In an emergency, it's where you'll find a life vest. But on an average flight, it's where your feet and your stuff will battle for space. The space under an airline seat is limited, as any frequent traveler knows. As airlines start charging passengers for their carry-on suitcases, the space under the seat becomes more valuable than ever. In order to squeeze as much as possible down there, start with a bag that complies with the airline's size restrictions.

How Much Space is There?

Airlines don't actually share the dimensions of under-seat space on their planes. Those dimensions aren't relevant to the traveler, anyway. Anyone who has peeked under a typical coach seat has seen the removable life vest packages and metal crossbars that take up space down there.

It's the airline's baggage size restrictions that really matter when packing. Airlines categorize bags that fit under seats as personal items. A personal item that exceeds the airline's maximum dimensions probably won't fit completely under the seat, which means airline staff will insist it be stowed overhead. That's not a big deal in many instances, but it can be an issue on a completely full flight or if you've purchased an economy ticket that doesn't include access to overhead bins.

Size Restrictions

Below-seat dimensions vary slightly by airline. American Airlines accommodates personal items up to 18 inches by 14 inches by 8 inches in size. JetBlue's under-seat allowance is a little more restricted at 17 inches by 13 inches by 8 inches. Southwest has one of the most generous policies. It allows travelers to bring personal items of up to 18.5 inches by 13.5 inches by 8.5 inches onboard.

Space is even tighter on certain international airlines. On Air Canada, personal items may not be larger than 17 inches by 13 inches by 6 inches. Air France is even stricter, limiting passengers to bags that are 16 inches by 12 inches by 6 inches, or smaller.

Size standards are subject to change as planes are reconfigured. Space allowance may even vary from seat to seat, with window seat residents sometimes getting a little less room than their middle- or aisle-seat counterparts. Get specific guidance from the airline before flying with a crated dog in the cabin. The crate must fit under the seat, so it's essential to have accurate measurements for both the space under the seat and the crate itself.

The Best Bag to Stash

Officially, airlines consider personal items to be things like purses, briefcases and laptop bags. Unofficially, they generally don't care what type of bag you bring as long as it fits in the allotted space. A backpack, tote bag or duffel bag is a good choice. It's always best to choose a soft-sided bag to go under the seat, which gives you the flexibility to compress the bag and wedge it into a tighter-than-expected space. Make sure the bag you choose zips or otherwise closes completely so nothing falls out while it's under the seat. Keep in mind that both your feet and the feet of another passenger might kick the bag during flight, so avoid storing crushable food or delicate electronics in there.