How to use the stair calculator
Enter your total rise — the floor-to-floor height the staircase needs to climb. Then choose whether you know the run per step or the total horizontal run you have to work with, and whether you'd rather set a target riser height (around 7-7.5 in / 178-190 mm is typical) or a fixed number of steps. The calculator works out the rest — riser height, run, stringer length and angle — and redraws the diagram as you type.
The stringer length formula
The stringer is the diagonal board the treads and risers are cut into, so its length is the hypotenuse of a right triangle formed by the total run and the vertical stringer height. On a flush mount, stringer height equals the total rise. On a standard mount, it's the total rise minus one riser, since the last step is the upper floor itself.
Standard vs. flush mount
A standard mount stringer stops one riser below the upper floor — the floor itself is the final "step" — so it needs one fewer tread than riser. A flush mount stringer carries a full tread level with the upper floor, so risers and treads are equal in number. Pick whichever matches how your landing is framed.
Building code and comfort guidelines
- Riser height: 7.75 in (197 mm) maximum under the IRC
- Tread run: 10 in (254 mm) minimum under the IRC
- 2×Riser + Run: 24-25 in (61-64 cm) is the classic comfort range
- Angle: 30-40° is comfortable to climb; steeper feels like a ladder
- Width: 36 in (914 mm) minimum, not counting handrails
- Headroom: 6 ft 8 in (2032 mm) minimum, measured from the tread nosing straight up
These are common U.S. residential guidelines (2015 IRC R311.7). Local codes can be stricter — always confirm with your building department before cutting.
Materials: how many stringers do I need?
Use at least 2 stringers for any staircase — one on each side. Add a middle stringer for every additional 24 in (61 cm) of stair width so the treads don't flex underfoot. This calculator works the stringer count out from your stair width automatically, rounds the cut length up to a standard board length, and adds a waste allowance to the tread and riser lumber.
Common questions
How do I calculate stair rise and run?
Divide your total rise (floor-to-floor height) by a comfortable riser height, around 7-7.5 inches, and round to a whole number of steps. Then divide that riser height back into the total rise so every step matches exactly. The run (tread depth) is typically 10-11 inches. This calculator does both directions automatically.
What is the standard rise and run for stairs?
Most residential stairs use a riser height of 7 to 7.75 inches (178-197 mm) and a tread run of 10 to 11 inches (254-280 mm). The IRC caps riser height at 7.75 inches and requires at least 10 inches of run.
How do I calculate stair stringer length?
Stringer length is the hypotenuse of a right triangle: the stringer height (vertical) and the total run (horizontal) are the two legs. Stringer length = √(stringer height² + total run²). This calculator works it out automatically and rounds up to a standard board length.
What is the difference between a standard and flush stringer mount?
On a standard mount, the top step is the upper floor itself, so the stringer needs one fewer tread than riser. On a flush mount, the top tread is a separate board level with the upper floor, so risers and treads are equal in number.
How many stringers do I need for a staircase?
Use at least 2 stringers for any staircase, and add a middle stringer for every additional 24 inches (about 61 cm) of stair width beyond that to keep the treads from flexing. This calculator works out the stringer count from your stair width automatically.