Guide
How to Calculate Truck Axle Weights
Multiply the load's weight by the distance from the front axle to its center of gravity and divide by the wheelbase: that is the load on the rear axle. The rest goes on the front axle. Add each axle's empty weight and you have both axle weights. Then compare them with the federal 20,000 lb single-axle, 34,000 lb tandem and 80,000 lb gross limits, the Federal Bridge Formula, and the state limits that often govern instead.
How load position splits weight between axles
Treat the truck as a beam resting on two supports: the front (steer) axle and the rear (drive) axle. A load sitting between them pushes down on both, and the axle closer to the load carries more of it. The exact share follows from balancing moments — weight times distance — about either axle:
If the load sits 3 ft ahead of the rear axle on a 20 ft wheelbase, the front axle picks up 3/20 of it and the rear axle the other 17/20. A load directly over the rear axle puts all of itself there; a load halfway between the axles splits evenly. The gross never changes when the load moves — only the split does, which is how an axle ends up overweight on a truck whose gross is legal.
Measure from the center of the front axle, not the bumper or the front of the body, and use the load's center of gravity. For one uniform load that is roughly its middle; for several heavy items, work out each one separately and add the results.
Worked example: 12,000 lb, 3 ft ahead of the rear axle
A straight truck with a 20 ft wheelbase weighs 7,400 lb on the front axle and 6,100 lb on the rear axle empty. It carries 12,000 lb of cargo whose center of gravity sits 3 ft ahead of the rear axle — that is, 17 ft behind the front axle. These are example figures for illustration, not typical weights for any particular truck.
- Rear axle's share of the cargo = 17 ÷ 20 = 0.85
- Cargo on the rear axle = 12,000 × 0.85 = 10,200 lb
- Cargo on the front axle = 12,000 − 10,200 = 1,800 lb (check: 12,000 × 3 ÷ 20 = 1,800)
- Front axle total = 7,400 + 1,800 = 9,200 lb
- Rear axle total = 6,100 + 10,200 = 16,300 lb
- Gross = 9,200 + 16,300 = 25,500 lb
To reproduce it in the axle weight calculator, enter wheelbase 20 ft, cargo center of gravity 17 ft from the front axle, curb weights 7,400 lb front and 6,100 lb rear, and cargo 12,000 lb. It returns 9,200 lb front, 16,300 lb rear, 25,500 lb gross, and 1,800 / 10,200 lb of cargo on front / rear. Now suppose the rear axle is rated at 15,000 lb by its manufacturer: type that into the optional rear-axle rating field and the calculator flags the rear axle as 1,300 lb over rating, even though 16,300 lb is well under the 20,000 lb federal single-axle limit. The lower of the two always governs.
Moving the load instead of removing it
Same truck, same 12,000 lb, same hypothetical 15,000 lb rear-axle rating; only the cargo's center of gravity moves. Every row can be reproduced in the calculator by changing the second field and nothing else.
| Cargo CG behind front axle | Cargo on front / rear | Front axle | Rear axle | Against a 15,000 lb rear rating |
|---|---|---|---|---|
| 10 ft (midway) | 6,000 / 6,000 | 13,400 | 12,100 | Under |
| 12 ft | 4,800 / 7,200 | 12,200 | 13,300 | Under |
| 14 ft | 3,600 / 8,400 | 11,000 | 14,500 | Under |
| 15 ft | 3,000 / 9,000 | 10,400 | 15,100 | 100 lb over |
| 16 ft | 2,400 / 9,600 | 9,800 | 15,700 | 700 lb over |
| 17 ft (the example) | 1,800 / 10,200 | 9,200 | 16,300 | 1,300 lb over |
| 20 ft (over the rear axle) | 0 / 12,000 | 7,400 | 18,100 | 3,100 lb over |
Gross weight is 25,500 lb on every row. Each foot the load moves forward shifts 600 lb (12,000 ÷ 20) from the rear axle to the front, so moving it from 17 ft to 14 ft brings the rear axle under its rating without taking anything off the truck. Check the front axle's rating too. Past the rear axle the front share turns negative: the load lifts weight off the steer axle.
