I used to measure by hand, but I'm lazy when it comes to the more mundane aspects of my hobby, and the tool designed for the job is good for the lazy... and it happens to be accurate as well.
Width is easy. Don't squeeze on the calipers too much- done.
Circumference is a little more difficult with a slightly out of round example, like almost every single oring I've ever measured, but geometry solves that in 30 seconds.
You can slap that oring down on a ruler and measure without worries:
Ellipse Calculator for the math challenged.
There are also print-outs that I've stumbled upon in the past, where you place the oring on the picture that matches best. Or measure the best you can, make an educated guess, and then order one size bigger and one size smaller than your measurement, just in case. My OCD couldn't handle that, so geometry was the solution until I purchased tools.
Whatever is good for you, your brain, and your budget.
I happen to have a collection of tools that have probably only been used once, so I'm certainly not the person to ask if one should purchase a tool or not, because the answer is always "yes".
That's the kind of person I am. I'm a little dense, but I have an excellent collection of all sorts of tools that are "used: like new" that would make a garage sale tool collector think they'd died and gone to heaven.
