Bridge Formula Weight Calculator

What Does The Federal Bridge Formula Calculate?

The federal framework in 23 CFR 658.17 applies weight limits to the Interstate System, with stated exceptions and grandfather provisions. FHWA’s illustrative guide writes the relationship as W = 500 × [(L × N) ÷ (N − 1) + 12N + 36]. W is group weight in pounds rounded to the nearest 500; L is outer-axle distance in feet; N is the number of consecutive axles in that group.

Do not enter overall vehicle length as L. Start by drawing and numbering the actual axles, recording each spacing and linking those measurements to the scale record. An advertised trailer description does not establish the spacing of the assembled combination.

What Does A Four-Axle, 35-Foot Example Show?

Using FHWA’s bridge-formula method: For four consecutive axles with 35 feet between the outer axles, the calculation is 500 × [(35 × 4) ÷ 3 + 48 + 36]. The unrounded result is approximately 65,333 pounds; the nearest-500-pound result is 65,500 pounds. A measured 68,000 pounds on that group exceeds that result by 2,500 pounds. These are calculated example values, not a weighing or an approval.

This reproduces the numerical situation in FHWA’s published example. It demonstrates why a group check matters; it is not a vehicle-specific legal determination. A load cannot be declared compliant just because the arithmetic for one selected group works.

Which Other Weight Checks Still Apply?

The federal regulation separately addresses single axles, tandems and gross weight, along with exceptions. Its familiar Interstate figures are 20,000 pounds for a single axle, 34,000 for a tandem and 80,000 gross. Applicable bridge-formula limits can be lower. Posted structures, another road network, tire or equipment limits and permit-specific conditions require their own review.

Record the scale weights by axle or group rather than retaining only the gross total. If the evidence does not show how the weight is distributed, resolve that missing information before relying on a group calculation. A dispatch note saying “under 80,000” does not answer the axle-spacing question.

Does An Overweight Permit Replace The Formula?

There is no universal yes-or-no answer for every permit and road. The federal regulation provides for state special permits for qualifying nondivisible vehicles or loads, while other exceptions have defined conditions. Section 658.5 defines nondivisibility; a shipper calling an object heavy does not itself satisfy that definition.

Identify the actual issuing authority, load category and authorized combination. Check the approved weights, axle arrangement, road sequence, dates and conditions together. Do not infer that a permit for one configuration authorizes another configuration, or that approval on a state route extends to an unlisted local bridge. Ask the authority to resolve a mismatch.

What Should You Send With A Weight Question?

Prepare one coherent packet: an axle diagram with measured spacings, axle and gross scale weights, vehicle configuration, loaded dimensions, the relevant permit and the exact road or structure in question. Label an estimate as an estimate. Keep revised scale or configuration information with the same movement record so an earlier figure is not mistaken for the current one.

Ask a specific question: which group or road condition needs review, and what change or authorization is required? Do not solve an unknown restriction by choosing an unapproved alternate road. A bridge-formula worksheet helps explain the question; it does not grant authority.

Where Does RouteAll Fit?

After the issuer provides the movement permit, import it into RouteAll and compare the extracted route and permit details with the current document. Share the reviewed route with the escort crew. RouteAll does not replace a certified scale record, axle-spacing measurement, bridge engineering analysis or issuing authority’s decision.

Before You Roll

Resolve the group weights, actual configuration and road authority while parked. Carry the current permit and supporting records. If loading, axle arrangement or routing changes, revisit the affected checks before departure.