Truck Weight Limit Checker (Bridge Formula)

What the Bridge Formula Actually Does

The Federal Bridge Formula, codified at 23 U.S.C. 127, sets the maximum weight allowed on any group of consecutive axles based on how many axles are in the group and how far apart the outermost two are spread: W equals 500 times the quantity L times N divided by N minus 1, plus 12 times N, plus 36, where W is the maximum weight in pounds, L is the spread in feet, and N is the number of axles in the group.

This calculator runs that formula on the axle count and spread you enter, compares the result against the federal 80,000 pound gross cap (since the formula can sometimes compute a number higher than that cap for widely spread groups, and the lower of the two always governs), and checks your actual axle group weight against whichever limit applies.

Why This Is a Separate Check From a Flat Axle Limit

Standard flat limits (generally 20,000 lbs on a single axle, 34,000 lbs on a tandem) are a useful first-pass check, but they don't account for how close together axles are spaced. Two axles spaced very close together can be limited by the bridge formula to well below 34,000 lbs, even though that figure is the flat tandem limit, which is exactly the scenario this calculator is built to catch and the flat-limit check alone misses.

The Role of Axle Spread

Spreading axles farther apart increases the weight the bridge formula allows for the same number of axles, which is the entire engineering logic behind the formula: more spread distributes the load over more of a bridge span at once, reducing stress at any single point. This is why spreading a trailer's axles or adding a spread axle configuration can sometimes legally increase how much weight a given axle group can carry.

A Worked Example

Take a tandem axle group (N = 2) spread 4 feet apart (L = 4). The formula gives 500 times the quantity 4 times 2 divided by 1, plus 24, plus 36, which works out to roughly 34,000 lbs, close to the standard flat tandem limit. Now spread that same tandem to 8 feet: the formula allows a noticeably higher weight, showing exactly how spread, not axle count alone, drives the limit up or down.

What This Calculator Doesn't Replace

This tool checks the federal bridge formula and the 80,000 lb gross cap, but individual states can and do impose their own weight limits, permit requirements, and bridge law variations on top of the federal baseline, particularly for oversize or overweight permit loads. Always confirm against the specific state's DOT regulations for any route that pushes close to a legal limit, rather than relying on the federal formula alone.