Bridge Formula Calculator
Legal Weight for Any Axle Group Under 23 CFR 658.17
Quick answer
What does the Federal Bridge Formula allow?
W = 500 x (LN/(N-1) + 12N + 36), rounded to the nearest 500 lb. W is the most a group of N consecutive axles may carry, and L is the distance in feet from the group's first axle to its last. On the Interstate System the result is also capped at 20,000 lb per single axle, 34,000 lb per tandem and 80,000 lb gross (23 CFR 658.17).
Updated October 2026 by the HTS Logistics broker team (FMCSA MC# 1178470).
Calculate the legal weight for an axle group
Measure L from the center of the first axle to the center of the last axle in the group, in feet (inches are converted and the formula uses the exact value; FHWA rounds L to the nearest foot in its published table). This calculator gives the federal legal limit only. State limits on non-Interstate roads and every overweight permit can differ.
How to check a truck against the formula
- Check every group of two or more consecutive axles, not only the whole truck: the drive tandem, the trailer group, and every combination in between (steer axle to last trailer axle included).
- For each group, measure L between its first and last axle and count N, then compare the scale weight on that group with W.
- Check single axles against 20,000 lb and tandems against 34,000 lb, and the whole vehicle against 80,000 lb gross.
- If any group is over, the load either moves to a rig with more axles or longer spacing, or it needs an overweight permit in each state on the route. See oversize and overweight permits by state.
Example: a 5-axle tractor-semitrailer with 51 ft from the steer axle to the last trailer axle may carry W = 500 x (51 x 5/4 + 60 + 36) = 79,875 lb, which rounds to 80,000 lb. Shorten that spacing to 45 ft and the limit drops to 76,000 lb, so the same cargo would be overweight. Heavy machinery rarely fits these limits: a Cat D8 dozer ships at about 76,000 lb on its own, which is why dozers and large excavators move on multi-axle lowboys under overweight permits.
Bridge Formula weight table
Maximum legal weight (lb) on a group of N axles at spacing L, computed from 23 CFR 658.17 and rounded to the nearest 500 lb. Values above 80,000 lb still face the 80,000 lb gross cap on the Interstate System. Two axles 8 ft apart or less are a tandem (34,000 lb).
| L (ft) | 2 axles | 3 axles | 4 axles | 5 axles | 6 axles | 7 axles |
|---|---|---|---|---|---|---|
| 4 | 34,000 | |||||
| 5 | 34,000 | |||||
| 6 | 34,000 | 40,500 | ||||
| 7 | 34,000 | 41,500 | ||||
| 8 | 34,000 | 42,000 | ||||
| 9 | 39,000 | 43,000 | 48,000 | |||
| 10 | 40,000 | 43,500 | 48,500 | |||
| 11 | 41,000 | 44,500 | 49,500 | |||
| 12 | 42,000 | 45,000 | 50,000 | 55,500 | ||
| 13 | 43,000 | 46,000 | 50,500 | 56,000 | ||
| 14 | 44,000 | 46,500 | 51,500 | 57,000 | ||
| 15 | 45,000 | 47,500 | 52,000 | 57,500 | 63,000 | |
| 16 | 46,000 | 48,000 | 52,500 | 58,000 | 63,500 | |
| 17 | 47,000 | 49,000 | 53,500 | 58,500 | 64,000 | |
| 18 | 48,000 | 49,500 | 54,000 | 59,500 | 65,000 | 70,500 |
| 19 | 49,000 | 50,500 | 54,500 | 60,000 | 65,500 | 71,000 |
| 20 | 50,000 | 51,000 | 55,500 | 60,500 | 66,000 | 71,500 |
| 22 | 52,000 | 52,500 | 56,500 | 62,000 | 67,000 | 73,000 |
| 24 | 54,000 | 54,000 | 58,000 | 63,000 | 68,500 | 74,000 |
| 26 | 56,000 | 55,500 | 59,500 | 64,500 | 69,500 | 75,000 |
| 28 | 58,000 | 57,000 | 60,500 | 65,500 | 71,000 | 76,500 |
| 30 | 60,000 | 58,500 | 62,000 | 67,000 | 72,000 | 77,500 |
| 32 | 62,000 | 60,000 | 63,500 | 68,000 | 73,000 | 78,500 |
| 34 | 64,000 | 61,500 | 64,500 | 69,500 | 74,500 | 80,000 |
