How to Calculate Stair Rise and Run (Step-by-Step Formula Guide)

Every staircase depends on two numbers: stair rise and run. Rise is how far up you climb. Run is how far forward you travel to get there. Once you know these two numbers, the rest of the layout follows simple math. This guide shows you how to calculate stair rise and run by hand, step by step, with a real example.

Step 1: Measure Total Rise

Total rise is the exact vertical distance from the bottom floor to the top floor or landing. Measure after the flooring is installed, not from bare subfloor. Use a laser level, or a straight board and a level if the floor is not built yet. A half-inch error here throws off every step above it.

Step 2: Find the Number of Risers

Divide total rise by your target riser height. Most codes cap riser height at 7¾ inches. Builders typically aim for 7 to 7¼ inches instead, for extra comfort.

Number of risers = Total rise ÷ Target riser height

Round the result to the nearest whole number. You cannot build a fractional step. Example: 108 inches of total rise divided by 7.5 inches equals 14.4. Round that to 14 risers.

Step 3: Recalculate the Exact Riser Height

Divide total rise by your rounded riser count. This gives the real, exact riser height every step must share.

Exact riser height = Total rise ÷ Number of risers
108 in. ÷ 14 = 7.71 in. per riser

Uniform risers matter more than hitting a specific target number. The IRC allows no more than ⅜ inch of variance between the tallest and shortest riser in a flight. Inconsistent steps cause most stair trips.

Step 4: Calculate Total Run

Total run equals the number of risers minus one, multiplied by tread depth. There is always one fewer tread than risers, because the top landing acts as the final step.

Total run = (Risers − 1) × Tread depth
(14 − 1) × 10 in. = 130 in.

Step 5: Get the Stringer Length

The stringer is the diagonal board that carries the treads. It forms the hypotenuse of your rise and run triangle, so it follows the Pythagorean theorem.

Stringer length = √(Total rise² + Total run²)
√(108² + 130²) = √28,564 ≈ 169 in.

Step 6: Check the Comfort Rule

Builders have used a simple comfort check for centuries. Multiply riser height by two, then add tread depth. The result should land between 24 and 25 inches. Our example checks out: (2 × 7.71) + 10 = 25.4 inches. That feels comfortable to climb, which confirms your stair rise and run numbers are dialed in.

Common Mistakes With Stair Rise and Run

Most calculation errors start with measuring at the wrong point. Always measure total rise from finished floor to finished floor, not subfloor to subfloor. Skipping the recalculation in Step 3 is another common mistake. Builders sometimes use the rough target riser height instead of the exact recalculated number. That single shortcut creates one uneven step in the flight. Run the numbers twice before you cut a single stringer.

Frequently Asked Questions

What is a comfortable riser height for stairs?

Aim for 7 to 7¼ inches. Code sets 7¾ inches as the maximum, not the target.

Why does my calculation give a decimal number of risers?

Total rise rarely divides evenly by your target riser height. Round to the nearest whole number first. Then recalculate the exact riser height from that rounded count. That final number is what you build to.

Do I need to do this math by hand?

No. Our Standard Stair Calculator runs all six steps instantly and flags anything outside IRC limits. Doing the math once by hand still helps, since it shows you exactly what the calculator checks.