About this calculator

Screen measured axle weights and center spacing against the modeled federal Interstate baseline. The tool examines consecutive groups as well as single/tandem-group caps and the gross cap.

Inputs

How to use this calculator

Axle

  1. Axle center position from first axle (ft)Measure center-to-center distance along the vehicle; positions must increase.
  2. Scale weight on this axle (lb)Enter a measured axle weight, not a calculated load transfer.

Enter the shipment or scenario values, then select Calculate result.

Method

Formula and variables

For every consecutive group, combine axles whose outer centers span ≤40 in and compare with the 20,000 lb single-axle-group limit. Two or more consecutive axles spanning >40 through 96 in use the 34,000 lb tandem cap. Other groups of N ≥ 2 use W = 500 × (L × N ÷ (N − 1) + 12N + 36), with L the outer axle-center spacing in feet, rounded to the nearest 500 lb; exact half-increment ties round down. Also screen 80,000 lb gross. The two-consecutive-tandem exception applies only to a qualifying four-axle group with at least 36 ft between its outer axles.

Axle center position from first axle (ft)
Measure center-to-center distance along the vehicle; positions must increase.
Scale weight on this axle (lb)
Enter a measured axle weight, not a calculated load transfer.

Interpret the result

A numeric pass means the entered measurements pass this federal baseline screen. It does not establish state, permit, route or posted-bridge legality.

Outputs: Federal axle-group weight limits and over-limit amounts

Federal Interstate planning screen only. It does not determine state grandfather rights, permits, non-Interstate limits, posted-bridge restrictions, route legality, or vehicle-specific exceptions. Enter actual scale weights and axle centers. This is not legal advice or a route-compliance determination.

Worked example

Check the arithmetic

This example uses the starting values shown above. The arithmetic below is calculated independently of the interactive result.

Work through the calculation

  1. Total weight = 3 × 14,000 = 42,000 lb. For all three axles, L = 10 ft and N = 3.
  2. Bridge limit = 500 × (10 × 3 ÷ 2 + 12 × 3 + 36) = 43,500 lb.
  3. The two adjacent pairs carry 28,000 lb each and span 4 ft and 6 ft, within the modeled 34,000 lb tandem cap. Individual axles and 42,000 lb gross also pass their caps.

Compare the displayed result

Expected displayed result: Within federal baseline screen

FAQs

Does a passing outer axle group prove every group passes?

No. Consecutive interior groups can have tighter limits. The tool checks those groups separately, including applicable single/tandem caps and the limited two-tandem exception.

Sources

These references support the methods or rules used on this page. Check the version effective for your shipment date and any applicable carrier terms or item-specific provisions.

Example inputs and options

Starting values are examples unless a field identifies a dated rule. Unit selectors convert only fields marked for conversion. Rates and model options keep the units and basis shown in their labels.

Axle center position from first axle (ft)
Example: 0
Scale weight on this axle (lb)
Example: 14000