Stair Calculator: Rise, Run, and Stringer Dimensions for Any Staircase
Calculate stair rise, run, number of steps, stringer length, and total rise for interior or exterior stairs. Get code-compliant dimensions before you build.
Stairs look deceptively simple until you're cutting stringers and realize one step is noticeably shorter than the rest. Stair geometry is one of those things where the math has to be right — unequal step heights are a tripping hazard and a code violation.
The CalcHub Stair Calculator calculates the number of steps, individual rise and run dimensions, stringer length, and total horizontal run for any total rise you enter.
The Key Measurements
Total Rise: Vertical distance from finished floor to finished floor (or ground to landing). This is fixed — it's the space you're working with. Rise per step: Vertical height of each individual step. Code allows a maximum of 7¾" (IRC). Run per step: Horizontal depth of each tread. Code minimum is 10" (IRC). Comfortable stairs typically have 10–11" run. Stringer length: The diagonal length of the structural stringer board. Total run: Horizontal footprint of the entire staircase.Code Requirements (IRC Residential)
| Dimension | Minimum | Maximum | Recommended |
|---|---|---|---|
| Rise per step | 4" | 7¾" | 7–7.5" |
| Run per tread | 10" | — | 10–11" |
| Headroom clearance | 6'8" | — | 6'8"+ |
| Width (residential) | 36" | — | 36–42" |
| Handrail height | 34" | 38" | 36" |
Calculation Example
Total rise: 8 ft 9 in (105 inches)Step 1: Find number of risers
105 ÷ 7.5 (target rise) = 14 risers (round to nearest whole number)
Step 2: Actual rise per step
105 ÷ 14 = 7.5 inches per riser — perfect
Step 3: Calculate run
Using 10.5" run per tread, 13 treads (always 1 fewer tread than risers):
13 × 10.5" = 136.5 inches (11 ft 4.5 in) total run
Step 4: Stringer length
√(105² + 136.5²) = √(11,025 + 18,632) = √29,657 = 172 inches (14 ft 4 in)
Your stringer boards need to be at least 14'4" long.
Stringer Sizing
Stringers are typically cut from 2×12 lumber. After notching for the treads and risers, you need at least 3.5 inches of solid wood remaining at the narrowest point of the notch (the "effective depth"). If your rise and run combination eats too deep into the 2×12, you may need to sister a second stringer alongside the notched one, or use a deeper engineered board.
| Total Rise | Typical Stringer Length Needed |
|---|---|
| 3 ft (deck step) | 4–5 ft |
| 7 ft (interior floor-to-floor) | 10–11 ft |
| 9 ft (common interior) | 12–14 ft |
| 10 ft | 14–16 ft |
Platform vs. Straight Stairs
If your total run is more than about 14 feet, building codes and practical design often call for a landing midway. A landing breaks the stair into two runs and reduces the fall distance in case of a tumble. The landing must be at least as wide as the stair width and at least 36" deep.
What if my total rise doesn't divide evenly into whole steps?
It won't, and that's fine. The goal is equal-height steps — calculate rise per step to several decimal places (e.g., 7.32") and cut every step to that exact dimension. The calculator gives you the precise rise and run so you can set your saw accurately.
Can I make stairs narrower than 36" for a deck or outdoor use?
Deck stairs connecting to grade often have different local requirements — some allow 36", others require 36" minimum. Commercial occupancies require 44" minimum. Always check your local building code.
How much does stringer lumber cost?
16-foot 2×12 treated lumber runs roughly $40–$60 per piece depending on market. You need one stringer for every 16–24 inches of stair width (minimum 3 for a 36" wide stair).
Related Tools
- Lumber Calculator — estimate board feet for treads and stringers
- Deck Calculator — if the stairs connect to a deck
- Concrete Calculator — footing at the bottom of the stair