Stair Riser Calculator

Stair construction must balance comfort, safety, and building code requirements. The two critical measurements are the riser height (vertical) and tread depth (horizontal).

Key formulas:

• Number of risers = Total rise / Desired riser height (round to nearest whole number)
• Actual riser height = Total rise / Number of risers
• Total run = (Number of risers − 1) × Tread depth
• Stringer length = √(Total rise² + Total run²)

Building code requirements (IRC - International Residential Code):

• Minimum riser height: 4 inches (102 mm)
• Maximum riser height: 7.75 inches (196 mm)
• Minimum tread depth: 10 inches (254 mm)
• Riser variation: No more than 3/8 inch (9.5 mm) difference between tallest and shortest riser
• Maximum rise in one flight: 151 inches (3835 mm) between floors or landings
• Minimum headroom: 6 feet 8 inches (2032 mm)
• Minimum stair width: 36 inches (914 mm)

The flight limit is the one that surprises people. IRC R311.7.3 will not let a single unbroken flight climb more than 151 inches, so a 14-foot floor-to-floor needs a landing partway up no matter how good the riser height comes out. This calculator checks it.

The comfort rule: The “rule of 17-18” states that the riser height plus tread depth should equal 17 to 18 inches. For example: 7" riser + 11" tread = 18" (comfortable).

Another comfort formula is: 2 × Riser + Tread = 24 to 25 inches. This is widely used by architects and builders across Europe and North America.

Common comfortable dimensions:

• 7" riser with 11" tread (residential standard)
• 7.5" riser with 10" tread (steeper but code-compliant)
• 6" riser with 12" tread (gentle, ideal for elderly)

Stringer considerations: A standard 2×12 stringer works for most residential stairs.

Two separate things get muddled here. The 151-inch flight limit above is code, and it is not negotiable. A rule of thumb about breaking long flights at 13 or 14 risers is advice, usually about how far you want to fall and how a long unsupported stringer behaves, and no inspector will cite you for it. This page reports the code limit as pass or fail and leaves the rule of thumb to you.

Lumber comes in even lengths, so a stringer is cut from a 2×12 in 8, 10, 12, 14, 16 or 20 feet. The calculator rounds up to the next size you can actually buy rather than to the next whole foot.