Trucker Tools
Federal Bridge Formula Calculator
The federal bridge formula sets the most weight any group of two or more axles can carry: W = 500 x (LN/(N-1) + 12N + 36), where L is the feet between the group's outer axles and N is the number of axles. Round to the nearest 500 lb, with an exact half rounded down, and never over 80,000 lb.
How the calculator works
- L is rounded down to the whole foot for the main answer, matching FHWA's whole-foot table. The result at the exact length is shown too; no official fractional-foot rule was found, so the whole-foot number is the conservative one.
- Groups spanning 40 in or less use the 20,000 lb single limit; over 40 in and up to 96 in use the 34,000 lb tandem limit.
- Results over 80,000 lb are capped at 80,000.
- For a whole truck, including the 36 ft tandem exception, use the axle weight calculator.
Bridge table
Row L = 8 applies to groups 97 to 107 inches long (8 ft when rounded down). FHWA confirms 38,000 lb for 2 axles at 97 inches.
| L (ft) | N = 2 | N = 3 | N = 4 | N = 5 | N = 6 | N = 7 |
|---|---|---|---|---|---|---|
| 8 | 38,000 | 42,000 | 47,500 | 53,000 | 59,000 | 64,500 |
| 9 | 39,000 | 42,500 | 48,000 | 53,500 | 59,500 | 65,000 |
| 10 | 40,000 | 43,500 | 48,500 | 54,000 | 60,000 | 66,000 |
| 11 | 41,000 | 44,000 | 49,500 | 55,000 | 60,500 | 66,500 |
| 12 | 42,000 | 45,000 | 50,000 | 55,500 | 61,000 | 67,000 |
| 13 | 43,000 | 45,500 | 50,500 | 56,000 | 62,000 | 67,500 |
| 14 | 44,000 | 46,500 | 51,500 | 56,500 | 62,500 | 68,000 |
| 15 | 45,000 | 47,000 | 52,000 | 57,500 | 63,000 | 68,500 |
| 16 | 46,000 | 48,000 | 52,500 | 58,000 | 63,500 | 69,500 |
| 17 | 47,000 | 48,500 | 53,500 | 58,500 | 64,000 | 70,000 |
| 18 | 48,000 | 49,500 | 54,000 | 59,000 | 65,000 | 70,500 |
| 19 | 49,000 | 50,000 | 54,500 | 60,000 | 65,500 | 71,000 |
| 20 | 50,000 | 51,000 | 55,500 | 60,500 | 66,000 | 71,500 |
| 21 | 51,000 | 51,500 | 56,000 | 61,000 | 66,500 | 72,000 |
| 22 | 52,000 | 52,500 | 56,500 | 61,500 | 67,000 | 73,000 |
| 23 | 53,000 | 53,000 | 57,500 | 62,500 | 68,000 | 73,500 |
| 24 | 54,000 | 54,000 | 58,000 | 63,000 | 68,500 | 74,000 |
| 25 | 55,000 | 54,500 | 58,500 | 63,500 | 69,000 | 74,500 |
| 26 | 56,000 | 55,500 | 59,500 | 64,000 | 69,500 | 75,000 |
| 27 | 57,000 | 56,000 | 60,000 | 65,000 | 70,000 | 75,500 |
| 28 | 58,000 | 57,000 | 60,500 | 65,500 | 71,000 | 76,500 |
| 29 | 59,000 | 57,500 | 61,500 | 66,000 | 71,500 | 77,000 |
| 30 | 60,000 | 58,500 | 62,000 | 66,500 | 72,000 | 77,500 |
| 31 | 61,000 | 59,000 | 62,500 | 67,500 | 72,500 | 78,000 |
| 32 | 62,000 | 60,000 | 63,500 | 68,000 | 73,000 | 78,500 |
| 33 | 63,000 | 60,500 | 64,000 | 68,500 | 74,000 | 79,000 |
| 34 | 64,000 | 61,500 | 64,500 | 69,000 | 74,500 | 80,000 |
| 35 | 65,000 | 62,000 | 65,500 | 70,000 | 75,000 | 80,000 |
| 36 | 66,000 | 63,000 | 66,000 | 70,500 | 75,500 | 80,000 |
| 37 | 67,000 | 63,500 | 66,500 | 71,000 | 76,000 | 80,000 |
| 38 | 68,000 | 64,500 | 67,500 | 71,500 | 77,000 | 80,000 |
