"I wear size 14 in U.S. dresses. What size is that in salwar kameez?"
Salwar kameez size doesn't translate well to US and UK sizes, partly because the kameez shape and fit differs from most US dresses, and partly because kameez size considers only one measurement—the bustline. You can find your kameez size by using a tape measure to measure yourself (a.k.a. body measurements) or to measure a salwar suit that fits (a.k.a. garment measurements).
Method 1: Body measurements
You'll need a tailor's measuring tape, which is available for a few dollars at craft stores, fabric shops, and many grocery and drug stores. (You can make do with a length of ribbon that doesn't stretch and a ruler, but your measurement will not be as accurate.)
Find the 1" mark on your measuring tape, and hold it in front of your body at bust level. Use your other hand to pull the rest of the measuring tape behind your back, round the other side of your body, and back to the front. Use a mirror to make sure the tape is snug (not tight) around your back, isn't twisted, and stays level with the fullest part of your bust. Find the measurement where the end you started with overlaps with the rest of the measuring tape as it comes around to the front.
Write down your bust measurement. Now add some room to breathe (ease in US terms, and loosing if you're talking to an Indian tailor): add 2" for close fit, 3" for average fit, and 4-5" for loose fit. If the result is an odd number, round up to the next even number. The result is your salwar kameez size.
Not sure if you measured right? Roughly, a bust measurement of 34-38" corresponds to US size XS to M, a bust measurement of 39-44 corresponds to size L or XL, and a measurement of 45-52 corresponds to 1X to 3X. If your measurement doesn't match the size you would expect, double-check your measuring tape to make sure you're using the side with inches, not centimeters, you started at the 1" end, and that the tape hasn't twisted.
The salwar kameez size system
The kameez size is literally the measurement (in inches) around the kameez at the widest part of the bust area, which is usually just below the sleeves. In many ways, this size system is more reliable than US sizes, which vary between stores and even between brands in the same store.
Ready-made suits come in even numbers.... size 36, 38, 40, and so on. Some manufacturers change to half sizes (measured only across the front) for sizes above 46, so a kameez that's 50 inches around at the bust might be labeled as size 25. Size labels in salwar kameez shops are notoriously inaccurate, and a tape measure comes in handy for checking size when you're deciding which suits are worth trying on.
If American clothing in your size generally fits right off the rack, you'll probably be able to order salwar kameez by size alone and expect it to fit. However, if you're petite, tall, long-waisted, broad-shouldered, or curvier than average in the bust or hip, the tailor will need a few more measurements to make a suit that will fit you ell. That's the subject of an upcoming post.
Related post: Find your salwar kameez size from a kameez that fits