The federal limits, and where each one is written
These are the federal weight limits in 23 CFR 658.17, which apply to the Interstate System and reasonable access to it. Section 658.17(g) states that they are inclusive of all tolerances, so there is no built-in allowance on top of them.
| Limit | Maximum | What it covers | Where |
|---|---|---|---|
| Single axle | 20,000 lb | Any one axle, including one axle of a group; wheels whose centers fall within 40 in of each other count as a single axle | 23 CFR 658.17(c); definition in 658.5 |
| Tandem axle | 34,000 lb | Two or more consecutive axles whose centers are more than 40 in and not more than 96 in apart | 23 CFR 658.17(d); definition in 658.5 |
| Gross vehicle weight | 80,000 lb | The whole vehicle or combination, unless the Bridge Formula sets a lower figure | 23 CFR 658.17(b) |
| Bridge Formula | varies | Any group of two or more consecutive axles, by number of axles and spread | 23 CFR 658.17(e) |
| Two consecutive tandems | 34,000 lb each | Allowed when the first and last axles of the two tandems are 36 ft or more apart | 23 CFR 658.17(e) |
| Idle-reduction unit (APU) | up to 400 lb more | May be allowed on gross, axle, tandem or Bridge Formula limits, with written certification of the unit's weight and proof it works | 23 CFR 658.17(n) |
The steer axle is not exempt. Section 658.17(f) says a state may not limit steering-axle weight to less than 20,000 lb or the axle rating established by the manufacturer, whichever is lower — so the rating on your front axle can be the number that binds, exactly as in the worked example above.
The Federal Bridge Formula
Congress enacted the Bridge Formula in 1975 to limit how much weight a vehicle concentrates on a short length of bridge. FHWA states it as W = 500 × (LN ÷ (N − 1) + 12N + 36), where W is the maximum weight on the group to the nearest 500 lb, L is the distance in feet between the outer axles of the group, and N is the number of axles in the group. It applies to every group of two or more consecutive axles, not just the whole truck, and a vehicle can satisfy every single-axle, tandem and gross limit and still fail it. For axles 96 in apart or closer, the single-axle and tandem limits replace the formula. FHWA's published Bridge Table rounds down when the formula lands exactly halfway between 500 lb steps.
| Axles (N) | Spread (L) | Formula, unrounded | FHWA Bridge Table | Note |
|---|---|---|---|---|
| 2 | 97 in | 38,083 lb | 38,000 lb | The first spacing where two axles are no longer a tandem |
| 3 | 9 ft | 42,750 lb | 42,500 lb | Exactly halfway, so it rounds down |
| 3 | 20 ft | 51,000 lb | 51,000 lb | FHWA's own example: steer axle plus drive tandem |
| 4 | 23 ft | 57,333 lb | 57,500 lb | FHWA's single-unit truck example, axles 1–4 |
| 4 | 35 ft | 65,333 lb | 65,500 lb | FHWA's example of a violation at 68,000 lb |
| 4 | 36 ft | 66,000 lb | 66,000 lb | Two tandems at this spread may carry 34,000 lb each (68,000 lb) under the exception |
| 5 | 45 ft | 76,125 lb | 76,000 lb | Tractor-semitrailer with a short spread |
| 5 | 50 ft | 79,250 lb | 79,000 lb | Exactly halfway, so it rounds down |
| 5 | 51 ft | 79,875 lb | 80,000 lb | The shortest whole-foot spread at which five axles reach 80,000 lb |
FHWA's worked example shows why the inner groups matter. Its five-axle tractor-semitrailer carries 12,000 lb on the steer axle and 17,000 lb on each of the other four, 80,000 lb in all. Axles 1–3 (20 ft) and 1–5 (51 ft) pass, but axles 2–5, spread 35 ft, carry 68,000 lb against 65,500 lb allowed: a violation, fixed by removing load or increasing that spacing.
