Stair Stringer Calculator
Calculate rise, run, stringer length, angle, and cut dimensions for any staircase.
Step-by-Step Cut Dimensions
| Step # | Unit Rise (in) | Unit Run (in) | Cumul. Rise (in) | Cumul. Run (in) |
|---|
How to Use This Stair Stringer Calculator
Enter the total rise (vertical height from one floor to the next), the number of steps (risers), and the tread depth (horizontal run per step). The calculator instantly returns stringer length, unit rise/run, angle, and a full cut schedule for each step. You can also enter tread thickness, stringer board width, and minimum remaining board width to check if your lumber is adequate.
Why This Matters
Getting stair geometry right is one of the most critical steps in any deck, basement, or interior stair build. A miscalculated rise or run creates stairs that feel awkward at best and are a trip hazard at worst. Building codes in most jurisdictions require unit rise between 4″ and 7¾″ and unit run of at least 10″, with a combined "rule of 17–18" for comfort (rise + run should total 17–18 inches).
For a typical 9-foot basement: 108 inches ÷ 14 steps = 7.71″ rise, which is code-compliant. But push it to 13 steps and each riser becomes 8.3″ — a violation in most areas. The stringer length also dictates your lumber purchase: a common 13-step deck stair needs a stringer roughly 14–15 feet long, meaning you'll need 16-foot 2×12 stock.
The cut geometry matters for the saw operator too. Knowing the exact angle (typically 32–40° for comfortable stairs) lets you set your framing square or circular saw bevel correctly. This tool removes the guesswork.
How It's Calculated
The core stringer formulas are:
- Unit Rise = Total Rise ÷ Number of Risers
- Total Run = Unit Run × (Number of Risers − 1) (one less tread than risers)
- Stringer Length = √(Total Rise² + Total Run²)
- Angle = arctan(Total Rise ÷ Total Run)
- Min Board Remaining = Stringer Width − √(Unit Rise² + Unit Run²)
The tread thickness adjustment: the bottom riser is reduced by one tread thickness so the first step feels the same height as all others after treads are installed.
Tips & Common Mistakes
- Forget the tread thickness adjustment: If you don't subtract tread thickness from the bottom riser cut, your bottom step will be taller than the rest — a classic and dangerous error.
- Use 2×12 stock: Always start with 2×12 (actual 11.25″) for stringers. Thinner stock may not leave enough material below the cut notches.
- Check the 3.5″ rule: Building codes typically require at least 3½″ of solid wood remaining below the deepest notch on a cut stringer. This calculator checks that for you.
- Verify code before you build: Code-compliant rise is 4″–7¾″ and run is 10″+ per IRC. Always check your local building authority.
- Account for landing: The total run does NOT include the landing platform. Your deck or slab must be sized separately.
Frequently Asked Questions
How many stringers do I need for a staircase?
For stairs up to 36″ wide, two stringers (one on each side) are sufficient. For widths between 36″ and 48″, add a center stringer. Beyond 48″, space stringers no more than 16″ apart across the width. Most residential deck stairs use three stringers for a standard 36″ width.
What is the ideal rise and run for comfortable stairs?
The most comfortable stairs follow the "Rule of 17–18": unit rise + unit run should equal 17 to 18 inches. A 7″ rise with a 11″ run (total 18) is considered ideal for most people. Steeper stairs save horizontal space but are harder on knees; shallower stairs require more floor space.
Why does my stringer length differ from a direct measurement on my framing square?
The calculated stringer length is the theoretical hypotenuse from the top plumb cut to the bottom level cut. The actual board you purchase should be slightly longer (add 6–12 inches) to allow for the seat cut at the bottom and top attachment point. Always add waste when ordering lumber.
Can I use this for indoor stairs?
Yes. Interior stairs typically have a slightly lower rise (6.5–7.5″) and longer run (10–12″) than deck stairs for comfort. The formulas are identical — just enter your floor-to-floor height and desired number of steps. Interior stairs often use a closed-riser design but the stringer geometry is the same.