Bridge Law Calculator – Quick Check
Use the free calculatorAbout the Bridge Law Calculator
A quick, federal-only check with the Bridge Law Calculator. Estimate axle weights and axle group loads from your weights and dimensions, and check them against the federal bridge formula. Runs in your browser; nothing is sent anywhere.
Need state rules, doubles or lift axles? They are in the full version.
Results
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Need an engineered layout? Request a quote.
This free tool is for planning. For a documented, engineered layout (state rules, lift axles and structure), send your numbers.
Request a quote with these numbersHow the calculator works
Each unit is treated as a rigid beam. Loads are summed about the steer axle (tractor or truck) or the kingpin (trailer), and axles in a tandem share load equally, as with an equalized suspension. The trailer's kingpin load is carried by the tractor at the fifth wheel. Empty weights use the empty steer-axle weight you enter for the tractor, and axle weights of 2,500 lb each with the rest of the trailer body centered between kingpin and rear for the trailer.
What the federal bridge formula check includes
- 20,000 lb single axle and 34,000 lb tandem (axles more than 40 in and not more than 96 in apart)
- W = 500 × (LN/(N−1) + 12N + 36) on every group of consecutive axles, rounded to the nearest 500 lb
- The two-tandem exception (34,000 lb each with 36 ft or more between the first and last axle)
- 80,000 lb gross on the Interstate
What it does not do
State limits, lift and pusher axles, doubles, tire load limits, permits and seasonal restrictions are not included. It does not check structure. Results are estimates, not an engineering review; see the calculator disclaimer. For those, request an engineered weight distribution layout or a bridge formula and state compliance check.
Need state rules, doubles or lift axles?
The full Bridge Law Calculator adds 50-state rule sets, doubles and B-trains, lift and pusher axles, a truck and trailer library and printable weight sheets. The free calculator covers straight trucks up to 3 axles and tractor-trailers up to 5 axles on the federal bridge formula.