Weight
Legal at 80,000 and still over
The bridge formula governs the SPACING of your axles as well as the total on them, so a truck that grosses under the federal cap can still be over on a group. That is how a scale ticket arrives on a load you were sure was legal.
W = 500 x ( (L x N) / (N - 1) + 12N + 36 ), from 23 U.S.C. 127, rounded to the nearest 500 pounds and capped at the federal 80,000 pound gross.
What the formula is measuring
L is the distance between the outermost axles of the group and N is how many axles are in it. Five axles at 51 feet is the standard spread, and it is the shortest one the formula will allow the full 80,000 pounds on.
Legal on the Interstate system
80,000 lb
The formula allows 80,000 lb on this group.
Federal gross cap
80,000 lb
before any state permit
Tandem limit
34,000 lb
which is this formula at N=2, L=4 ft
W = 500 x ( (L x N) / (N - 1) + 12N + 36 ), rounded to the nearest 500 lb and capped at the federal 80,000 lb gross. This is why a legal 80,000 lb truck can still be over on a group: the formula governs spacing as well as total, and a tight group is limited far below the gross.
The whole table, from the formula
Maximum pounds on a group, by the number of axles in it and the distance between the outermost two. Every cell is the statute evaluated, not a transcription, so it cannot drift from the calculator above.
| Spacing | 2 axles | 3 axles | 4 axles | 5 axles |
|---|---|---|---|---|
| 8 ft | 38,000 | 42,000 | 47,500 | 53,000 |
| 12 ft | 42,000 | 45,000 | 50,000 | 55,500 |
| 16 ft | 46,000 | 48,000 | 52,500 | 58,000 |
| 20 ft | 50,000 | 51,000 | 55,500 | 60,500 |
| 24 ft | 54,000 | 54,000 | 58,000 | 63,000 |
| 28 ft | 58,000 | 57,000 | 60,500 | 65,500 |
| 32 ft | 62,000 | 60,000 | 63,500 | 68,000 |
| 36 ft | 66,000 | 63,000 | 66,000 | 70,500 |
| 40 ft | 70,000 | 66,000 | 68,500 | 73,000 |
| 44 ft | 74,000 | 69,000 | 71,500 | 75,500 |
| 48 ft | 78,000 | 72,000 | 74,000 | 78,000 |
| 51 ft | 80,000 | 74,500 | 76,000 | 80,000 |
| 56 ft | 80,000 | 78,000 | 79,500 | 80,000 |
| 60 ft | 80,000 | 80,000 | 80,000 | 80,000 |
Green is where the formula reaches the federal gross cap and stops. Note the two anchors every driver already knows: two axles at 4 feet gives 34,000 pounds, which is the tandem limit, and five axles need a 51 foot spread before the formula will allow the full 80,000.
Gross
80,000 lb
on the Interstate system, before a state permit
Steer axle
12,000 lb
though many tractors are rated higher and states vary
Single axle
20,000 lb
any one axle on its own
Tandem
34,000 lb
which is the formula at N=2 and L=4 ft
These are the federal limits for the Interstate system. States set their own rules for state and local roads, several grandfather higher limits, and permits change all of it. Confirm against the state you are running before you load to a number you read here.