Stair Calculator
Enter the height from one finished floor to the next. This divides it into equal risers, gives you the tread depth, the total run, the stringer length and the angle, and checks the result against the code limits.
How the math works
Start with the total rise, floor to finished floor. That number is fixed. Number of risers = total rise / the riser height you want, rounded to a whole number. Then the actual riser height = total rise / that count. You do not get to choose the riser height directly; you choose how many steps to divide the height into. Treads = risers - 1, because the upper floor is the top step. Total run = treads x tread depth. Stringer length = the square root of run squared plus rise squared, which is Pythagoras on the flight. Angle = arctan(rise / run). The code limits under the IRC: riser no more than 7.75 inches, tread no less than 10 inches, variation within a flight no more than 3/8 inch, width at least 36 inches, headroom at least 80 inches. Local amendments are common, so check before cutting.
Common questions
- How many stairs do I need for 9 feet?
- 108 inches divided by a 7.25 inch target gives 15 risers at 7.2 inches each, and 14 treads. At a 10.5 inch tread that is 147 inches of run, or 12 feet 3 inches of floor space.
- What is the maximum riser height?
- 7.75 inches under the International Residential Code for a home. Commercial stairs under the IBC are limited to 7 inches with an 11 inch tread. Some jurisdictions amend these, so check locally.
- Why is the number of treads one less than the risers?
- Because the floor at the top is itself the last step up. A flight with 15 risers has 14 treads plus the landing. Cutting 15 treads is the classic mistake, and it puts the whole stair one step out.
- What is the 2R plus T rule?
- Two riser heights plus one tread depth should come to between 24 and 25 inches. It comes from the length of a human stride on a slope, and stairs that hit it feel natural to walk. It is a guide, not a code requirement.
- Why do I cut the bottom of the stringer shorter?
- Because the first tread sits on top of the stringer and adds its own thickness. If the bottom riser is cut full height, that first step ends up one tread thickness taller than the rest. Take the tread thickness off the bottom cut.
- How much floor space do stairs need?
- Around 10 to 13 feet of run for a standard flight between floors, plus at least 36 inches of landing at the bottom. That landing is required, and it is where many tight retrofit plans fail.