Washington truck weight limits
On Interstate highways in Washington, federal weight limits apply. Four checks decide whether your truck is legal — and the one that surprises most drivers is the Federal Bridge Formula, which can fail a truck that passes every other limit.
Federal limits on Washington Interstates
| Limit | Maximum | Applies to |
|---|---|---|
| Single axle | 20,000 lb | Any one axle |
| Tandem axle group | 34,000 lb | Axles 40–96 inches apart |
| Tridem axle group | governed by the bridge formula | 3 axles in a group (no flat federal cap) |
| Gross vehicle weight | 80,000 lb | The whole rig |
| Bridge formula | varies by spacing | Every group of 2+ axles |
The bridge formula check
L is the spacing in feet between the outer axles of a group, N the number of axles. A standard 5-axle rig at 51 ft is capped at 79,500 lb by this formula — under the 80,000 lb gross limit. Full explanation with worked examples.
Washington-specific rules
Washington does not use a flat 80,000 lb gross cap off the Interstate — it uses a weight table based on wheelbase and axle count, so a rig with enough axles and length can run well above 80,000 lb. On the Interstate, federal limits govern: 20,000 lb single axle, 34,000 lb tandem, 80,000 lb gross, plus the Federal Bridge Formula. Off the Interstate, Washington sets gross by the lesser of three tests — tire size, axle weight, and the weight table (RCW 46.44.041) — and WSDOT or a local authority may extend the table by rule up to 115,000 lb (consistent with federal law). Tire width matters here: 600 lb per inch of tire width, and any axle over 10,000 lb needs four or more tires.
Washington exceptions to the federal limits
Washington determines legal weight by the lesser of tire size, axle weight, and the weight table (RCW 46.44.041):
| Measure | Limit |
|---|---|
| Single axle | 20,000 lb |
| Tire loading | 600 lb per inch of tire width |
| Tires per heavy axle | 4+ tires on any axle over 10,000 lb |
| Gross (non-Interstate) | set by the weight table (wheelbase + axles), extendable by rule to 115,000 lb |
| Gross (Interstate) | 80,000 lb (bridge formula) |
- Two consecutive tandem sets may each carry 34,000 lb where the extreme axles are 36 ft or more apart.
- On Interstates, federal limits govern — 80,000 lb gross plus the bridge formula; the extended weight table does not apply there.
Seasonal & designated-route rules in Washington
Washington posts seasonal load restrictions on frost-susceptible and mountain routes, and permanent posted-bridge limits statewide. Mountain-pass corridors also carry traction/chain requirements that can restrict heavy loads in winter. Check WSDOT's current road and bridge restrictions before running heavy, especially east of the Cascades and over the passes.
Overweight permits in Washington
Loads over the weight-table limits need an oversize/overweight permit from WSDOT (single-trip or annual), route-specific by bridge and pavement rating.
Fines & enforcement in Washington
Enforced by the Washington State Patrol Commercial Vehicle Enforcement at fixed scales and with portable/mobile enforcement. Because legal weight is the lesser of tire, axle and table limits, an axle can be over on the tire-width test even when the axle-weight test passes — a common and costly surprise.
What dispatchers should check in Washington
- No flat 80,000 off the Interstate — Washington's weight table (wheelbase + axles) governs, and can be extended by rule up to 115,000 lb; more axles and length buy more weight.
- Lesser-of-three — legal weight is the lowest of the tire-size, axle-weight and weight-table tests; check all three.
- Tire width — 600 lb/in, and 4+ tires on any axle over 10,000 lb.
- Interstate — federal 20,000 / 34,000 / 80,000 plus the bridge formula.
Washington truck weight — frequently asked questions
How much can a truck weigh in Washington?
On Washington Interstates the federal gross-weight limit is 80,000 lb, with a 20,000 lb single-axle limit, a 34,000 lb tandem limit and the Federal Bridge Formula. Washington sets its own limits on non-Interstate roads — see the Washington-specific rules on this page.
Does Washington follow federal truck weight limits?
On the Interstate system, yes — federal limits govern in every state. Off the Interstate, Washington applies its own axle, gross and permit rules, detailed on this page where they have been verified against the official state source.
What is the single-axle weight limit in Washington?
The federal single-axle limit of 20,000 lb applies on Washington Interstates. Some states set a different single-axle figure on non-Interstate roads — check the Washington-specific section above for the governing number.
What is the maximum tandem-axle weight in Washington?
On Washington Interstates the federal tandem limit is 34,000 lb. Several states allow a different tandem weight on non-Interstate roads — see the Washington exceptions above for the figure that governs.
Scaled heavy in Washington?
If the trailer tandems are over, the fix is usually a computed tandem slide, not unloading — see how sliding tandems shifts weight. And here's what an overweight ticket really costs before you decide to run it.
Run your scale-ticket weights through the federal and bridge-formula calculator in your browser. The full platform predicts axle loads from load placement before you load, computes the exact slide-to-legal fix, and applies Washington rules on top of the federal baseline.
Open the free calculatorOur calculator runs the federal single, tandem, gross and Bridge Formula checks in seconds. The full platform adds per-state limits, axle-load prediction and the slide-to-legal fix.
Try the free calculator