Federal bridge formula calculator
Enter where each axle sits and, if you have them, the axle weights. The calculator checks every group of two or more axles against the Bridge Formula and the federal limits.
Updated October 2026 · Free, no signup
Start from a setup
| Axle | Feet from axle 1 | Weight (lb) | Remove |
|---|---|---|---|
| 1 | |||
| 2 | |||
| 3 | |||
| 4 | |||
| 5 |
Bridge Formula check
- All 5 axles over 55.25 ft
- 80,000 lb
- Groups checked
- 10
- Groups over
- 0
- Outer bridge needed for 80,000 lb (5 axles)
- about 51 ft
| Axles | Limit | Weight | Basis |
|---|---|---|---|
| 1-2 | 49,000 lb | 29,000 lb | Bridge Formula, 2 axles over 19 ft |
| 1-3 | 53,500 lb | 46,000 lb | Bridge Formula, 3 axles over 23.25 ft |
| 1-4 | 76,000 lb | 63,000 lb | Bridge Formula, 4 axles over 51 ft |
| 1-5 | 80,000 lb | 80,000 lb | Bridge Formula, 5 axles over 55.25 ft |
| 2-3 | 34,000 lb | 34,000 lb | Tandem axle limit (658.17(d)) |
| 2-4 | 60,000 lb | 51,000 lb | Bridge Formula, 3 axles over 32 ft |
| 2-5 | 68,000 lb | 68,000 lb | Two tandems at 36 ft or more: 34,000 lb each (658.17(e)) |
| 3-4 | 58,000 lb | 34,000 lb | Bridge Formula, 2 axles over 27.75 ft |
| 3-5 | 60,000 lb | 51,000 lb | Bridge Formula, 3 axles over 32 ft |
| 4-5 | 34,000 lb | 34,000 lb | Tandem axle limit (658.17(d)) |
23 CFR 658.17 (eCFR, current 2026-10-01). Applies on the Interstate System; states set limits elsewhere and some keep grandfathered rules. Check your state.
Legal at 80,000 lb max on all 5 axles. We only offer loads that fit your truck's legal weight, and you confirm every one.
Get loads that fit my axlesThe rule, exactly as written
W = 500 x (LN / (N - 1) + 12N + 36)
W is the maximum weight on any group of two or more consecutive 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 calculator rounds to the nearest 500 lb, as FHWA's tables do, and also applies:
- 20,000 lb on any one axle (658.17(c))
- 34,000 lb on a tandem axle (658.17(d)); a tandem is two axles more than 40 and no more than 96 inches apart
- 80,000 lb gross (658.17(b))
- Two consecutive tandems may carry 34,000 lb each when the first-to-last distance is 36 feet or more (658.17(e))
Source: 23 CFR 658.17 (eCFR, current 2026-10-01) · checked 2026-10
Source: FHWA Bridge Formula Weights · checked 2026-10
Every group, not just the whole truck
A 5-axle truck has 10 groups of two or more consecutive axles, from axles 1-2 up to 1-5. Each one has its own limit. The calculator checks them all and shows which group, if any, is over, because the gross can be fine while a short inner group isn't.
Worked examples
| EXAMPLE | Axles | Outer distance | Bridge Formula | Notes |
|---|---|---|---|---|
| Tandem dump | 3 | 20 ft | 51,000 lb | Short trucks hit the bridge limit long before 80,000 |
| Tri-axle dump | 4 | 22 ft | 56,500 lb | A pusher or tag axle adds capacity by adding N |
| Drive + trailer tandems | 4 | 36 ft | 66,000 lb | 68,000 lb allowed by the two-tandem exception |
| 5-axle tractor-trailer | 5 | 51 ft | 80,000 lb | The common 80,000 lb setup |
What the rule doesn't cover
States may issue permits for nondivisible loads without regard to the axle, gross or Bridge Formula limits (658.17(h)). Grandfathered state limits can allow more on some routes (658.17(i)). Vehicles with a working idle reduction unit may get up to an extra 400 lb under 658.17(n), with certification. None of that makes a divisible load legal to run heavy without the state's rules behind it. Check your state before you rely on any exception, and never run overweight.
Questions about this calculator
What is the federal bridge formula?
W = 500 x (LN / (N - 1) + 12N + 36), from 23 CFR 658.17(e). W is the maximum weight in pounds on a group of two or more consecutive axles, L is the distance in feet between the outer axles of the group, and N is the number of axles. It spreads weight so bridges aren't overstressed.
How long does a truck need to be to haul 80,000 pounds?
For a 5-axle truck, the Bridge Formula reaches 80,000 lb at about 51 feet between the first and last axles. Shorter, and the formula sets a lower gross limit even if every axle group is legal.
What is the 36-foot rule?
Under 23 CFR 658.17(e), two consecutive sets of tandem axles may carry 34,000 lb each, 68,000 lb together, if the distance from the first to the last of those four axles is 36 feet or more, even where the formula gives less.
Does the bridge formula apply on all roads?
The federal limits apply to the Interstate System and reasonable access to it. States set limits on other roads, and some keep grandfathered axle tables from before 1975. Check your state for routes off the Interstate.
Do I count the steer axle?
Yes, for groups that include it. The calculator checks every group of consecutive axles, including those starting at the steer axle, and the steer axle alone is held to the 20,000 lb single-axle limit.
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