Discuss as:

The best -- and worst -- seats for economy fliers

Which U.S. airlines have the roomiest, leg-stretching seats on domestic flights for no extra fee and which carriers' seats are so cramped that they could be advertisements for deep vein thrombosis?

If you are among the millions of airline passengers who wait at the gate when more frequent fliers board early to snatch their seat upgrades or premium seats purchased for an additional fee, then here’s some advice for landing a roomy seat without having to shell out extra money: Fly JetBlue or Virgin America.

On the other hand, you should definitely avoid Cape Air, Spirit, and Allegiant if don’t want to feel crammed in .

Read the full report on best and worst economy seats on U.S. Airlines

This analysis comes from www.routehappy.com, a New York-based startup building a flight-amenities search engine. Routehappy crunched the numbers for domestic flights in September 2012, and put JetBlue, US Airways, Virgin America, Hawaiian Airlines, United and American in the roomy seat category for all or some of their flights, depending on the airline.

With some carriers, including JetBlue, Virgin America, Cape Air, Spirit and Allegiant, it is easy to figure out whether to choose or avoid them because they each operate only one aircraft type domestically, and seating is standard on every plane. However, when it comes to larger carriers such as American and US Airways -- which fly multiple types of aircraft in different configurations -- the decision-making becomes much more complex.

That’s why in the Routehappy rankings, carriers such as US Airways and American Airlines appear in both the roomy- and tight-seat categories.

Related: Flight cancellations surge at American Airlines

Routehappy ranked the airlines with the roomiest and tightest seats by number of flights per day within the U.S., and secondarily looked at the percentage of flights within each airline.

So while US Airways is a leader in flights per day with roomy seats, it is also seventh among airlines by flights per day with tight seats, and these cramped flights make up 5 percent of its September 2012 schedule.

What it comes down to -- with the exception of JetBlue, Virgin America, Cape Air, Spirit and Allegiant -- is you can’t look for seat comfort based on the airline livery and logos, but you have to take a close look at the route and the aircraft type being used.

For example, avoid US Airways and its Boeing 737-400 aircraft if you are flying Fort Lauderdale, Fla., to Charlotte, N.C., if you are in the market for a comfortable ride because the tight seats may cramp your style -- and your legs.

But the US Airways Airbus A321 is just the ticket when flying Charlotte, N.C., to LaGuardia in New York because the pitch is 32 inches and the seat cushions are 18 inches wide.

In other words, roomy.

More from Skift.com