Bridge Formula Axle Weight Checker
Enter the distance between your axle group’s outer axles and how many axles are in the group — get the maximum legal weight under the Federal Bridge Formula used on Interstate highways and in Texas.
Maximum allowable weight for this axle group
The caps that always apply (Interstate)
Passing the bridge formula does not override these caps — your legal weight is whichever limit is lowest: the formula result, the single/tandem caps, or 80,000 lbs gross.
Common combinations (from the formula, caps applied)
| Axles | Axle spacing (L) | Max weight |
|---|---|---|
| 2 (tandem) | 8 ft | 34,000 lbs — tandem cap |
| 2 | 10 ft | 40,000 lbs — two single axles |
| 3 | 20 ft | 51,000 lbs |
| 3 | 30 ft | 58,500 lbs |
| 4 | 36 ft | 66,000 lbs |
| 4 | 40 ft | 68,500 lbs |
| 5 | 36 ft | 70,500 lbs |
| 5 | 38 ft | 72,000 lbs |
| 5 | 40 ft | 73,000 lbs |
| 5 | 51 ft | 80,000 lbs — gross cap |
Why this formula exists
Bridges are damaged by weight concentrated over a short span, not just by total weight. The bridge formula forces heavy loads to spread out: the farther apart your axles are, the more weight the group may legally carry. That is why a long-wheelbase 5-axle rig can gross 80,000 lbs while the same weight on a short group is illegal.
Inner groups count too. Every group inside the truck — the drive tandem, the trailer tandem, steer-to-trailer — must pass the formula on its own. This checker tests one group at a time, so run it again for each inner group with that group’s own spacing and axle count.
Texas: Texas follows the federal bridge formula and the 20,000 / 34,000 / 80,000 caps on Interstate and National Highway System routes. Overweight permits (TxDMV) are the only legal way past these limits, and only for non-divisible loads.
Planning aid only — not legal advice. Results use the Federal Bridge Formula with results rounded to the nearest 500 lbs and the standard federal caps. Actual axle weights on a certified scale and the official FHWA / state bridge tables govern in any enforcement or permit decision.
How the bridge formula works
The federal bridge formula is the rule that sets the maximum weight an axle group can legally carry on the Interstate Highway System. It does not set one flat limit per axle. Instead, it weighs two things together: how many axles are in the group (N) and how far apart the outer axles sit (L, in feet). The formula is W = 500 × ((L × N) / (N − 1) + 12N + 36), where W is the maximum weight in pounds for the group. The checker above runs that exact calculation, then rounds the answer to the nearest 500 lbs and applies the legal caps.
A bridge is a span, and the load from a truck crosses it as the truck moves over it. When axles sit close together, their weight lands on a short stretch of the span at the same time, and the bridge has to carry that concentrated load. Spread the same axles further apart and the weight is shared across more of the structure, so the bridge can safely carry more. That is why the formula rewards spacing: add distance between the outer axles and the legal limit climbs, even though the truck and the number of axles have not changed. It is a structural rule, not a paperwork rule — it exists because short, heavy axle groups are what overload a bridge.
To use the formula, measure L as the distance in feet between the centres of the first and last axles in the group you are checking — the extremes of the group, not the distance between neighbouring axles. For a tandem, that is the distance between the two axle centres. For a five-axle trailer group, it is the distance from the centre of the first axle to the centre of the last. Measure on level ground, hub to hub, and round to the nearest foot. Enter that number and the axle count into the checker, and it works out the legal maximum for that group alone.
Tandems have their own hard limits on top of the formula. A two-axle group spaced 8 ft or less is capped at 34,000 lbs, even though the formula on its own would allow more — at exactly 8 ft the formula gives 38,000 lbs, so the cap cuts 4,000 lbs off the answer. Once the tandem spacing goes beyond 8 ft, the cap rises to 40,000 lbs. So the checker shows the lower of the formula result and the tandem cap for that spacing. If you run a spread-axle trailer, check where your spacing falls, because a few inches either side of 8 ft can change the legal limit by thousands of pounds.
The formula also has a ceiling. No axle group on the Interstate can legally exceed 80,000 lbs gross vehicle weight, and the checker applies that cap to every result. A long five-axle group at 51 ft of spacing works out to more than 80,000 lbs on the formula, but the Interstate gross limit pulls it back to 80,000 lbs. Shorten that same five-axle group to 38 ft and the formula itself is the limit, at 72,000 lbs. That cap keeps the formula from ever authorising a gross weight above the federal Interstate maximum.
This is different from a simple per-axle limit, the way many drivers first learn weight rules. A per-axle limit says each axle may carry a fixed amount, so more axles always means more weight in a straight line. The bridge formula does not work that way: two groups with the same number of axles can have very different legal limits, and a short group can be capped well below what its axles could carry on their own. Before you load, check both — the bridge formula for the group, and the GAWR for each axle — because whichever limit is lower is the one that governs. Our Hotshot Axle Weight Checker covers the per-axle side of that check.
Texas follows the federal bridge formula on Interstate highways, so the limits in this checker are the ones that apply when you are running on I-35, I-10, I-20 and the rest of the Texas Interstate network. State highways can have their own rules, and any move over the legal limits needs a permit before it rolls. If your numbers come out over the limit, run the Texas Permit Wizard to find out which permit the load needs and how to apply. Permit rules change, so always check the current requirements first.
The checker is free to use, with no sign-up, and it sits alongside the rest of the free tools on the Trucking Tools hub. Use it before you load, not after you are stopped at a scale.
Frequently asked questions
What is the bridge formula?
The federal bridge formula sets the maximum legal weight for an axle group on the Interstate Highway System: W = 500 × ((L × N) / (N − 1) + 12N + 36), where L is the distance in feet between the outer axles and N is the number of axles. Results are rounded to the nearest 500 lbs, and the tandem and 80,000 lb gross caps apply.
Why does axle spacing change the legal weight?
A bridge carries the load over a span. Closely spaced axles put their weight on a short stretch of the bridge at the same time, stressing the structure more. Axles spread further apart share the load across more of the span, so the bridge can safely carry more and the formula allows a higher limit.
What is the tandem axle limit?
A two-axle group spaced 8 ft or less is capped at 34,000 lbs; beyond 8 ft the cap rises to 40,000 lbs. The caps sit on top of the formula, so a tandem is always limited to the lower of the formula result and the cap for its spacing.
Does Texas use the federal bridge formula?
Yes, on Interstate highways. Texas follows the federal bridge formula on Interstates such as I-35, I-10 and I-20. Moves over the legal limits need a permit first — use the Texas Permit Wizard to find out which permit applies.