How to Calculate Stair Rise and Run
Every stair layout starts with one measurement, the total rise, and four simple calculations. Here’s how to go from that number to a complete layout.
The terms
- Total rise: vertical distance from the landing surface to the top of the finished floor or deck surface
- Riser height (unit rise): height of one step
- Tread depth (unit run): horizontal depth of one step, nosing to nosing
- Total run: horizontal distance the stairs project from the deck
The formulas
- Number of risers = total rise ÷ maximum riser height, rounded up
- Riser height = total rise ÷ number of risers
- Number of treads = number of risers − 1 (the deck is the top step)
- Total run = number of treads × tread depth
Under the 2021 IRC the maximum riser height is 7¾” and the minimum tread depth is 10”.
Worked examples
| Total rise | Risers | Riser height | Treads | Total run (10” treads) | Total run (11” treads) |
|---|---|---|---|---|---|
| 24” | 4 | 6” | 3 | 30” | 33” |
| 30” | 4 | 7½” | 3 | 30” | 33” |
| 36” | 5 | 7 3/16” | 4 | 40” | 44” |
| 48” | 7 | 6 7/8” | 6 | 60” | 66” |
| 60” | 8 | 7½” | 7 | 70” | 77” |
| 72” | 10 | 7 3/16” | 9 | 90” | 99” |
Riser heights are rounded to the nearest 1/16”. Take 48”: 48 ÷ 7.75 = 6.19, rounded up to 7 risers. 48 ÷ 7 = 6.86”, about 6 7/8”.
Comfort rules of thumb
Code sets the limits, but stairs feel best inside these ranges:
- 2R + T = 24–25”: two risers plus one tread. A 7” riser with a 10½” tread gives 24½”.
- R + T = 17–18”: one riser plus one tread.
- Riser 6½–7½”, tread 10½–11½” for most outdoor stairs.
If your numbers land outside these ranges, try a lower target riser height. You’ll get one more step, each step will be shorter, and the run gets longer.
Measuring total rise on sloped ground
When the ground slopes away from the deck, the stairs land lower than the ground at the rim, which adds rise, which adds run, which moves the landing farther out still. Settle it in two passes:
- Measure the rise straight down at the rim and calculate a first total run.
- Measure again at the point that far out from the deck, using a long level or a string line from the deck surface.
- Recalculate with the new rise. One more pass is rarely needed.
If the stair lands on a new concrete pad, measure to the top of the pad, not the dirt.
Stringer length
The stringer is the hypotenuse of a triangle formed by the total rise and the total run plus one more tread depth for the top notch:
Stringer length ≈ √(total rise² + (total run + tread depth)²)
For a 36” rise with 10” treads: √(36² + 50²) ≈ 61.6”, so an 8 ft 2x12 is plenty. Allow a few extra inches for the end cuts.
Let the calculator do it
The stair stringer calculator runs all of these formulas, draws the stringer, and checks the result against code. You can also browse solved layouts for common rises.