Federal Bridge Formula Checker

Most bridge pages only check the outer span. The law checks every group of consecutive axles — so does this.

FHWA formula, FHWA rounding All axle groups + 36-ft two-tandem exception Free — no signup

Your axles, front to back

Standard 5-axle setup shown — edit for your rig. Spacing = distance to the NEXT axle, center to center, in feet (decimals fine: 4 ft 6 in = 4.5).

What a pass means: this passes the federal Interstate axle, gross-weight, and Bridge Formula checks calculated from the numbers you entered. State roads, permits, grandfathered limits, tire ratings, posted bridges, and state tandem-position laws can all produce a different answer. Measured spacing beats guessed spacing.
Formula, limits, rounding, and the 36-ft two-tandem exception: FHWA Bridge Formula Weights & 23 CFR 658.17. FHWA's own example (51 ft, 5 axles → 80,000 lb) is one of this page's automated tests. Reviewed July 2026

How the bridge formula really works

The federal formula is W = 500 × (LN/(N−1) + 12N + 36) — L is the feet between the outer axles of the group, N is how many axles are in it, and W (taken to the nearest 500 lb) is the most that group may carry. On top of that sit three hard caps: 20,000 lb on any single axle, 34,000 lb on a tandem (axles spaced more than 40 inches and not more than 96 inches apart, per FHWA), and 80,000 lb gross on the Interstate.

The part that catches people: the formula applies to every group of two or more consecutive axles. Axles 1–3 (steer + drives) are a group. Axles 2–5 are a group. A rig can be fine on gross and fine on the full span and still fail on an interior group — that's a real citation, and it's why this checker enumerates all of them and shows you the tightest one.

The two exceptions worth knowing

First, FHWA's rounding works in your favor sometimes: the classic 5-axle rig at 51 ft computes to 79,875, but the official table says 80,000 — the formula result is taken to the nearest 500 lb. Second, two consecutive tandems may carry 34,000 lb each (68,000 combined) when the distance from the first axle of the first tandem to the last axle of the second is at least 36 feet — even where the raw formula would say less. This checker applies both.

Worked example: 5 axles, 51-ft outer span: 500 × (51×5/4 + 60 + 36) = 79,875 → nearest 500 → 80,000 lb allowed, which matches FHWA's published table exactly.