| 39 | 69,000 | 65,000 | 68,000 | 72,500 | 77,500 | 80,000 |
| 40 | 70,000 | 66,000 | 68,500 | 73,000 | 78,000 | 80,000 |
| 41 | 71,000 | 66,500 | 69,500 | 73,500 | 78,500 | 80,000 |
| 42 | 72,000 | 67,500 | 70,000 | 74,000 | 79,000 | 80,000 |
| 43 | 73,000 | 68,000 | 70,500 | 75,000 | 80,000 | 80,000 |
| 44 | 74,000 | 69,000 | 71,500 | 75,500 | 80,000 | 80,000 |
| 45 | 75,000 | 69,500 | 72,000 | 76,000 | 80,000 | 80,000 |
| 46 | 76,000 | 70,500 | 72,500 | 76,500 | 80,000 | 80,000 |
| 47 | 77,000 | 71,000 | 73,500 | 77,500 | 80,000 | 80,000 |
| 48 | 78,000 | 72,000 | 74,000 | 78,000 | 80,000 | 80,000 |
| 49 | 79,000 | 72,500 | 74,500 | 78,500 | 80,000 | 80,000 |
| 50 | 80,000 | 73,500 | 75,500 | 79,000 | 80,000 | 80,000 |
| 51 | 80,000 | 74,000 | 76,000 | 80,000 | 80,000 | 80,000 |
| 52 | 80,000 | 75,000 | 76,500 | 80,000 | 80,000 | 80,000 |
| 53 | 80,000 | 75,500 | 77,500 | 80,000 | 80,000 | 80,000 |
| 54 | 80,000 | 76,500 | 78,000 | 80,000 | 80,000 | 80,000 |
| 55 | 80,000 | 77,000 | 78,500 | 80,000 | 80,000 | 80,000 |
| 56 | 80,000 | 78,000 | 79,500 | 80,000 | 80,000 | 80,000 |
| 57 | 80,000 | 78,500 | 80,000 | 80,000 | 80,000 | 80,000 |
| 58 | 80,000 | 79,500 | 80,000 | 80,000 | 80,000 | 80,000 |
| 59 | 80,000 | 80,000 | 80,000 | 80,000 | 80,000 | 80,000 |
| 60 | 80,000 | 80,000 | 80,000 | 80,000 | 80,000 | 80,000 |
FAQ
What is the federal bridge formula?
The federal bridge formula is W = 500 x (LN/(N-1) + 12N + 36). 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 in the group (23 CFR 658.17(e); FHWA).
How is the bridge formula rounded?
To the nearest 500 lb. When a result falls exactly halfway, FHWA's table rounds down: its own example of 3 axles 9 feet apart works out to 42,750 lb and is listed as 42,500 lb. No group may go over 80,000 lb in total, whatever the formula says, under federal rules for the Interstate.
Which axle groups do I check on a 5-axle truck?
FHWA says the formula applies to every group of two or more axles, but on a 5-axle tractor-semitrailer the critical groups are axles 1 through 3, 1 through 5 and 2 through 5. If those pass, the others normally will too. Check each single axle and each tandem against its own limit as well.
When does the bridge formula not apply?
Axles 40 inches or less apart use the 20,000 lb single axle limit, and axles over 40 and up to 96 inches apart use the 34,000 lb tandem limit, instead of the formula. Two consecutive tandems 36 feet or more apart may carry 34,000 lb each. States can also issue permits for nondivisible loads (23 CFR 658.17).
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Sources
- 23 CFR 658.17, Weight (eCFR)
- FHWA, Bridge Formula Weights (FHWA-HOP-19-028)
- 23 CFR 658.5, Definitions (eCFR)
Disclaimer: For general information. Federal limits from 23 CFR 658 and 49 CFR; state limits, permits and enforcement vary. Check your state's CDL manual and DOT before you rely on a number. Not legal or tax advice.
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