Federal bridge formula calculator
Enter the number of axles and the distances between them to see the maximum weight each group of axles may carry on the Interstate System. Add your scale weights to check them.
The formula
23 CFR 658.17(e) sets the Bridge Gross Weight Formula: W = 500 [ LN/(N − 1) + 12N + 36 ], where W is the overall gross weight on any group of two or more consecutive axles to the nearest 500 pounds, L is the distance in feet between the extreme axles of the group, and N is the number of axles in the group.
The other federal limits it works with
- 20,000 lb on any one axle (658.17(c)).
- 34,000 lb on a tandem: axles more than 40 and not more than 96 inches apart (658.17(d), 658.5).
- 80,000 lb gross, whatever the formula gives (658.17(b)).
- Two consecutive sets of tandems may carry 34,000 lb each if the first and last axles are at least 36 feet apart (658.17(e)).
- A vehicle with a working idle-reduction unit may be allowed up to 400 lb more, with certification of the unit’s weight (658.17(n)).
What it does not cover
These are the federal limits for the Interstate System and reasonable access to it. States set limits on other roads, some keep higher grandfathered limits, and oversize or overweight permits for loads that cannot be divided are issued by the states. Check the state’s rules for your route. The calculator uses the distances exactly as you enter them.