Stair Calculator Guide: Risers, Treads & Stringer Layout (IRC)

Stairs are the one place in framing where the math is also the law. Risers and treads must satisfy the building code and feel right underfoot, and both come from two measurements: total rise (floor to floor) and total run (available horizontal distance).

Solving the riser count

Divide the total rise by a comfortable target 7.5 inches is the standard starting point and round to a whole number of risers. Then divide the total rise by that count to get the exact unit rise. A 105-inch total rise divides by 7.5 into exactly 14 risers at 7.5 inches each.

The number of treads is always risers minus one, because the top tread is the upper floor or landing itself. Those 14 risers need 13 treads cut into the stringers.

The three IRC numbers

The International Residential Code (R311.7) sets hard limits that every stair in a dwelling must meet. These are maximums and minimums, not targets:

• Riser height: no more than 7¾ inches
• Tread depth: no less than 10 inches
• Comfort rule: 2 × riser + tread should land between 24 and 25 inches
• Riser/tread uniformity: the largest and smallest in a flight must be within ⅜ inch

Stringers and stock

Stringers are cut from 2×12s, and the total stringer length rounds up to standard stock 10, 12, 14, or 16 feet. Stairways wider than 36 inches need a fourth stringer down the middle to keep treads from flexing. Always check headroom (6 ft 8 in minimum), stairway width, landings, and handrails on site the riser/tread math is only part of a compliant stair.