Loading Calculator...
Please wait a moment
Please wait a moment
Calculate optimal stair dimensions with code compliance checking. Follow the 2R+T rule and building codes (IRC, IBC, or custom standards) to design safe and comfortable stairs.
Let us know if this calculator was useful
1545 people found this calculator helpful
The 2R+T rule is a comfort and safety formula for stair design, where R is the riser height and T is the tread depth. This formula helps ensure that stairs are proportional and comfortable to climb.
The standard for residential stairs is: 2R + T = 24 to 25 inches
For commercial stairs, the range may be slightly different. This formula creates a natural stride that feels comfortable for most people climbing stairs.
Where:
R = Riser height (inches)
T = Tread depth (inches)
Given: Total rise = 120 inches
Number of risers = 120 / 7.75 = 15.48 ≈ 15
Riser height = 120 / 15 = 8.00"
2R + T = 24
Tread = 24 - (2 × 8) = 8"
Result: 15 risers, 14 treads, 8" riser, 8" tread
Most common residential formula
Blondel formula (European standard)
Product formula for comfort zone
| Code / Type | Riser Height | Tread Depth | 2R+T |
|---|---|---|---|
| IRC Residential | 7.0" - 8.25" | 10.0" - 14.0" | 24" |
| IBC Commercial | 7.0" | 11.0" | 24" |
| ADA Accessible | 7.0" max | 11.0" min | 24" |
| Steep Stairs | up to 9.5" | 7.5" - 9.0" | 24" |
A riser is the vertical part of a step, while a tread is the horizontal part you step on. The riser height is the vertical distance between one step and the next, and the tread depth is how far back each step extends.
The 2R+T rule ensures stairs are proportional and comfortable to climb. It creates a natural stride that accommodates most people's walking patterns and reduces fatigue when climbing or descending stairs.
For residential stairs, the ideal riser height is typically 7 to 8 inches. Commercial stairs may use 7 inches exactly. Steeper stairs (9-10 inches) are used in tight spaces but are less comfortable.
Building codes typically require a minimum tread depth of 10 inches for residential stairs and 11 inches for commercial or public stairs. This ensures safe footing for people of different shoe sizes.
Total rise is measured from the finished floor surface below to the finished floor surface above. Measure vertically at the point where the stair will meet each floor level, including any flooring materials.
Adjust your stair design by changing the number of steps, riser height, or tread depth. Most building codes allow some flexibility within specified ranges. Consult your local building department for specific requirements in your area.
No, building codes require all risers in a single flight of stairs to be uniform, with a maximum variation of only 1/4 inch. This is a critical safety requirement to prevent tripping hazards.
Yes, because the top landing acts as the final tread. If you have 15 risers, you have 14 treads plus the top landing. Count landing as the final step surface.
Check out similar construction calculators