Bridge Formula Calculator
Pick how many axles are in the group and measure the distance between the first and last one. Get the max legal weight for that group under the federal bridge formula — statutory caps included, before the scale house does the math for you.
Pick your axles, enter the span, hit CHECK MY MAX
How this calculator works
The scale house doesn't just check your gross — it checks how your weight is spread out. The federal bridge formula (23 CFR 658.17) exists because 80,000 lb crossing a bridge on axles bunched close together beats that bridge up far worse than the same 80,000 stretched over 51 feet. The calculator takes your two inputs — how many axles are in the group, and the feet between the first and last of them — runs the formula, then applies the hard caps the formula can't override: 20,000 lb on a single axle, 34,000 lb on a tandem (two axles within 8 ft), 80,000 lb gross on the Interstate without a permit.
L = outer span in feet · N = axles in the group · W = max group weight in lb
The rule of thumb falls straight out of the math: more axles, spread farther, carry more. The ticket prints the raw formula result and flags when a statutory cap binds instead — and it reminds you that official FHWA tables round to the nearest 500 lb, which matters exactly once (see Example 2).
Two worked examples
Example 1 — a tridem, tight vs spread
Three axles spread over 9 ft: 9 × 3 ÷ 2 = 13.5, plus 12 × 3 = 36, plus 36 makes 85.5 — times 500 is a 42,750 lb ceiling for the group. Stretch those same three axles over 20 ft and the formula reads 500 × (30 + 36 + 36) = 51,000 lb. Same axles, same tires, 8,250 more legal pounds — spacing is money, which is why spread-axle trailers and jeep dollies exist.
Example 2 — the famous 80,000 on a 5-axle rig
Standard 18-wheeler, steer to rear trailer axle measuring 51 ft, all 5 axles in the group: 51 × 5 ÷ 4 = 63.75, plus 60, plus 36 gives 159.75 — times 500 is 79,875 lb. Not 80,000. The official FHWA table rounds to the nearest 500, which is the only reason a standard semi runs the full 80,000 gross legally. Pull that outer span in to 48 ft — short-wheelbase day cab, tandems slid forward — and the formula drops to 78,000: two thousand pounds of payload gone to geometry.
From the road
Where bridge law actually bites drivers isn't the outer span — it's the interior group you never measured. Every consecutive set of axles has to pass, including your drives-plus-trailer-tandems as a four-axle group, and federal law only lets two consecutive tandems carry 34,000 each when the first drive axle to the last trailer axle stretches at least 36 feet. Slide your tandems all the way forward chasing a drive-axle problem and you can shrink under that line: every group legal on its own, the combination in violation. It's the ticket nobody at the fuel desk can explain.
This is also the real reason the West runs longer rigs and spread axles, and why the reflexive "slide it all the way up" habit can cost you at an Eastern scale. Before a heavy, oddly-balanced load — coils, machinery, anything on a stepdeck — sketch the axle picture first: the tandem slide calculator handles the everyday drives-vs-tandems case, and our guide to sliding tandems and state bridge laws covers kingpin limits and where the formula does and doesn't follow you off the Interstate. This ticket is an estimate — verify with the FHWA table and the DOT of the state you're rolling through.
FAQ
What is the federal bridge formula?
The federal rule (23 CFR 658.17) that caps any group of axles at W = 500 × (LN/(N−1) + 12N + 36), where N is the axle count and L is the outer spacing in feet. It runs alongside the 20,000 single, 34,000 tandem, and 80,000 gross limits on the Interstate.
Why does axle spacing change how much I can haul?
Bridges feel concentrated weight harder than spread-out weight. Same pounds over a longer stretch means each section of bridge carries less at once, so the law allows more. Spread the axles, raise the limit.
What is the outer bridge measurement?
That's L: center of the first axle to center of the last axle in the group, in feet. On a standard 5-axle semi, that's steer axle to rear trailer axle — usually 51 ft or better.
Do all states use the federal bridge formula?
On the Interstate, yes. Off it, most states follow the federal formula but some run their own tables or grandfather rights. Check the state DOT before you bet a citation on it.
Can sliding my tandems cause a bridge violation?
Yes. Sliding forward shortens the drives-to-tandems span, and two consecutive tandems only get 34,000 each when first drive axle to last trailer axle is 36 ft or more. Slide under that and the four-axle group can be illegal while every individual group passes.
What happens if I fail bridge law at a scale?
A citation even under 80,000 gross, fines scaling by pounds over, and the scale can park you until weight moves — a slide if that fixes it, offloading if it doesn't. Nondivisible loads need the overweight permit before the trip.