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Stair Calculator
Stair Calculator
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Stair Calculator

Calculate stair rise, run, number of steps, and check IRC building code compliance.

DIY difficulty

HardTypical time: 1–2 weekends
  • ›Stairs are structural — errors are safety hazards, not just cosmetic
  • ›IRC code compliance (riser height, tread depth) must be exact
  • ›Cutting stringer boards precisely is the hardest step

Must-have tools

Circular sawSpeed squareLevelFraming square

How to calculate stair rise, run, and step count

  1. Measure the total rise — the floor-to-floor vertical height in inches. Be precise; this single number drives everything else.

  2. The calculator divides the total rise into even steps, targeting a riser height between 7 and 7.75 inches per IRC code.

  3. Review the riser height and tread depth. The IRC requires tread depth of at least 10 inches (excluding nosing).

  4. Check the compliance indicator. If your rise/run is outside code limits, try adjusting the riser height manually to find a compliant combination.

Common uses

  • Planning interior stairs for a new construction or addition
  • Building exterior deck stairs down to a yard or patio
  • Checking whether an existing staircase meets building code

Frequently asked questions

What's the standard rise and run for stairs?
IRC code allows risers from 4 to 7.75 inches and requires treads of at least 10 inches. Most builders target 7-inch risers with 11-inch treads for a comfortable, code-compliant stair.
How do I calculate the number of steps?
Divide the total rise (floor to floor) by your target riser height, then round to the nearest whole number. Re-divide the total rise by that count to get the exact riser height per step.
How long does the stringer need to be?
The stringer length equals the square root of (total rise² + total run²). For a 9-step stair with 63 inches of rise and 99 inches of run, that's about 118 inches — nearly 10 feet.
What is the 18-inch rule for stairs?
The "18-inch rule" is that riser height + tread depth should sum to roughly 17–18 inches for a comfortable stair. A 7" riser + 11" tread = 18" — the most common comfortable combination.
How wide should residential stairs be?
IRC requires a minimum clear width of 36 inches for residential stairs. Most builders use 36–42 inches for interior stairs. Exterior deck stairs are often narrower at 36 inches.

Common mistakes to avoid

  • ✕Measuring floor-to-floor height to the subfloor — total rise must be to finished floor surface. A 1/4" error throws off every step.
  • ✕Making all stringers identical — center stringers on wide stairs are often notched differently than outer ones.
  • ✕Ignoring headroom — IRC requires 6'8" clearance above any stair nosing. Basement stairs frequently fail this.
  • ✕Not adjusting for finish flooring — if stairs are calculated before flooring is installed, the first and last risers change when flooring is added.

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Not sure whether to DIY this project?

Read our project-by-project guide on when to hire a contractor vs. do it yourself.

Read the guide →

About this calculator

Enter the total floor-to-floor rise to calculate the number of steps, individual riser height, total run, and stringer length. Includes IRC building code compliance check (riser 4–7¾", tread min 10").

All results are estimates. Verify quantities with your local supplier and confirm measurements before purchasing materials. Requirements vary by region and project type.

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