Work out the number of risers, the exact riser height, tread count, total run and stringer length for any staircase, and check the result against the International Residential Code limits automatically.
Measure the total rise from finished floor to finished floor, after flooring is down on both levels. Measuring to the rough subfloor is the single most common cause of uneven steps.
A staircase is not designed from a picture; it is designed from one measurement and one code limit. The measurement is the total rise, the vertical distance from the finished floor at the bottom to the finished floor at the top. The limit is the maximum riser height, which is 7¾ inches under the International Residential Code.
Everything else follows. Divide the rise by the limit and round up: rounding up guarantees no single riser exceeds the maximum. Then divide the rise back through that whole number to get the exact riser height, and that number is the one you cut to, on every step, because the risers in a flight must be identical.
Treads are always one fewer than risers. The top riser lifts you to the level of the floor above, and that floor is already a walking surface, it does not need a tread cut into the stringer.
| Requirement | Limit | Code section |
|---|---|---|
| Maximum riser height | 7¾ in (196 mm) | R311.7.5.1 |
| Minimum riser height | 4 in | R311.7.5.1 |
| Minimum tread depth | 10 in (254 mm) | R311.7.5.2 |
| Maximum riser variation within a flight | ⅜ in | R311.7.5 |
| Minimum headroom (nosing to ceiling) | 6 ft 8 in | R311.7.2 |
| Minimum stair width | 36 in clear | R311.7.1 |
| Handrail height | 34–38 in | R311.7.8.1 |
| Guard opening (baluster gap) | Under 4 in sphere | R312.1.3 |
The IRC is a model code: most US jurisdictions adopt it, often with local amendments. Check with your building department before you cut.
Two rules of thumb show up in every carpentry reference, and both are conventions rather than legal requirements. The first is riser + tread = 17 to 18 inches. A 7-inch riser with an 11-inch tread gives 18 and feels generous; a 7½-inch riser with a 10-inch tread gives 17½ and is the practical code minimum.
The second is 2 × riser + tread ≈ 24 to 25 inches, which describes how far your foot travels rather than how high it climbs. A 7½-inch riser with a 10-inch tread gives 25 and sits at the top of the range.
Neither rule overrides the code. You can satisfy both and still fail the ⅜-inch uniformity rule, which is the one that actually causes falls: a single shallow step at the top of a flight is where people trip, because the body has already learned the rhythm.
The riser and tread limits, the ⅜-inch uniformity rule, the headroom and width minimums and the guard-opening rule all come from the International Residential Code, Section R311.7 (stairways) and R312 (guards), as published by the International Code Council and reproduced in the IRC's Chapter 3 building planning provisions. The riser-plus-tread and 2R+T comfort rules are traditional carpentry conventions, presented here as guidance rather than code.
Divide the total rise in inches by your maximum riser height and round up. A 108-inch rise divided by 7.75 gives 13.94, which rounds up to 14 risers. Rounding up is the safe direction, it guarantees no single riser is taller than the maximum.
7¾ inches under the International Residential Code (R311.7.5.1), with a 4-inch minimum. The tread must be at least 10 inches deep (R311.7.5.2). Most US jurisdictions adopt the IRC, but check your local amendments, because some are stricter.
It is a traditional guideline rather than a code rule: risers should be no taller than about 7 inches and treads at least 11 inches deep. It is a comfortable target for a main interior stair, but the legal limits are 7¾ inches and 10 inches.
The top riser brings you up to the level of the floor above, and that floor is already a walking surface. It does not need a separate tread cut into the stringer, so a 14-riser stair has 13 treads.
Because uneven steps cause falls. The IRC caps the variation between any two risers in a flight at ⅜ inch. People climb stairs by rhythm, and a single short or tall step breaks that rhythm, which is why measuring to the finished floor matters so much.