# Stair Tread & Riser Calculations

Standards for safe stairs are a major preoccupation for building regulations. Falls on stairs kill 4,000 people a year in the United States, and another 2 million people are injured. The riser-tread relationship is a significant factor in stair safety. Stair dimensions are calculated using a ratio for the proportion of the riser to the tread that is determined to be safe and comfortable. Interior stairs have a different ratio than exterior stairs.

The dimensions of the tread and riser determine the angle of a staircase. Treads and risers are calculated to meet a ratio so that as the riser becomes steeper, the tread decreases in width. Stair dimensions are regulated in building codes, expressed in terms of minimum and maximum tolerances. In 2003 the International Code Council recommended that treads shouldn't exceed 10 inches and risers shouldn't be higher than 7 3/4 inch. Building regulations are variable, but don't deviate much from that standard.

## Interior Stairs Tread and Riser Ratio

The typical interior stair dimension is calculated as the formula, riser (R)+ tread (T) = 17 inches minimum, or 18 inches maximum, not including the nosing. The nosing is the projecting edge of a tread. Risers can be too low and treads too long for comfortable use of the stairs; a riser that's 7 inches is considered an ideal height with a 10-inch wide tread. The riser may be set to a different height within the maximum, but the calculations are based on the same tread-riser ratio.

## Exterior Stairs Tread and Riser Ratio

Exterior stairs are built to a different ratio than interior stairs. Outdoor stairs are viewed as potentially more hazardous because of rain and snow. A riser higher than 7 inches is not recommended outside, though interior stair dimensions can be higher than that. The ratio for exterior stairs is two risers + the tread = 26 inches. The optimum riser for exterior stairs is 6 1/2 inches, making the tread 13 inches wide.