What the calculator works out
Give this stairs calculator your total floor-to-floor height and a target step height, and it sizes the whole staircase: how many risers you need, the exact height of each one, how many treads that leaves, how deep each tread is for the run you have, the resulting angle, and whether the numbers land inside common code limits.
The one input people underestimate is total run — the horizontal distance the stair is allowed to take up. Rise is fixed by the building; run is a design choice, and it is what determines whether your treads are deep enough to be legal and comfortable.
Rise, run, riser, tread
- Total rise — vertical distance from the lower finished floor to the upper finished floor.
- Total run — horizontal distance from the face of the first riser to the face of the last.
- Riser — the vertical face of one step, and its height.
- Tread — the horizontal surface you step on, and its depth front to back.
A flight always has one more riser than treads, because the top riser brings you up to the landing, which is not itself a tread.
The stair formulas
With
Worked example
The calculator’s defaults are a total rise of 108 inches, a total run of 120 inches, and a target riser of 7.5 inches.
The comfort figure is
Code limits and comfort rules
Typical residential figures from the International Residential Code:
| Dimension | Limit |
|---|---|
| Maximum riser height | 7¾ in |
| Minimum tread depth | 10 in |
| Riser-to-riser variation | ≤ 3/8 in |
| Minimum headroom | 6 ft 8 in |
| Minimum stair width | 36 in |
Three long-standing comfort formulas, any one of which is a useful cross-check:
to — the stride rule. to — a simpler sum. to — the product rule.
These are guidelines, not law. Your local building department’s adopted code and amendments are what actually govern the job — the stair code basics guide goes into the details.
Stair stringer length
The stringer is the sawtooth board the treads sit on, and its length
For the 108-inch rise and 120-inch run above, the stair stringer works out at
- Buy longer than the calculation. The cuts waste material at both ends, and a stringer that comes up half an inch short is firewood.
- Drop the bottom of the stringer by one tread thickness. Cutting the notches to the raw numbers makes the first step taller than the rest by exactly the thickness of the tread stock — the single most common stringer mistake.
A stair tread is the horizontal board you step on; a riser is the vertical face between treads. Open-riser stairs leave that face off, which changes the look but not the rise and run arithmetic.
Adjusting when the numbers don't work
If the tread comes out too shallow, as in the example above, your options are:
- Give the stair more run. The cleanest fix when the space allows it.
- Use a landing and a turn. An L-shaped or U-shaped stair fits the same rise into a smaller footprint.
- Accept a steeper stair only where code permits it — some spaces allow steeper "secondary" stairs, but main stairs rarely qualify.
Reducing the riser count does not help the tread problem; it makes each riser taller and the stair steeper still.
Common mistakes
- Measuring rise to the subfloor. Total rise is finished floor to finished floor. Forgetting the flooring thickness at one end throws off every riser.
- Dividing run by the riser count. Tread depth uses the number of treads, which is one fewer.
- An odd first or last step. If the stringer sits on a slab that later gets tiled, the bottom riser ends up shorter than the rest — account for finished surfaces at both ends.
- Ignoring nosing. The tread’s nosing overhang adds depth for your foot but does not count toward the code tread measurement.
- Forgetting headroom and the handrail. A stair that pencils out can still fail if the opening above it is too low or there is no room for a graspable rail.