Bridge Formula B Calculator
Calculate the maximum gross weight allowed on any group of consecutive axles under the FHWA Federal Bridge Formula. Used by dispatchers, drivers, and permit officers on every Interstate move.
Calculate max allowable gross weight
How Bridge Formula B works
The Federal Bridge Formula (also written Federal Bridge Gross Weight Formula or just Bridge Formula B) was established by the Federal-Aid Highway Act of 1975 to protect bridges from being overstressed. It defines the maximum gross weight that any group of consecutive axles can carry on the Interstate System.
- W = maximum allowable gross weight, in pounds, rounded down to the nearest 500 lb
- L = distance in feet between the outer axles of any group
- N = number of axles in the group
The result is subject to three additional federal limits:
- 80,000 lb total gross weight on the Interstate (without a permit)
- 20,000 lb on any single axle
- 34,000 lb on any tandem (two-axle) group
States may set their own limits above or below the federal numbers for non-Interstate routes. Some states (notably Michigan, with its 164,000 lb gross) allow much higher weights with the right axle configuration.
Standard configurations
For reference, here are common tractor-trailer combinations and the gross weight Bridge Formula B allows for them:
| Configuration | Axles | L (ft) | Max W (lb)* |
|---|---|---|---|
| 3-axle straight truck | 3 | 18 | 49,500 |
| Tractor + tandem-axle trailer (5-axle) | 5 | 51.2 | 80,000 |
| Tractor + tridem-axle trailer (6-axle) | 6 | 54 | 86,000 |
| Tractor + double 28' trailers (7-axle) | 7 | 61 | 95,500 |
| Tractor + double 48' trailers (9-axle) | 9 | 85 | 119,500 |
* Bridge Formula B value rounded down to the nearest 500 lb per 23 CFR §658.17. The 5-axle / 80,000 lb row reflects the federal 80,000 lb gross cap, which is binding for a standard 5-axle tractor-trailer at 51.2 ft (the formula itself yields slightly above 80,000 at this spread, but the cap controls).
When you need to compute this
Most over-the-road dispatchers never run Bridge Formula B by hand — the truck is engineered around 80,000 lb and the rule is built in. But the formula matters in three real-world scenarios:
- Permit moves over 80,000 lb gross. Overweight permits require you to demonstrate axle weights comply with the state's bridge formula equivalent. Your permit officer will run this calculation against the trailer configuration on your application.
- Re-rigging a load for axle compliance. A load that grosses fine but exceeds Bridge Formula B on one axle group can be brought into compliance by sliding the trailer tandems or adding a booster axle. This calculator tells you how much you can shift.
- Spec'ing a heavy-haul tractor or trailer. When buying or building heavy-haul equipment, the axle spacing on every group is set by Bridge Formula B — closer spacing means lower allowable weight per group.
Stop calculating axle weights by hand.
OverSizeTMS encodes the per-state bridge formula and axle-group rules. Enter your trailer configuration once, and the platform checks every load against every state on the route automatically — flagging axle groups that need re-rigging before you dispatch.
See how it works →Related resources
- Oversize / overweight permit requirements by state — legal limits, escort thresholds, and admin code citations for all 50 states + DC
- FHWA Bridge Formula Weights brochure (original federal reference, PDF)
- 23 CFR Part 658 — federal weight and width limitations on the Interstate