The Federal Bridge Formula limits the weight on groups of axles in order to reduce the risk of damage to highway bridges. Allowable weight depends on the number of axles a vehicle has and the distance between those axles. Some definitions are needed to use the Bridge Formula correctly.
- Gross weight – The weight of a vehicle or combination and any load thereon. The Federal gross weight limit on the Interstate System is 80,000 pounds.
- Single axle weight – The total weight on one or more axles whose centers are not more than 40 inches apart. The Federal single-axle weight limit on the Interstate System is 20,000 pounds.
- Tandem-axle weight – The total weight on two or more consecutive axles more than 40 inches but not more than 96 inches apart. The Federal tandem-axle weight limit on the Interstate System is 34,000 pounds.
