How stairs are laid out
Every step in a staircase must be the same height, so the total rise has to be divided evenly. Start from the total rise and a comfortable riser height, round to a whole number of steps, and then work out the exact riser height. Treads are one fewer than risers because the top floor acts as the last step.
Risers = total rise ÷ ideal riser, rounded (never above the code maximum)
Riser height = total rise ÷ risers · Treads = risers − 1
Total run = treads × tread depth · Stringer = √(rise² + run²)
Example: a total rise of 108 inches with a 7.25 inch ideal riser gives 14.9, so 15 risers of exactly 7.2 inches. That's 14 treads; at 10.5 inches each the total run is 147 inches (12 ft 3 in). The stringer is √(108² + 147²) = about 182.4 inches, or 15 ft 2 3/8 in, so 16 ft lumber works.
US residential code basics
| Item | Typical IRC requirement |
|---|---|
| Maximum riser height | 7 3/4 in |
| Minimum tread depth | 10 in |
| Nosing (with treads under 11 in) | 3/4 to 1 1/4 in |
| Minimum stair width | 36 in |
| Minimum headroom | 6 ft 8 in |
| Riser height variation | no more than 3/8 in between steps |
These are common values from the International Residential Code. Your local code may differ or have been amended, so confirm with your building department before building.
Stringers and comfort
- Use 2 × 12 lumber for cut stringers in most cases, and space stringers no more than about 16 inches apart for wood treads.
- Subtract the tread thickness from the bottom of the stringer so the first step isn't taller than the rest.
- The 2R + T rule (twice the riser plus the tread) is a good comfort check: aim for 24 to 25 inches.
Frequently asked questions
What is the standard riser height for stairs?
In the US, the International Residential Code allows a maximum riser height of 7 3/4 inches and a minimum tread depth of 10 inches for homes. Many builders aim for risers around 7 to 7 1/2 inches for more comfortable stairs. Local codes may differ.
How do I calculate the number of stairs?
Measure the total rise from finished floor to finished floor, then divide by your target riser height and round up. Divide the total rise by that number of steps to get the exact riser height, which must not exceed your code's maximum.
Why is there one fewer tread than risers?
The top step lands on the upper floor, so it doesn't need a tread of its own. A stair with 14 risers has 13 treads.
What is the 2R + T rule?
It's a comfort rule of thumb: twice the riser height plus the tread depth should be about 24 to 25 inches. A 7 inch riser with a 10.5 inch tread gives 24.5, which feels natural to climb.
How long should a stair stringer be?
The stringer is the diagonal of a right triangle with the total rise and total run as its sides. The calculator works it out with the Pythagorean theorem. Buy lumber at least a foot longer to allow for the top and bottom cuts.