It's made from the same material as most microfiber panties so there's some stretch to it. Unstretched, the band measures about 10" from side to side (not circumference) and the fullest part measures 14" (again, not circumference). From the top of the shoulder to the band, it's approximately 12". Here's a picture with a standard size computer mouse, for comparison.
And one without so you can see the happy little "underwire" that is not really any sort of underwire.
It just gives you general guidelines of where it should sit on your person. Once I put it on, and it was stretching for all its worth, it wasn't so happy. Do you see those bands of stitching that run from side to side in the picture above? Well, like stitching does, there was less "give" along those lines so my "fullest" part was compressed. Also, because the stitching runs up and over--rather than in a circumference parallel to the ground--the fullest measurement was compromised. By a good two inches. But, I digress.
Inside is the spider web of stitching.
DO NOT WASH IT!!
...at least not in the washing machine
So, is it accurate?
In a nutshell? No, not by correct bra-fit standards. But...it is by Soma standards, using the compromised numbers that it measured.
The four measurements that it calculated were:
Top line: 45.9
Second line: 44.6
Third line: 42.3
Bottom line: 38.8
Which a result of bras in size 40DD
When I use accurate measuring techniques, I get:
Loose Underbust: 39
Snug Underbust: 37.5
Tight Underbust: 35.5
Standing Bust: 47.5
Leaning Bust: 49
Lying Bust: 48.5
Which results in size 38GG/H UK sizing
If you were to turn that 38GG in a 40 band it would be 40G which is FOUR cups larger than what the Innofit put me at. Now, to be fair, the Innofit DID NOT add four inches to the band measurement. It merely rounded to the nearest whole band size which would be a 40. That really isn't that far off of what I measure my own band at, if you left off the "Tight" numbers.
Unfortunately, if the Bottom line had measured at 36, it would have still put me in a 40 band because the Innofit is based off Soma's size guide and that guide is terribly inaccurate. Also, because of the compression of my own bra--which you're required to wear under the Innofit for best accuracy--and the added compression of the Innofit, the cup size was not accurate, as I already mentioned.
All in all, the journey continues to get proper bra-fit education out to the masses.