Staircase Calculator

Staircase calculation determines the number of steps (risers), the height of each step, and the horizontal depth of each tread to create stairs that are safe, comfortable, and code-compliant.

The Key Formulas:

Number of risers = Total rise / Ideal riser height (round to nearest whole number, then add one if that leaves the step above the code maximum)

Actual riser height = Total rise / Number of risers

Total run = (Number of risers − 1) × Tread depth

A useful cross-check on tread depth: 2 × riser + tread should land near 25 inches (630 mm). A 7 inch riser with an 11 inch tread gives 25, which is why that pair turns up on so many staircases.

Code Requirements (IRC — US Residential):

Worked Example:

Floor-to-floor height: 108 inches (9 feet). Target riser height: 7.5 inches. Tread depth: 10 inches.

• Number of risers: 108 / 7.5 = 14.4 → round to 14 risers
• Actual riser height: 108 / 14 = 7.71 in (under the 7.75 in maximum ✓)
• Number of treads: 14 − 1 = 13
• Total horizontal run: 13 × 10 = 130 inches (10.8 ft of floor space needed)
• Diagonal stair length: √(108² + 130²) = 169 inches (14.1 ft)

Note: There is always one fewer tread than riser (the upper floor is the final “tread”). That single missing tread is the most common arithmetic slip in stair layout, and it throws the total run off by a full step.

Practical Tips:

• All risers must be within 3/8 inch (9.5 mm) of each other: uneven risers cause falls
• The landing at top and bottom should be at least as wide as the stair width
• Use a stair gauge on a carpenter’s square to mark consistent riser and tread lines on the stringer