Where can you get the full story on airline seat space?

With airline coach seats getting so tight, even an extra inch or two of legroom and side-to-side space can be important on any flight that lasts more than a hour or so. Although you can get quite a bit of information about seat dimensions online, you don't get as much as you really need.

Two measurements fix the amount of seat room you get. First, airlines measure legroom by seat pitch, the front-to-rear spacing of the seat. Among larger U.S. lines, pitch in ordinary coach ranges from a very tight 30 inches to an almost-generous 34 inches. Second, airlines measure seat width as the width of the seat cushion. Coach seat cushion widths range from a very tight 17 inches to 18-and-a-half inches.

All big U.S. airlines post seat pitch and width on their websites, as do most small U.S. lines. However, only a few of the foreign lines provide that information to the public.

Several third-party websites specialize in providing airline-seating information:

• Seat Guru is by far the most detailed. It lists data for 15 U.S. airlines, plus 10 of the larger foreign lines. The website is extremely well programmed, with detailed seat maps of each aircraft each line flies in the U.S. The seat maps are interactive: When you place the cursor over a seat, the map displays comments about that particular seat.
• SimplyQuick covers more airlines than Seat Guru, but it provides much less detail for each.
• Flat Seats provides details on airlines that offer truly lie-flat seats in business and first class—data mainly of interest to the expense-account set.

With all that info, you'd think you could really get a fix on airline seating, but, sadly, you can't. The airlines' own data—and therefore the data that goes into Seat Guru and other sites—misses out on two vital counts.

First, seat pitch on many planes is given as a range, without noting which specific seat rows have more and which have less legroom. The only exception is JetBlue, which tells you that rows 11 to 26 have the extra space.

Second, width measurements are even worse. Seat cushion width isn't the correct way to measure width. What matters is total width at shoulder level, which should be measured as the side-to-side distance between centers of adjacent seats.

Cushion width comparisons grossly understate the differences between coach and premium economy-, business- or first-class seats. In 737s and 757s, for example, most U.S. lines quote coach seats as 17 inches wide and first class as 21 inches. However, the actual difference in shoulder width is several inches greater, because first-class seats typically have a separate, much wider armrest for each seat, as well as a small table between the armrests.

Also, take what some airlines say with a grain of salt. AirTran, for example, says the seats in its 737s are 18 inches wide rather than the usual 17. But, realistically, it's hard to imagine how those AirTran seats are really an inch wider than seats on other 737s where it matters, at shoulder level. Where did they get the extra six inches per row that are required to accommodate wider seats?

Is imperfect information better than none? Sure. But despite airline postings and Seat Guru's great website, there's no place you can currently go to find a truly accurate picture.