Under the International Residential Code, a riser can be at most 7 3/4 in tall and a tread at least 10 in deep. For a comfortable stair, aim for risers around 7 to 7 1/2 in and treads around 10 1/2 to 11 in, so that twice the riser plus the tread comes to about 24 to 25 in. Within one flight, risers and treads may not vary by more than 3/8 in.
The code limits
| Rule | IRC limit | Section |
|---|---|---|
| Riser height | 7 3/4 in maximum | R311.7.5.1 |
| Tread depth | 10 in minimum, nosing to nosing | R311.7.5.2 |
| Variation within a flight | 3/8 in maximum | R311.7.5.1, R311.7.5.2 |
| Nosing projection | 3/4 to 1 1/4 in, not needed if tread is 11 in or more | R311.7.5.3 |
| Headroom | 6 ft 8 in minimum | R311.7.2 |
| Width | 36 in minimum | R311.7.1 |
| Handrail height | 34 to 38 in above the nosings | R311.7.8 |
A stair at the code limits, 7 3/4 in by 10 in, is legal but steep. It’s what you find in many basements. Anything you use every day deserves something gentler.
Three rules of thumb for comfort
Stair designers use simple formulas that match the length of a normal stride:
- 2 × riser + tread = 24 to 25 in. The oldest rule, attributed to the French architect Blondel.
- Riser + tread = 17 to 18 in.
- Riser × tread = 70 to 75.
They agree closely in the normal range. Here’s what the first rule gives with 2R + T = 25 in:
| Riser | Tread | R + T | R × T | Angle |
|---|---|---|---|---|
| 6 1/2 in | 12 in | 18.5 | 78 | 28.4° |
| 6 3/4 in | 11 1/2 in | 18.25 | 77.6 | 30.4° |
| 7 in | 11 in | 18 | 77 | 32.5° |
| 7 1/4 in | 10 1/2 in | 17.75 | 76.1 | 34.6° |
| 7 1/2 in | 10 in | 17.5 | 75 | 36.9° |
| 7 3/4 in | 9 1/2 in | 17.25 | 73.6 | 39.2° |
The 7 1/2 in riser with a 10 in tread passes all three rules and is about the steepest comfortable combination. The 7 in riser with an 11 in tread is a classic, easy stair.
Worked example: floor to floor of 105 1/2 in
Measure the total rise from finished floor to finished floor: here 105 1/2 in.
- Divide by a target riser of 7 1/2 in: 105.5 ÷ 7.5 = 14.07, so round up to 15 risers.
- Exact riser: 105.5 ÷ 15 = 7.033 in, about 7 1/16 in.
- Treads: one fewer than risers, 14. At 10 1/2 in each, the total run is 147 in, or 12 ft 3 in.
- Comfort: 2 × 7.03 + 10.5 = 24.57 in. Right in the comfortable band.
With 14 risers instead, each would be 7 9/16 in, which is legal, and the stair would be shorter by one tread. The stair calculator runs these checks for any rise.
Measure the rise last, and to finished floors
The single most common stair mistake is building to the subfloor. If the top floor gets 3/4 in hardwood and the bottom gets tile on a mortar bed, the finished rise differs from the framing rise, and the first or last step ends up a different height from the rest. That’s both a trip hazard and a code violation, since all risers must be within 3/8 in of each other.
Outdoor and deck stairs
Deck stairs follow the same IRC rules for risers, treads and handrails. Outside, slightly shallower risers and deeper treads (around 6 to 7 in by 11 to 12 in) feel safer in wet weather, and the bottom step needs a solid, level landing rather than soft ground.