Baggage fees have been around for almost six years now, and while travelers have gotten used to them, they are still one of the most irritating fees that airlines charge.

Passengers don’t like the fees, but the airlines make a lot of extra money off of them, so unfortunately, they are here to stay. The airlines are expected to make up to $8 billion this year from all of the different fees combined, with baggage fees making up a big chunk of the extra revenue.

On domestic flights, most airlines charge $25 for the first checked bag and $35 for a second checked bag. In addition to paying fees to check bags, fliers can also get hit with fees if a bag is overweight and/or oversize.

Most airlines allow fliers to check a bag up to 50 pounds and up to 62 linear inches, with anything larger incurring an additional fee. Linear inches is an airline industry measurement that’s length, plus width, plus height.

If you get to the airport and your bag is a couple of pounds over, you can always take a few items out and stuff them in your carry-on, but if your bag measures more than 62 linear inches, you will be stuck paying an oversize fee.

The next time you buy luggage, you’d be wise to bring along a tape measure, so you don’t get hit with a surprise oversize bag fee when it comes time to fly.

On domestic flights, US Airways charges $175 for oversize bags, while American, Delta and United charge $200. Southwest seems like a bargain at $75.

For overweight bags, Southwest charges $75 for bags weighing 51 to 100 pounds. US Airways charges $90 for bags weighing 51 to 70 pounds and $175 for bags weighing 71 to 100 pounds. On American, Delta and United, the fees are $100 for bags weighing 51 to 70 pounds and $200 for bags weighing 71 to 100.

The fees are charged each way and are in addition to the checked-bag fee. If a bag is overweight and oversize, passengers must pay both fees, plus the checked-bag fee. For example, on American, Delta or United, to check a bag that weighs 60 pounds and measures 70 linear inches, it would cost $25 for the checked-bag fee, $100 for the overweight-bag fee and $200 for the oversize-bag fee, for a total of $325. And that’s each way.