Federal bridge formula calculator for every axle group
Enter the number of axles and the distance between each one, and optionally the scale weights. The calculator checks every group of consecutive axles against the federal bridge formula, the tandem limit and the 80,000 lb cap.
Built and checked by Mike Shelton · Updated · Free, no sign-up
Axle 1
Axle 2
Axle 3
Axle 4
Axle 5
1 axle group over the limit
Overall 51 ft from first to last axle; maximum gross 80,000 lb.
| Axles | Spread | Limit | Basis | Weight |
|---|---|---|---|---|
| 1 to 2 (2) | 17.5 ft | 47,500 | Bridge formula | 28,500 |
| 1 to 3 (3) | 21.8 ft | 52,500 | Bridge formula | 45,500 |
| 1 to 4 (4) | 46.9 ft | 73,500 | Bridge formula | 62,000 |
| 1 to 5 (5) | 51 ft | 80,000 | Bridge formula | 78,500 |
| 2 to 3 (2) | 4.3 ft | 34,000 | Tandem axle limit | 34,000 |
| 2 to 4 (3) | 29.4 ft | 58,000 | Bridge formula | 50,500 |
| 2 to 5 (4) | 33.5 ft | 64,500 | Bridge formula | 67,000 |
| 3 to 4 (2) | 25.1 ft | 55,000 | Bridge formula | 33,500 |
| 3 to 5 (3) | 29.2 ft | 58,000 | Bridge formula | 50,000 |
| 4 to 5 (2) | 4.1 ft | 34,000 | Tandem axle limit | 33,000 |
Formula 23 CFR 658.17(e); FHWA Bridge Formula WeightsChecked October 2026 · Tandem exception 23 CFR 658.17(e)Checked October 2026 · Gross 23 CFR 658.17(b)Checked October 2026 · Single axle 23 CFR 658.17(c)Checked October 2026 · Interstate limits; states may differ on other roads and with permits. Pairs under 8 ft are treated as tandems.
Your axle setup tops out at 80,000 lb. We check every load's weight against your axles before we call. Our dispatchers book freight your rig is built for at 5% of gross, and you approve every load.
Get loads that fit my axlesThe bridge formula
W = 500 [LN/(N-1) + 12N + 36], where W is the maximum weight in pounds on a group of axles, L is the distance in feet between the outer axles of the group, and N is the number of axles in the group. The result is rounded to the nearest 500 lb. 23 CFR 658.17(e); FHWA Bridge Formula WeightsChecked October 2026
The calculator applies it to every group of two or more consecutive axles, then applies three other federal limits:
- Single axle: 20,000 lb. 23 CFR 658.17(c)Checked October 2026
- Tandem axle: 34,000 lb for two axles spaced as a tandem. 23 CFR 658.17(d)Checked October 2026
- Gross weight: 80,000 lb for the whole vehicle. 23 CFR 658.17(b)Checked October 2026
- Two-tandem exception: 34,000 lb on each of two tandems when their outer axles are 36 ft or more apart. 23 CFR 658.17(e)Checked October 2026
Worked example EXAMPLE
A 5-axle tractor and van: steer to first drive axle 17.5 ft, drive tandem 4.3 ft, drive to trailer tandem 25.1 ft, trailer tandem 4.1 ft, 51 ft overall. Each tandem is limited to 34,000 lb, and the whole truck to 80,000 lb. The group from the first drive axle to the last trailer axle spans 33.5 ft, under the 36 ft needed for the two-tandem exception, so the formula sets that group's limit, which comes out below 68,000 lb.
Slide the trailer tandems back so the drive and trailer tandems are 36 ft or more apart, and the exception allows the full 68,000 lb on the four axles. That is why the slider position matters as much as the scale ticket.
Bridge formula limits for common spreads
Computed from the formula and rounded to the nearest 500 lb, before the 80,000 lb gross cap:
| Outer axle spread | 3 axles | 4 axles | 5 axles |
|---|---|---|---|
| 12 ft | 45,000 lb | 50,000 lb | 55,500 lb |
| 20 ft | 51,000 lb | 55,500 lb | 60,500 lb |
| 30 ft | 58,500 lb | 62,000 lb | 67,000 lb |
| 40 ft | 66,000 lb | 68,500 lb | 73,000 lb |
| 51 ft | 74,500 lb | 76,000 lb | 80,000 lb |
More axles and a longer spread raise the limit. That is why heavy haul combinations add axles, and why a short dump truck can be limited well below its rating.
Reading the results
Each row is one group of consecutive axles: which axles, how far apart the outer two are, the legal limit and why. If you enter weights, the calculator adds the weights in the group and marks any group over its limit in red. A red inner group with a green gross means the load is sitting in the wrong place; move weight by sliding the fifth wheel or the trailer tandems. A red gross means weight has to come off, or the move needs an overweight permit.
Where the bridge formula bites
- Dump trucks and short straight trucks: axles close together carry heavy loads, so the formula often sets a limit below what the truck could carry.
- Tankers: dense liquids reach the weight limit before the tank is full.
- Heavy haul: extra axles and boosters spread weight so each group stays within its permit.
- Log and grain trucks: heavy loads in short combinations often run near the limit.
Measuring axle spacing
Measure from the center of one axle hub to the center of the next, in feet and tenths. Measure with the trailer tandems where they will be for the trip. Small errors matter near the limit: a foot can change the allowed weight by about 500 lb for a large group. Keep the measurements on file for each truck and trailer setup so you don't have to repeat them.
Common bridge formula mistakes
- Checking only the gross. An inner group can be over even when the whole truck is under 80,000 lb.
- Measuring to the wrong point. Use the center of each axle, not the tires' edges or the frame.
- Forgetting the slider moved. Re-measure after sliding the trailer tandems.
Off the Interstate
States set their own limits on roads off the Interstate system, and some allow heavier configurations under laws older than the federal formula, or by permit. Seasonal limits in spring can lower allowed weights on some roads. The calculator shows federal Interstate limits only; check state rules for the roads on your route. Use the truck weight limit checker for a quick check of a scale ticket.
When you dispatch with us, your axle setup and ratings sit on your rig card, and we check every load's weight before we call. See oversize and heavy haul dispatch or send an application.
Questions about this calculator
01What is the federal bridge formula?
A rule that limits the weight on any group of two or more consecutive axles based on how many axles are in the group and how far apart the outer ones are: W = 500 [LN/(N-1) + 12N + 36]. Spreading weight over more axles and a longer distance protects bridges, so longer, wider-spaced groups may carry more.
02Why does my truck fail the bridge formula at 80,000 lb?
Usually because the axles are too close together. A short wheelbase or trailer tandems slid forward shortens the distance between the outer axles, and the formula then allows less than 80,000 lb for the whole vehicle or for an inner group. Sliding the tandems back, or carrying less, brings it into line.
03What is the tandem exception?
Two consecutive sets of tandem axles may carry 34,000 lb each, 68,000 lb together, when the first and last axles of the two tandems are at least 36 feet apart, even if the formula alone would allow slightly less. It is what lets a standard 5-axle tractor-trailer reach 80,000 lb.
04Does the bridge formula apply on every road?
The federal formula applies on the Interstate system and roads tied to it. States set their own limits on other roads, and some allow heavier trucks under older laws or permits. Check the state's rules for the roads you actually run, especially off the Interstate.