Risers for common floor heights (IRC)
| Total rise | Risers | Riser height | Run at 10 in treads |
|---|---|---|---|
| 30 in (2' 6") | 4 | 7.50 in (7 1/2) | 30 in |
| 36 in (3' 0") | 5 | 7.20 in (7 3/16) | 40 in |
| 42 in (3' 6") | 6 | 7.00 in (7) | 50 in |
| 48 in (4' 0") | 7 | 6.86 in (6 7/8) | 60 in |
| 54 in (4' 6") | 7 | 7.71 in (7 11/16) | 60 in |
| 60 in (5' 0") | 8 | 7.50 in (7 1/2) | 70 in |
| 72 in (6' 0") | 10 | 7.20 in (7 3/16) | 90 in |
| 84 in (7' 0") | 11 | 7.64 in (7 5/8) | 100 in |
| 96 in (8' 0") | 13 | 7.38 in (7 3/8) | 120 in |
| 108 in (9' 0") | 14 | 7.71 in (7 11/16) | 130 in |
| 120 in (10' 0") | 16 | 7.50 in (7 1/2) | 150 in |
| 144 in (12' 0") | 19 | 7.58 in (7 9/16) | 180 in |
Code limits
| Rule | Residential (IRC) | Commercial (IBC) |
|---|---|---|
| Riser height | 7¾ in max | 4 in min, 7 in max |
| Tread depth | 10 in min | 11 in min |
| Variation within a flight | ⅜ in max | ⅜ in max |
| Nosing projection | ¾ in to 1¼ in (treads under 11 in) | 1¼ in max |
| Rise of one flight | 151 in (12 ft 7 in) max | 12 ft max |
| Handrail | 4 or more risers | Both sides, most stairs |
| Headroom | 6 ft 8 in min | 6 ft 8 in min |
IRC section R311.7 (stairways); IBC section 1011. States and cities adopt different editions and often amend stair rules — confirm with your building department.
How the numbers are worked out
- Risers = total rise ÷ maximum riser height, rounded up. Every riser in a flight gets the same height.
- Treads = risers − 1 for a straight stair landing on the upper floor.
- Total run = treads × tread depth. Stringer length = √(rise² + run²), before trimming for the floor and nosing.
- 2R + T of about 24–25 in is a comfort rule of thumb, not a code requirement. Most comfortable stairs land between about 30° and 37°.
Frequently asked questions
How many stairs do I need?
Divide the total rise (floor to floor) by the maximum riser height and round up. Under the IRC the maximum riser is 7¾ in, so a 108 in rise needs 14 risers of about 7.71 in each.
Why are there fewer treads than risers?
In a straight stair the upper floor acts as the last step, so a stair with 14 risers has 13 treads. Total run is the number of treads times the tread depth.
What is the maximum rise of one flight?
The IRC limits a single flight to 151 in (12 ft 7 in) of vertical rise. Above that, add a landing. The commercial IBC limit is 12 ft.
Is the 2R + T rule a code requirement?
No. Twice the riser plus the tread at about 24 to 25 in is a comfort rule of thumb. The code only sets the maximum riser and minimum tread.
Does my local code use these numbers?
Most US jurisdictions adopt the IRC, but states and cities can amend it — some allow taller risers or shorter treads. Check with your building department before you cut stringers.