Tri-axle dump truck capacity
There is no one number. Legal weight comes from a published formula on the axles, and the load is what is left after the truck’s own weight.
Last updated 29 September 2026The three Interstate limits
The Federal Highway Administration’s publication on bridge formula weights states them for the Interstate system, under 23 U.S.C. § 127 and 23 CFR § 658:
| Limit | Pounds |
|---|---|
| A single axle | 20,000 |
| Tandem axles, 40 to 96 inches apart | 34,000 |
| Gross vehicle weight | 80,000 |
Those are the caps. Between them sits the rule that decides most trucks with more than two axles.
The bridge formula
W = 500 × (L × N ÷ (N − 1) + 12 × N + 36)
W is the overall gross weight, in pounds, allowed on any group of two or more consecutive axles, to the nearest 500 lb. L is the distance in feet between the outer axles of the group. N is the number of axles in the group. Source: FHWA-HOP-19-028.
Read it as a promise about spreading weight: the further apart the outer axles, the more the group may carry, and every extra axle adds to the allowance. The rule applies to every group of consecutive axles on the truck, not only the rear ones, and the lowest result governs.
Worked groups
These spacings are made up to show the arithmetic. They are not the spacings of any real truck; measure yours.
| Group | Working | Formula weight | Allowed on the Interstate |
|---|---|---|---|
| Two axles, 4 ft apart | 500 × (4 × 2 ÷ 1 + 12 × 2 + 36) = 500 × 68 | 34,000 lb → 34,000 lb | 34,000 lb |
| Two axles, 5 ft apart | 500 × (5 × 2 ÷ 1 + 12 × 2 + 36) = 500 × 70 | 35,000 lb → 35,000 lb | 34,000 lb |
| Three axles, 10 ft from first to last | 500 × (10 × 3 ÷ 2 + 12 × 3 + 36) = 500 × 87 | 43,500 lb → 43,500 lb | 43,500 lb |
| Four axles, 14 ft from first to last | 500 × (14 × 4 ÷ 3 + 12 × 4 + 36) = 500 × 102.667 | 51,333 lb → 51,500 lb | 51,500 lb |
| Five axles, 36 ft from first to last | 500 × (36 × 5 ÷ 4 + 12 × 5 + 36) = 500 × 141 | 70,500 lb → 70,500 lb | 70,500 lb |
The second row shows the cap at work. Two axles 5 ft apart come out of the formula at 35,000 lb, but a tandem is limited to 34,000 lb, so the smaller figure governs.
From legal weight to the load
Take a made-up truck with 4 axles and 18 ft between the first and the last, weighing 27,000 lb empty (a scale ticket gives you the real figure). The formula for the whole truck allows 54,000 lb, which is under the 80,000 lb cap, so its legal gross weight is 54,000 lb.
54,000 − 27,000 = 27,000 lb
Provided every smaller group of axles on the same truck also passes the formula. If one does not, the smaller allowance governs, and the load is lower.
What that weight means in yards depends on what is in the body:
| Material (density from the supplier’s ticket) | Load ÷ density | Cubic yards the weight allows |
|---|---|---|
| 2,700 lb per cu yd | 27,000 ÷ 2,700 | 10.0 cu yd |
| 1,800 lb per cu yd | 27,000 ÷ 1,800 | 15.0 cu yd |
Those two densities are inputs to the example, not recommendations. The point is the shape: the same legal load is many more yards of a light material than a heavy one, and the body volume is a second, independent ceiling. Whichever is reached first is the capacity. The dump truck capacity guide works the two-limit method in full, and the dirt hauling calculator does it for your own numbers.
“Tri-axle” and the other names
Single-axle, tandem and tri-axle trucks are usually named for the group of axles behind the steering axle, but people do not all count the same way, and a truck may have an extra axle that lifts. So the name alone does not tell you the two numbers the formula needs. Ask for, or measure, how many axles are in the group and the distance from the first to the last.
What this does not settle
- This is the Interstate ceiling. States set their own limits on other roads, some higher and some lower, and issue permits. The rule that applies is the one for the road the truck is on.
- It is a check on axle groups, not a promise about a particular truck. A truck can be under 80,000 lb and still fail on a group.
- Body volume is a separate limit. A body that could physically hold more than the weight allows is common with dense material.
- The figures are arithmetic on a published formula. They do not replace a scale ticket or the rules of the agency that enforces the road.
Why there is no capacity table here
A table of “typical tri-axle capacities” would need a typical axle spacing, a typical empty weight, a typical body and a typical material, and none of those is typical. Two searches for sources on truck-class capacities returned only secondary publishers whose figures contradict each other. A manufacturer-specific table, with each figure tied to a published specification, is a different thing and may follow.
- Bridge Formula Weights (FHWA-HOP-19-028, August 2019) — Federal Highway Administration, Office of Freight Management and Operations
- 23 CFR § 658.17 — Weight — Code of Federal Regulations, via Legal Information Institute, Cornell Law School
- Every worked group and load is arithmetic on the formula above — Arithmetic — verifiable by hand
Put it to work
All construction calculators →Find the legal load, turn it into yards for your material, then count the loads.