When it comes to baggage fees, most of us know we’ll probably have to pay something. Is there any way to pay less or avoid them altogether? Sometimes there is.

Here are five things to know about bag fees before you book your next flight.

1. Bag fees can vary a lot from airline to airline.

The following fees are one-way prices for economy passengers. As for those fees shown in a range of prices, what you pay will vary depending on when the fee is paid, the duration of the flight and other factors.

•Frontier: Carry-on is $35 to $60; 1st checked-bag is $30 to $60

•Hawaiian: Carry-on is free; 1st checked-bag is $25 to $30

•Allegiant: Carry-on is $10 to $75; 1st checked-bag varies by route

Bag fees can also vary quite a bit, even within a single airline. Check out these Spirit fees for a trip from Los Angeles to New York in July.

•Carryon: From $27 to $65

•1st checked bag: From $31 to $65

Tip: Usually, the cheapest time to pay bag fees (and this is true for Spirit) is when you book the flight. If you wait, the price usually goes up with the biggest fee reserved for those who pay at the airport on departure day.

2. Overweight bags can be very expensive

If your bag weighs even a single pound over what the airline allows (typically 50 pounds), you will be penalized with a steep overweight fee. Here’s an example from American Airlines.

If a checked bag weighs 51 pounds or more, it will cost an additional $100 one way. If the bag weighs more than 70 pounds and you’re flying to China, Japan, South Korea or Hong Kong, you will pay an extra $450. Tip: Pack light, and weigh bags before leaving for the airport.

3. Free bags still exist

Southwest is the sole U.S. airline that still offers all passengers three bags for free: a carry-on and two checked bags. Anything over that allowance and you’ll be charged $75 per bag. You will also pay $75 for any overweight luggage.

4. Careful. Not all carry-ons are free, even on major airlines

You will get a free carry-on in economy class on American, Delta or United. However, if you fly the fairly new Basic Economy class on United, you are not allowed to bring a carry-on into the cabin. Instead, you check it and pay a fee. The good news is American used to do something similar in its Basic Economy class but later dropped the carry-on restriction — maybe United will, too.

5. How to figure out the best deal

Don’t fall into the trap of thinking you got a good airfare price, and since your bag is free or very cheap, this is the deal for you. It may be! But you won’t know for sure until you compare fares and do the math. You know, find multiple good deals, check for baggage fees, then add those in and do your final comparison. In this era of fees, it’s the only way of knowing if that deal is really a deal.

Final tip: Once in a while, a checked bag may actually be a little cheaper than a carry-on, but saving a couple of bucks may not be worth it in that case. You see, the beauty of traveling with a carry-on is it’s a bag an airline can’t lose.

Rick Seaney is the CEO for FareCompare. Opinions expressed in this column do not necessarily reflect the views of ABC News.