Stair Calculator: Rise, Run, and Stringer Length
Designing stairs that are both comfortable and safe requires getting the right balance between rise (step height) and run (tread depth). Our stair calculator helps you find the optimal number of steps, riser height, tread depth, and stringer length based on your floor-to-floor height.
Basic Stair Terms
- Riser — the vertical part of a step (height)
- Tread / Run — the horizontal part you step on (depth)
- Nosing — the front edge of the tread that overhangs the riser below
- Total rise — floor-to-floor vertical height
- Total run — total horizontal length of the stairs
- Stringer — the diagonal structural support for the stairs
- Landing — a flat platform breaking a flight of stairs
Calculating Steps
Start with total rise (floor to floor). Decide your ideal riser height (typically 7 inches):
Round to the nearest whole number, then divide total rise by the number of risers to get the actual riser height. Number of treads = number of risers − 1.
The Comfort Rule (2R + T)
The classic stair design rule of thumb:
Stairs that fall in this range feel natural to walk up and down. A 7" rise with a 10" tread gives 24" — right in the sweet spot.
Building Code Guidelines
Typical IRC (International Residential Code) requirements:
- Maximum riser height — 7 ¾ inches
- Minimum tread depth — 10 inches (with nosing)
- Nosing projection — ¾" to 1 ¼"
- Maximum riser variation — ≤ 3/8" between tallest and shortest riser
- Minimum headroom — 6' 8"
- Handrail — required for stairs with 4+ risers
Stringer Length
The stringer is the diagonal support. Calculate using the Pythagorean theorem:
Example: 9 ft Ceiling
9 ft floor-to-floor (108 inches), 10" treads, 1" nosing:
- Ideal rise: 7"
- Number of risers: 108 ÷ 7 = 15.4 → 15 risers
- Actual riser height: 108 ÷ 15 = 7.2"
- Treads: 15 − 1 = 14 treads
- Total run: 14 × (10 + 1) = 154"
- Stringer: √(108² + 154²) ≈ 188"
FAQ
What is a comfortable stair angle?
30-35 degrees is most comfortable. Steeper stairs (40°+) feel more like ladders.
How many steps per floor?
For an 8 ft ceiling: roughly 14-16 risers. For 9 ft: 16-17 risers.
When do I need a landing?
For residential stairs, a landing is typically required if the vertical rise exceeds 12 ft between floors.
Note: Always verify with your local building code. Requirements vary by jurisdiction.