Federal Bridge Formula and Axle Weights
Per-axle, tandem, and bridge-formula limits from axle weights and spacings.
Example
You enter
- Axle weights (lb) 12000, 17000, 17000, 17000, 17000
- Axle spacings (ft) 12, 4, 30, 4
You get
- Total weight 80000
- Interstate cap (lb) 80000
Checked against published worked examples from: FHWA.
Details, formula, and sources
Formula or tableFederal Bridge Formula B: W = 500 × (LN / (N − 1) + 12N + 36), to the nearest 500 lb (FHWA rounds an exact half down), where W is the maximum allowed gross weight (lb) on a group of N consecutive axles spaced L feet apart. Single-axle limit 20 000 lb; tandem-axle limit 34 000 lb. Maximum gross 80 000 lb on Interstate routes.
Source and edition23 CFR 658.17 by section (Federal-aid Highway Act of 1956 / FHWA Bridge Formula).
Free accessFree at ecfr.gov.
What governsMath aid for personal verification. The ELD on the truck and the carrier tariff are the legal record. State limits may be lower than federal.
API field namesaxle_weights_lb, axle_spacings_ft, total_weight_lb, interstate_cap_lb
- Single / tandem caps 20 000 lb / 34 000 lb23 CFR 658.17
- Gross-weight cap 80 000 lb on Interstates unless permitted higher23 CFR 658.17