable to climb.
Why must every riser be the same height?
People set their stride from the first few steps. A riser even 3/8 inch different is a common cause of trips and falls, which is why codes limit the variation between risers.