Stair Calculator

Plan a Safe Staircase Before You Cut a Board

A staircase is unforgiving: get the riser height wrong by even a quarter inch and every step feels off, or worse, fails inspection. This stair calculator takes the one measurement you can always get — the total floor-to-floor rise — and turns it into a complete cutting plan: the number of steps, the exact riser height, the total horizontal run, and the diagonal stringer length you need to buy lumber for. It also checks your design against the comfort and code rules carpenters use every day.

The Four Numbers That Define Every Stair

Every flight of stairs is built from the same relationships:

Number of risers = round(Total rise ÷ Target riser) Actual riser = Total rise ÷ Number of risers Total run = (Number of risers − 1) × Tread depth Stringer length = √(Total rise² + Total run²)

The reason you round the riser count and then divide again is precision. You rarely get a clean whole number, so the calculator finds the closest sensible step count and back-solves the exact riser height every step will actually be — because all risers in a flight must be equal within about 3/8 inch.

A Worked Example: A Typical 9-Foot Floor

Say you are building stairs between two floors that are 9 feet apart, which is 108 inches of total rise. You aim for a comfortable 7-inch riser and an 11-inch tread.

• Risers: 108 ÷ 7 = 15.43, which rounds to 15 risers.
• Actual riser: 108 ÷ 15 = 7.2 inches — comfortably inside the 5–8 inch range.
• Total run: (15 − 1) × 11 = 14 × 11 = 154 inches (about 12 feet 10 inches of floor space).
• Stringer length: √(108² + 154²) = √(11,664 + 23,716) = √35,380 ≈ 188 inches.

Check the comfort rule: 2 × 7.2 + 11 = 25.4 inches, right at the top of the ideal 24–25 inch band. Now you know to buy three stringer boards at least 188 inches long (round up to the next stock length so you have room for the plumb and seat cuts), plus 14 tread boards.

Why the Comfort Range Matters

The calculator flags any actual riser outside 5 to 8 inches. Risers taller than 8 inches force an awkward high step and are a trip hazard; risers under 5 inches make the flight unnecessarily long and easy to scuff. Most residential codes cap the riser around 7.75 inches and require a minimum tread near 10 inches, so staying mid-range keeps you both safe and compliant. If your number drifts out of bounds, nudge the target riser up or down by a fraction and watch the step count and stringer length update.

Practical Tips From the Job Site

• Measure rise to finished floor. Include the thickness of any flooring you will add on top and bottom, or your top step will be off.
• Buy stringers long. Always size lumber to the next whole length above the calculated stringer so you can cut the top plumb cut and bottom seat cut cleanly.
• Keep risers equal. Inspectors measure the difference between the tallest and shortest riser; the calculator gives you one uniform value to mark with a framing square.
• Mind the headroom. A steeper stair saves floor space but can pinch headroom at the top — check at least 6 feet 8 inches of clearance.

Pair It With Your Other Estimates

Once your stair geometry is locked, you can plan the rest of the build. Use the concrete calculator to size a footing or landing pad at the base of the stairs, and the brick calculator if you are framing a masonry stairwell around the flight. With the rise, run, and stringer length in hand, your lumber order and cut list practically write themselves.