The formula is W = 500 × (L × N ÷ (N − 1) + 12 × N + 36), where W is the maximum weight in pounds on the axle group, L is the distance in feet between the outer axles of the group and N is the number of axles in the group.
The result is rounded to the nearest 500 pounds. It sits on top of the 20,000 pound single axle and 34,000 pound tandem limits and the 80,000 pound gross limit. Spreading axles farther apart raises the weight a group may carry, which is why trailer axle position matters on a scale.
Regulation: 23 CFR 658.17.