Bridge Formula Explained for Truck Drivers

Contents

• What the bridge formula is
• The formula
• Why axle spacing matters
• How drivers should use it
• Related reference
• Sources

What the bridge formula is

FHWA’s bridge formula is a federal weight rule that sets a maximum allowable weight for each axle group based on two inputs: the number of axles in the group and the distance between the outer axles. It is designed to protect bridge structures from concentrated loads that would overstress a single span.

The formula is:

W = 500 × ( LN / (N − 1) + 12N + 36 )

• W — maximum allowable weight in pounds for the group
• L — distance in feet between the outermost axles of the group
• N — number of axles in the group

For a standard drive tandem (N = 2, L = 4 ft), the formula produces 500 × (8 + 24 + 36) = 34,000 lb — which is exactly the familiar tandem limit. That number is not arbitrary: it is what the formula produces for that spacing and axle count.

For a five-axle combination with 48 ft of overall spacing (N = 5, L = 48), the formula produces 78,000 lb — below the 80,000 lb gross ceiling. A truck at this spacing cannot legally reach 80,000 lb gross even if every individual axle group is within its own limit. Sliding trailer tandems to lengthen the outer spacing to 51 ft raises the formula result to 79,875 lb, which fits under the gross ceiling.

Why axle spacing matters

The formula rewards spreading weight over more distance. A heavier load can be carried legally when the axles supporting it are spaced farther apart, because the stress on any one point of a bridge deck is lower.

For drivers, this has two practical consequences:

Tandem slide affects bridge compliance, not just axle readings. When you slide trailer tandems to reduce a heavy drive reading, you also change the overall axle spacing. Depending on the starting wheelbase, this can help or hurt bridge formula compliance on the full combination.

Gross weight does not predict bridge formula results. A truck at 77,000 lb gross can fail the bridge formula on a short-wheelbase configuration. A truck at 79,500 lb can pass the bridge formula on a longer wheelbase. Read the spacing, not just the total pounds.

How drivers should use it

1. Read steer, drive, trailer, and gross weight separately from the scale ticket.
2. Compare each axle group to the applicable limit (single axle, tandem, bridge).
3. If any tandem is at or near 34,000 lb, confirm the full-combination spacing is sufficient to clear the bridge formula — especially if the wheelbase is on the shorter end.
4. After any tandem or fifth-wheel adjustment, re-weigh. Spacing changes with tandem position, so the bridge formula result changes too.
5. Check the state page for non-Interstate route rules and permit requirements before moving over legal weight.

Related reference

• Bridge Formula Weights — quick-reference card with the formula output for common axle configurations (drive tandem, five-axle at various spacings). Use this when you need the threshold number without working through the calculation.
• Bridge Formula Calculation Examples — three worked examples with real numbers, including the case where an axle group fails while gross weight passes.
• How to Slide Tandems — the step-by-step driver workflow for adjusting axle distribution after reading a scale ticket, including a before-and-after example with numbers.

Sources

This page uses FHWA’s Bridge Formula Weights page and FHWA’s Federal Size and Weight Regulations page. Use the source links on this page to verify current federal wording.