Stair Calculator: Risers, Treads and Stringer Length
Enter the floor-to-floor rise, the riser height you are aiming for and the tread depth. The calculator rounds to a whole number of equal risers, then reports the run, stringer length and angle, and compares the result with commonly used riser and 2R+T ranges. Local building codes differ, so confirm yours before building. After switching units, re-enter your measurements.
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Formula
risers = round(total rise / target riser), at least 1riser height = total rise / riserstreads = risers - 1total run = treads x tread depthstringer length = sqrt(total rise^2 + total run^2)angle = atan(total rise / total run)2R + T = 2 x riser height + tread depth
Common guide values used for the checks: a riser of at most 7.75 in (196.85 mm; the IRC has no minimum, 7 in is just a common guide, so shorter risers are informational only) and 2R + T of 24 to 25 in (609.6 to 635 mm), with 1 in = 25.4 mm exactly. Local codes differ.
Worked example
Total rise 108 in, target riser 7 in, tread 10 in:
- 108 / 7 = 15.43, so 15 risers.
- Riser height 108 / 15 = 7.2 in, at or below the 7.75 in maximum.
- 14 treads, so total run = 14 x 10 = 140 in.
- Stringer = sqrt(108^2 + 140^2) = sqrt(31,264) = 176.82 in, at an angle of 37.65 degrees.
- 2R + T = 2 x 7.2 + 10 = 24.4 in, within 24 to 25 in.
Frequently asked questions
How do I work out the number of stair risers?
Divide the total rise by the riser height you want and round to the nearest whole number. For 108 in at about 7 in, 108 / 7 = 15.4, so use 15 risers of 108 / 15 = 7.2 in. All risers must be equal.
Why is the number of treads one less than the risers?
The top floor acts as the last tread. With 15 risers there are 14 treads, so a 10 in tread gives a total run of 140 in.
What is the 2R + T rule?
Twice the riser plus the tread should be about 24 to 25 in (610 to 635 mm) for a comfortable stride. A 7.2 in riser with a 10 in tread gives 2 x 7.2 + 10 = 24.4 in.
Do these ranges match my local building code?
Not necessarily. The IRC sets a 7.75 in (197 mm) maximum riser and no minimum riser; 7 in is only a common rule of thumb, so a shorter riser is shown as information and does not fail the overall check. Many codes also cap risers near 7.75 in and set minimum treads around 10 in (254 mm), but limits, headroom and handrail rules differ by country and region. The stringer length here is the straight-line distance and does not include extra stock for the cuts. Check with your local authority.