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

There are two comfort rules in circulation and this page reports both, because a stair can satisfy one and miss the other. Riser + tread should land between 17 and 18 inches. 2 × riser + tread should land between 24 and 25 inches. A 7 inch riser with an 11 inch tread gives 18 and 25, which is why that pair turns up on so many staircases.

Code Requirements (IRC, US Residential):

That last row is the one people miss. IRC R311.7.3 caps a single flight at 151 inches of vertical rise, so anything taller than 12 ft 7 in has to be broken by a landing. It is not a comfort guideline, it is a hard limit, and a 14-foot rise fails it no matter how good the riser height looks.

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