| 36 | 66,000 | 63,000 | 66,000 | 70,500 | 75,500 | 81,000 |
| 38 | 68,000 | 64,500 | 67,500 | 72,000 | 77,000 | 82,000 |
| 40 | 70,000 | 66,000 | 68,500 | 73,000 | 78,000 | 83,500 |
| 42 | 72,000 | 67,500 | 70,000 | 74,500 | 79,000 | 84,500 |
| 44 | 74,000 | 69,000 | 71,500 | 75,500 | 80,500 | 85,500 |
| 46 | 76,000 | 70,500 | 72,500 | 77,000 | 81,500 | 87,000 |
| 48 | 78,000 | 72,000 | 74,000 | 78,000 | 83,000 | 88,000 |
| 50 | 80,000 | 73,500 | 75,500 | 79,500 | 84,000 | 89,000 |
| 52 | 82,000 | 75,000 | 76,500 | 80,500 | 85,000 | 90,500 |
| 54 | 84,000 | 76,500 | 78,000 | 82,000 | 86,500 | 91,500 |
| 56 | 86,000 | 78,000 | 79,500 | 83,000 | 87,500 | 92,500 |
| 58 | 88,000 | 79,500 | 80,500 | 84,500 | 89,000 | 94,000 |
| 60 | 90,000 | 81,000 | 82,000 | 85,500 | 90,000 | 95,000 |
Red values exceed the 80,000 lb Interstate gross cap. Blank cells are spacings too short to be practical for that many axles.
Bridge Formula questions
How does the Bridge Formula work?
The Federal Bridge Formula limits the weight on any group of two or more consecutive axles: W = 500 x (LN/(N-1) + 12N + 36), where W is the maximum weight in pounds, L is the distance in feet between the first and last axle of the group, and N is the number of axles in the group. The result is rounded to the nearest 500 lb. Spreading the same weight over more axles or a longer spacing raises the legal limit.
What is the federal weight limit for a single axle and a tandem?
On the Interstate System, 20,000 lb on a single axle and 34,000 lb on a tandem axle, with 80,000 lb gross vehicle weight (23 CFR 658.17). A single axle means all axles whose centers fall within 40 inches of each other; a tandem is two or more consecutive axles whose centers are more than 40 inches and not more than 96 inches apart (23 CFR 658.5).
What is the 36-foot exception for two tandems?
Two consecutive sets of tandem axles may carry 34,000 lb each (68,000 lb total) when the distance between the first and last axles of the two tandems is 36 feet or more, even though the formula gives 66,000 lb at exactly 36 feet for 4 axles. This is why a standard tractor-trailer can carry 34,000 lb on the drives and 34,000 lb on the trailer tandem.
Does the Bridge Formula apply to oversize permit loads?
The formula sets the legal (non-permit) limit on the Interstate System and reasonable access roads. A load that cannot meet it, such as a heavy excavator or dozer, needs an overweight permit from each state on the route, and states set their own axle limits for permitted loads and often require more axles or a route survey. The formula is still the quickest way to see how far over legal a configuration is.
Why can't the gross weight go above 80,000 lb even when the formula allows more?
The formula can produce values above 80,000 lb for long groups of 6 or 7 axles, but 23 CFR 658.17(b) caps gross vehicle weight at 80,000 lb on the Interstate System. Above that, a non-divisible load needs an overweight permit, and some states allow higher legal weights on their own roads under grandfather rights.
Sources
Facts on this page were checked in October 2026 against the sources below. Rules and rates change; we confirm current requirements on every move.
Need an overweight load planned?
Send the machine, its weight and both ZIP codes. We match it to a rig with enough axles and order the overweight permits for each state on the route.
- FMCSA-licensed property broker, MC# 1178470
- Permits, escorts and route surveys arranged for each move
- Carriers checked for authority, insurance and safety record
- Commercial shippers, dealers, contractors and auction buyers
Prefer to talk? Call (631) 683-8275.