How a staircase is set out
A stair is designed from the floor-to-floor height down. That height has to divide into equal risers — every step the same, because the one odd step is the one people trip on. Once the riser is fixed, the tread depth decides how much floor the stair eats.
Formula
Number of risers = floor height ÷ preferred riser, roundedActual riser = floor height ÷ number of risersTreads = risers − 1·Total run = treads × tread depthStringer = √(height² + run²)- Comfort rule:
2 × riser + treadshould land between 22" and 25"
Worked example
A floor height of 120 inches with a 7 inch target gives 17 risers of 7.06 inches each. With 10 inch treads, that is 16 treads and 160 inches — 13.3 feet — of floor space, and a stringer of 200 inches. The comfort rule gives 24.1, comfortably inside the range.
How to use the stair calculator
- Measure floor to floor, from the finished floor below to the finished floor above — not just the ceiling height.
- Set the riser you want. 6.5 to 7.5 inches suits most homes; public stairs are shallower.
- Set the tread depth and check the total run fits the space you have.
Rules worth keeping
- Every riser in a flight must be the same height, within a few millimetres. Uneven steps are the most common cause of falls.
- Keep at least 6 ft 8 in of headroom above the nosing line, and a landing after about 12 to 16 steps.
- Local building rules may set their own limits on riser and tread. Check them before building anything permanent.
Frequently asked questions
What is a comfortable riser height?
Between 6.5 and 7.5 inches for a home. The usual test is that twice the riser plus the tread should come to between 22 and 25 inches.
How many steps for a 10 foot ceiling?
A 120 inch floor-to-floor height with a 7 inch target gives 17 risers of about 7.06 inches each, with 16 treads.
Why is there one fewer tread than riser?
The top riser lands on the floor above, which acts as the last tread. So a flight of 17 risers has 16 treads in between.