Stair Calculator — Rise, Run & Stringer Length

How to Build Stairs: The Complete Guide

Building stairs correctly is one of the most math-intensive tasks in construction. Get the calculations wrong and you'll end up with uneven risers, a code violation, or worse — a safety hazard. This guide walks you through every step, from measuring total rise to cutting stringers.

Step 1: Measure Total Rise Accurately

Total rise is the vertical distance from the finished floor at the bottom to the finished floor at the top. This is the most critical measurement — every other calculation depends on it.

• Measure from finished floor to finished floor, not subfloor to subfloor.
• Account for flooring thickness if it hasn't been installed yet.
• Measure at the exact location where the stairs will be.
• Double-check by measuring from multiple points — floors are rarely perfectly level.

Step 2: Calculate Number of Risers

Divide total rise by your target riser height (7.25 inches is ideal). Round to the nearest whole number. Then divide total rise by that number to get the exact riser height.

Example: 108 inches ÷ 7.25 = 14.9 → round to 15 risers. Actual riser height: 108 ÷ 15 = 7.2 inches.

Step 3: Determine Total Run

Total run = number of treads × tread depth. Remember: you always have one fewer tread than risers (the top floor acts as the last tread).

Example: 15 risers = 14 treads. At 10 inches each: 14 × 10 = 140 inches total run (11 feet 8 inches).

IRC Building Code Requirements (2021)

Requirement	Minimum	Maximum
Riser Height	—	7¾ inches (196mm)
Tread Depth	10 inches (254mm)	—
Stair Width	36 inches (914mm)	—
Headroom	80 inches (2032mm)	—
Nosing	¾ inch	1¼ inch
Riser Variation	—	⅜ inch between any two risers
Handrail Height	34 inches	38 inches

The Comfort Rule: 2R + T = 24–25

The most comfortable stairs follow this formula: (2 × Riser Height) + Tread Depth = 24 to 25 inches. A 7.25-inch riser with a 10.5-inch tread gives you 25 — right in the sweet spot. Stairs that feel "too steep" or "too shallow" almost always violate this rule.

How to Cut Stair Stringers

1. Use 2×12 lumber (pressure-treated for exterior stairs). After cutting, you need at least 3.5 inches of solid wood remaining at the narrowest point.
2. Set a framing square to your riser height and tread depth. Use stair gauges to lock the measurements.
3. Mark each step on the stringer, starting from one end.
4. Cut the bottom of the first stringer to account for tread thickness (typically subtract 1–1.5 inches from the first riser).
5. Test-fit the first stringer before cutting the rest. Check that the first and last riser heights are equal.
6. Use the first stringer as a template for the remaining ones.

How Many Stringers Do You Need?

Stair Width	Stringers Needed
Up to 24 inches	2 stringers
24–36 inches	3 stringers
36–48 inches	3–4 stringers
Over 48 inches	4+ stringers (every 16 inches)

Common Stair-Building Mistakes

• Not accounting for finish flooring: If the top floor gets ¾-inch hardwood but you measured to the subfloor, your last riser will be ¾ inch too short.
• Forgetting the bottom riser adjustment: The bottom riser must be shortened by the tread thickness, or the first step will be taller than the rest.
• Uneven risers: Even a ¼-inch variation creates a noticeable trip hazard. Measure and cut precisely.
• Using 2×10 stringers: 2×12s give you the required 3.5 inches of solid wood after cutting. 2×10s only work for short risers and shallow treads.