How this calculator works
Enter the total rise — the vertical floor-to-floor height the stairs need to climb — along with the maximum riser height you want to allow and your chosen tread run (the depth of each step). The calculator rounds the riser count up so no single riser exceeds your maximum, then divides the total rise evenly across that many risers to get the exact riser height, since equal risers are essential for a safe, comfortable stair.
From there it works out the total horizontal run (tread run times one fewer than the riser count, since the top riser lands on the upper floor rather than needing its own tread), the stringer length using the Pythagorean theorem on the total rise and total run, and the stair's angle from horizontal. Stringer length is the board length needed for each diagonal support before notching out the risers and treads.
The formula
Riser count = ceiling(total rise ÷ max riser height)Exact riser height = total rise ÷ riser countTotal run = tread run × (riser count − 1)Stringer length = √(total rise² + total run²); angle = atan(total rise ÷ total run)7.75 inches is a commonly cited maximum riser height under the US International Residential Code (IRC), and 10 inches a commonly used minimum tread run — both are provided as defaults, not universal rules. Building codes vary by jurisdiction and stair type (interior, exterior, exit stairs); always check your local code before building.
Worked example: 108 in total rise, 7.75 in max riser, 10 in tread
- Riser count: 108 ÷ 7.75 = 13.94, rounded up to 14 risers.
- Exact riser height: 108 ÷ 14 ≈ 7.71 in — under the 7.75 in maximum, as required.
- Total run: 10 × (14 − 1) = 130 in.
- Stringer length: √(108² + 130²) = √28,564 ≈ 169.0 in, at an angle of about 39.7°.
Frequently asked questions
Why does riser count round up instead of down?
Rounding up guarantees every riser stays at or below your chosen maximum height. Rounding down would produce fewer, taller risers that could exceed the maximum and create a trip hazard, since risers on the same flight must be equal in height.
What is the 2R + T comfort rule?
It's a widely cited rule of thumb, not a code requirement: twice the riser height plus the tread run should land close to 24 to 25 inches for a comfortable stride. A 7.5 in riser with a 10 in tread gives 2 × 7.5 + 10 = 25 in, right at the top of that range; much outside it and stairs start to feel cramped or oddly long-strided.
Why is total run based on one fewer tread than the riser count?
The top riser brings you up to the level of the floor above, which already has its own surface to step onto — it doesn't need an additional tread cut into the stringer. A stair with 14 risers therefore has 13 treads.
Does this account for headroom or stringer thickness?
No. This calculator covers riser height, tread run, total run and stringer length only. Minimum headroom clearance above the stair nosing and the extra stringer board width needed after notching are separate checks required by most building codes.
Do interior and exterior stairs use the same riser limits?
Not always — many codes allow slightly different riser and tread dimensions for exterior stairs versus interior stairs, and stricter limits often apply to stairs serving as a required exit. Confirm the applicable limit for your specific stair type with your local building department.