Stair Calculator
Design a staircase from your floor-to-floor height: number of risers and treads, exact riser height, total run, stringer length and angle.
How to work out stairs
A staircase is decided by one measurement: the total rise, floor to floor, finished surface to finished surface. Divide it by a comfortable riser height (about 7.5 in) and round to a whole number of risers — that count is fixed, and the actual riser height is then the total rise divided by the count, so every step comes out identical. Identical is not a nicety; a single step 1/4 in different from its neighbours is the one people trip on.
Risers = round(total rise ÷ target riser) · Actual riser = total rise ÷ risers · Treads = risers − 1
Example: a total rise of 96 in at a 7.5 in target gives 12.8 → 13 risers of exactly 7.38 in each, 12 treads, and at 11 in per tread a total run of 132 in (11 ft). The stringer works out to 163.22 in at a 33.87° angle.
The comfort rule
Stairs feel right when 2 × riser + tread lands between 24 and 26 inches — the length of a natural walking stride, split by the step. The example above gives 2 × 7.38 + 11 = 25.77 in: comfortable. If your result lands outside the band, adjust the tread depth, not the riser — risers are locked by the count.
| Check | Usual residential rule (US IRC) |
|---|
| Maximum riser height | 7.75 in (196 mm) |
| Minimum tread depth | 10 in (254 mm) — 11 in is more comfortable |
| Riser variation within a flight | No more than 3/8 in between tallest and shortest |
| Comfort (2 × riser + tread) | 24–26 in |
This is a dimension tool — it designs the staircase rather than counting materials, so there is no shopping list and no waste allowance. Stringer stock is typically 2×12 lumber: the stringer length above, times the number of stringers (two for narrow stairs, three past about 3 ft wide), is the one number to take to the lumber yard.
Frequently asked questions
Why does the calculator change my riser height?
Because steps must divide the total rise exactly. 96 in cannot be built from 7.5 in risers (12.8 of them), so the count rounds to 13 and every riser becomes 7.38 in — equal steps are a safety requirement, not a preference.
What is the difference between a riser and a tread?
The riser is the vertical face of a step — its height. The tread is the horizontal surface you stand on — its depth front to back. A flight with 13 risers has 12 treads, because the top landing is the last “tread”.
How long must my stringer board be?
The calculator gives the sloped length of the flight itself. Buy the next standard board length up (stringers are cut from 2×12 stock) and confirm the layout with a framing square before cutting — the first stringer is the template for the rest.
My stairs will be outdoors (deck stairs). Does anything change?
The geometry is identical. Outdoor steps often use a slightly shallower riser and deeper tread for comfort in boots, and deck stairs land on a concrete pad — the top of that pad is the bottom of your total rise.
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