Stair Calculator
Calculate stair dimensions including risers, treads, total run, and stringer length based on building codes.
The total vertical height from the lower floor to the upper floor
Ideal range: 7″ to 7.75″. IRC max is 7-3/4″
IRC minimum is 10″. Standard is 10-11″
IRC minimum is 36″ for residential stairs
About This Calculator
This stair calculator helps you determine the correct dimensions for building a staircase that meets residential building codes. Enter the total rise (the vertical height between floors) and the calculator will determine the number of risers, actual riser height, total run, and stringer length.
Building codes (IRC):The International Residential Code sets maximum riser height at 7-3/4 inches (7.75″) and minimum tread depth at 10 inches. The minimum stair width for residential stairs is 36 inches. All risers in a flight must be within 3/8″ of each other in height to prevent tripping hazards.
The comfort formula:The rule of 2R + T = 24 to 25 inches is a widely used guideline for comfortable stairs. “R” is the riser height and “T” is the tread depth. A common comfortable combination is a 7.5″ riser with a 10″ tread (2 × 7.5 + 10 = 25″). Stairs outside this range may feel too steep or too shallow.
Stringer calculation:The stringer is the diagonal board that supports the treads and risers. Its length is calculated using the Pythagorean theorem: stringer = √(total rise² + total run²). When purchasing lumber for stringers, add several inches to the calculated length for cutting allowance.
Headroom:Building codes require a minimum of 6 feet 8 inches (80″) of headroom measured vertically from the tread nosing to any overhead obstruction. When planning stairs in an existing structure, check headroom at every point along the staircase, especially at the top where the floor opening may limit clearance.