Why your state's limits often govern
The federal numbers are a rule about the Interstate System. FHWA puts it plainly: off the Interstate Highway System, states may set their own commercial vehicle weight standards. Three more things move the number you actually have to meet:
- Grandfather rights. When the Interstate limits were adopted in 1956 and amended in 1975, states were allowed to keep higher limits they already had, so Interstate limits in some states are higher than the federal figures (23 CFR 658.17(i)). A state may not enforce Interstate limits below the federal ones (658.17(f)).
- Permits. States may issue special permits without regard to the axle, gross or Bridge Formula limits for nondivisible vehicles or loads (658.17(h)). What a permit allows is set by its own terms.
- Posted bridges. FHWA's Bridge Table notes that every bridge must be inspected, rated for safe load-carrying capacity, and posted or restricted if required. A posted limit is the limit on that structure, whatever the Bridge Table allows.
To find yours, go to the state department of transportation or the state agency that enforces commercial vehicle size and weight, and look for its published weight limits and its oversize/overweight permit office. Check every state on the route, including the roads off the Interstate at each end of the trip.
Where this goes wrong
- Counting on a scale tolerance. 23 CFR 658.17(g) makes the federal weights inclusive of all tolerances. The only exception is an allowance for portable wheel-load scales, never more than 5%, and a fine schedule that charges nothing up to a threshold such as 1,000 lb counts as an unauthorized tolerance.
- Using the two-axle method on a tractor-semitrailer. Trailer weight reaches the tractor through the fifth wheel, and both the fifth-wheel position and the trailer tandem position change how weight divides between axle groups. The same lever rule applies at each step, but the calculator models only a two-axle straight truck.
- Sliding tandems and forgetting the Bridge Formula. Moving the trailer tandem forward shortens the spread of every group it belongs to, and a shorter spread lowers what the formula allows.
- Leaving out fuel, driver and equipment. They belong in the empty axle weights. As fuel burns, its weight comes off the axles in the same lever proportion as any other load, according to where the tanks sit.
- Checking gross and stopping there. Being under 80,000 lb says nothing about an axle, a tandem or an inner axle group.
- Treating the estimate as the answer. The method assumes you know the center of gravity and empty axle weights exactly. A certified scale is the only weight a DOT officer accepts.
Where these numbers come from
Every federal limit on this page is in the regulation itself; the Bridge Table values and worked examples are FHWA's.
- 23 CFR 658.17 — Weight — The 20,000 lb single-axle, 34,000 lb tandem and 80,000 lb gross limits, the Bridge Formula, the tolerance rule, permits, grandfather rights and the APU allowance.
- 23 CFR Part 658 — Truck Size and Weight — The whole part, including the definitions of single-axle and tandem-axle weight in 658.5.
- FHWA — Bridge Formula Weights — The formula, the Bridge Table with its round-down policy, the two-tandem exception and the worked examples quoted above.
- FHWA Freight Size and Weight Program — The federal Interstate limits and the statement that states set their own standards off the Interstate.
How do I calculate the weight on each axle of a truck?
Multiply the cargo weight by its center of gravity's distance from the front axle and divide by the wheelbase: that is the cargo on the rear axle. Subtract it from the cargo weight for the front axle's share, then add each axle's empty weight.
Is 80,000 lb always legal on five axles?
No. Each axle, each tandem and every axle group must also pass, and the Bridge Formula needs a 51 ft outer spread before five axles reach 80,000 lb in FHWA's table. Off the Interstate, the state's own limits apply.
Does the axle calculator check federal or state limits?
No. It splits the load by lever arm and compares the result only with the axle ratings you type in. Compare its output against the limits on this page and your state's limits yourself, and confirm on a certified scale.