People keep growing larger and airline seats keep growing smaller. That's the unhappy truth, at least in economy or coach class—where the vast majority of you sit. No wonder so many of us are interested in finding out as much as you can about what limited seat space you have. A reader recently asked:

"I know that SeatGuru and some other sites post seat legroom and width information for dozens of airlines, but are the numbers accurate and can I rely on them to help select an airline?"

The short answer is, "Yes, at least in general. The data those sites post are as accurate as the data the airlines furnish, and that information provides a reasonably good—although imperfect—guide to seat space." Our reader actually raises several key questions about airline seating. Here's what we know right now.

How do airlines measure seat space?

Most of the world's major airlines (and some you've never heard of) publish two measures of seat space:

• Front-to-rear space is measured by "pitch," defined as the distance between any given point on a seat to the identical point on the seat in the next row forward or to the rear.

• Side-to-side space is generally measured by the width of the seat cushion.

Are these the right measures for economy class?

Not quite, but they're all that are available right now.

Pitch is a reasonably accurate way to measure how much front-to-rear space you have, for your legs and at reading-working level. However, pitch ignores the fact that some seatbacks are thicker than others, so, at any given pitch, thinner backs give you the effect of more legroom and working room than those with thick backs. Unfortunately, nobody has come up with a better measure, so we're probably stuck with pitch as the standard.

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Seat cushion width is a good measure of width for women but a lousy measure for men. (No sexism intended; we're built differently.) Hip width is the critical seating dimension for a vast majority of women, and cushion width reflects hip room accurately.

For most men, however, the critical width is at shoulder level, and seat cushion width doesn't get at that measure. Instead, what the airlines should measure is width between centers of adjacent seats—easy to do if you have the engineering drawings for the seats. For a quick check, however, width between the midpoints of the armrests is a reasonably good alternative, at least in economy.

Sadly, I see no likelihood that the big airlines will switch to shoulder-width data within the foreseeable future. We're stuck with the cushion data.

How well do economy class seats accommodate today's travelers?

Very poorly. The only people who find adequate space in economy are anorexic jockeys and teen-age female gymnasts from eastern Europe.

Width is a particular problem. The most comprehensive anthropometric study of airline seating space I could find concluded that to be comfortable, American women require seat cushions at least 20 ½ inches wide and American men require shoulder width of at least 22 inches. Moreover, that study—by ICE Ergonomics and two universities, no longer available online—was published 10 years ago, and Americans have grown even wider since then. No ordinary economy class seating in the world meets those standards, and neither Airbus nor Boeing narrow—body jets are capable of providing adequate width in the preferred (by airlines, not travelers) six—abreast configuration.

Legroom isn't as easily quantified as width, because of the varying geometries of different seat designs. But subjective reports suggest that adequate legroom requires a minimum of 34-inch pitch for a reasonable level of comfort. Only a very few ordinary economy class seats meet that standard, although airlines could easily do so if they wished.

How about premium economy, business, and first classes?

Cushion width measurements substantially understate the differences between economy and all premium classes. Even though a premium seat cushion often measures only an inch or two wider than economy, premium seats typically have armrests 4-6 inches wide, or maybe even separate armrests plus a small table between adjacent seats, providing far more than just an inch or two more shoulder room.

You can still rely on pitch numbers for domestic premium classes. But pitch doesn't work for international lines' various angled-seat designs. Instead, use the stretch-out measure many of them provide.

Where can I find seat information?

Many of the world's airlines show seat pitch and width on their websites. Alternatively, several independent websites compile data from dozens of airlines:

• SeatGuru was, as far as I know, the first such site, and it's still a leader. It posts detailed seat maps plus width and pitch data. Currently, SeatGuru provides info for each plane model flown by just under 100 airlines, with more steadily added. It provides a dynamic display of special details as you move your cursor over different seats on the seat map. SeatGuru bests the airlines' own sites in terms of ease of use and detail.

• SeatExpert, now part of Randy Petersen's frequent flyer empire, is similar, but it posts only about 50 airlines.

• SeatMaestro, based in Europe, provides information on 141 airlines, including many small Asian and European lines that the sites based in the U.S. do not.

• Seat Plans, from the publishers of the excellent London-based Business Traveller magazine, also covers dozens of airlines—notably some of the smaller European lines that you won't find anywhere else. It also posts lots of traveler reports, although they're weighted heavily to business class.

Other sites publish less detailed seating data, not as useful as the four detailed sites. And, as far as I know, all the sites post information they get from airlines—nobody provides any independent information.

Overall, I suggest you start with SeatGuru. But for smaller foreign airlines, check with a few of the others.

Which planes are the best in economy?

If you don't have time for detailed research, here are some general guidelines about economy seats:

• Width: Most lines' widest seats are generally in 777s, followed by Embraer 170/175/190/195s; then by most Airbus models, MD11s, MD80s, 767s, and 717s; then by 737s, 747s, 757s, and most older regional jets. The very tightest are in those few 767s with 8-across seating, 777s with 10-across, and Airbus widebodies with 9-across.

• Legroom: Among the domestic lines, JetBlue is tops with no extra charge, and if you're willing to pay a small premium, the front seats on JetBlue have outstanding legroom. Otherwise, United's Economy Plus cabins are better than anything else. Check SeatGuru or one of the other sites for international lines.