How axle weights limits are determined

Manufacturers Axle Rating

You can’t carry more than your equipment can handle. Both your truck and your trailer should have a manufacturer’s rating posted on them. Make sure to keep the weight under those limits. Mind that just because your equipment can handle the weight does not mean the states will allow you to carry it.

Manufacturers Tire Rating

Same as above, do not carry more than your tire manufacturer's rating allows.

Axle Weight Limits

There are two limits you have to consider, max legal and max permitted.

Maximum Legal Axle Weight is the weight under which you can move without purchasing permit, e.g. anything under the federal bridge formula or the state legal axle limit (state limits are often higher than the federal ones).

Max Permitted Axle Weight is the weight above which the state will not allow you to move even with a permit.

Always make sure to know both of those limits for the states you are going through.

Federal Limits

There is only a federal regulation for the maximum legal weight, the federal government does not issue overviewght permits, so they do not provide maximum permitted limits, those are determined by each individual state.

The federal limits you need to know are:

Federal Bridge Formula. You can read about it and use the federal bridge calculator here.

Maximum legal gross and axle weight limits. In addition to Bridge Formula weight limits, Federal law states that single axles are limited to 20,000 pounds, and axles spaced more than 40 inches and not more than 96 inches apart (tandem axles) are limited to 34,000 pounds. Gross vehicle weight is limited to 80,000 pounds (23 U.S.C. 127).

Almost everything else you need to know are the state level regulations.