Stair Stringer Calculator
Calculate stair stringer dimensions — rise per step, run per step, stringer length, angle, total run, and comfort formula. Includes IRC code compliance check and printable cut diagram.
Cut List
| Item | Qty | Size |
|---|---|---|
| Stringer boards | 2 | 2×12 × 16' |
| Treads | 13 | 36" wide × 11" deep (with 1" nose) |
| Risers (optional) | 14 | 36" wide × 7 3/4" tall |
Stringer Side Profile
How It Works
1. Enter total rise — measure the vertical distance from the finished lower floor to the finished upper floor in inches.
2. Set run per step — the horizontal depth of each tread (10" is the IRC minimum and most common).
3. Choose stringer material — 2×12 is standard. The calculator checks that enough wood remains after notching.
4. Review results — the calculator divides total rise evenly across risers, computes stringer length using the Pythagorean theorem, and checks all dimensions against IRC building code limits.
The comfort formula (2R+T) evaluates whether the stairs will feel comfortable. Values between 24" and 25" are ideal. The SVG diagram shows the stringer profile with notch cuts labeled.
Frequently Asked Questions
How do I calculate stair stringer length?
Stringer length is the hypotenuse of a right triangle formed by the total rise (vertical height) and total run (horizontal distance). Use the formula: stringer length = √(total rise² + total run²). For example, a stair with 108" rise and 130" run needs a stringer of about 169" (14.1 ft).
What size lumber do I need for stair stringers?
Most stair stringers use 2×12 lumber. After cutting the notches for treads and risers, you need at least 3.5" of solid wood remaining below the notch line. A 2×10 can work for stairs with a rise under 6" per step, but 2×12 is the standard recommendation for residential stairs.
How many stringers do I need?
Use 2 stringers for stairs up to 36" wide. For stairs wider than 36", add a center (third) stringer. Some codes require a center stringer for any stair over 30" wide. For very wide stairs (over 48"), you may need 4 stringers.
What is the maximum stair rise per step allowed by code?
The IRC (International Residential Code) limits riser height to 7-3/4" (197 mm) maximum. The minimum tread depth is 10" (254 mm). The maximum variation between any two risers in a flight is 3/8". These requirements are found in IRC Section R311.7.5.
What is the comfort formula for stairs?
The comfort formula is 2R + T, where R is the riser height and T is the tread depth. The ideal range is 24" to 25". For example, a 7" rise with a 10" tread gives 2(7) + 10 = 24", which is comfortable. Stairs outside this range feel either too steep or too shallow.