The deflection ratio expresses the maximum amount a member is allowed to bend and still remain in compliance with local building codes. BC Framer® software calculates the amount that each member will deflect based on the length of the member, the loads, and the bending characteristics of the selected product.

The deflection ratio is often referred to as the "L over" value, because it is expressed as "L/N", where "L" is the span converted to inches and "N" is a number determined by the local building codes.

In the U.S., this value is calculated in inches. For example, a member limited to a deflection ratio of L/300 is only allowed to deflect downward by one-three hundredths of its length. Therefore, a beam 16 feet (192 inches) long should not deflect more than 192/300, or 0.64 inches.