Hello there,
In preparation for a tour this April from Denver to Philadelphia, I'm swapping the mountain bike handlebars on my Scott hybrid for more touring-friendly drop bars. The problem is, I'm having trouble determining the mounting width (it's not written anywhere on the bar), so I have no idea whether I need 31.8 or one of the older, thinner models. I suppose I could bug my local mechanic, but I was wondering if there was a way I could do it on my own. I no longer have the owner's manual, and the Scott website does not feature the specs for my particular bike. I have a funny feeling I might need a new stem, too, but first I'd like to try to find a pair of drop bars that fit.
Any suggestions?

Yes there is a way to do it on your own. You could measure it. I recommend using calipers.

I suppose I might be able to root around and find some common item that would at minimum determine whether it is 31.8, but too much bother. You should not have a hard time locating a pair of calipers to borrow for a minute, whether from an auto shop, friend, or even at a hardware store.

But what about the shift and brake levers? Unless you have spare brifters laying around that's a bigger issue.

You can get cheap plastic calipers for a couple bucks at any hardware store. I wouldn't do any fine machining using them, but they are good enough to determine what diameter handlebars and seatposts, spindle lengths, etc. you have. I keep a cheap caliper and a 6' tape measure in my glove box and another set in my tool box.

Wrap a strip of paper around the bars at the widest diameter near the stem and mark the over lap with a pen. Then measure the distance between the marks. A 25.4 mm bar will have the marks 79.8 mm apart and a 31.8 mm bar will have the marks 99.9 mm apart.

Pic? It's pretty easy to eyeball.

Use a quarter, if it's 31.8 it will be just about the same size as a quarter (American currency), other sizes will be significantly smaller, and more finicky to measure.

You could also just take a ruler and put it slightly to the left or right of the stem clamp and if it's one inch, it's a 25.4mm bar (which I think it is) and if it's significantly (1/4 inch or so) larger, then it's a 31.8mm bar.

+1, this is straightforward, easy to do without special tools, and plenty accurate for the job. The other option is to put a ruler next the stem. You should be able to easily eyeball the difference between 1" and 1-1/4".

Changing the bar is easy but getting drop bars to play with hybrid bikes is not trivial. Watch out for:
Cable routing
Inline cable tensioner
Reach to primary handhold (brake hoods)
Road compatible front mech indexing.

Most Euro tourists are using trekking/butterfly style bars to get extra handholds with hybrid/mtb compatibility.

Buying a set of calipers is the correct solution.

Wow! What an outpouring of support! Thank you one and all. Now all I have to do is track down a set of 25.4 drop bars!

Thanks a lot! Ordered some new STIs just last week. But now that I've determined my current handle bars are 25.4, I'm having a tough time tracking down drop bars that aren't 31.8, so I guess I'll be in the market for a new stem, too. Anyway, thanks again!

www.jensonusa.com/!EWidxS85RkCQFT6cPjzBbw!/Dimension-Double-Groove-Road-Bar?utm_source=FRGL&utm_medium=organic

A large number of people tour the world on trekking bars, their figure 8 bend offers a variety
of hand holds , and they swap the MTB controls directly without even needing re Cabling .

as your research should uncover reading prior posts, here, some people don't like how
the left STI road lever interfaces with a mountain type front derailleur.

I have same size drops on a hybrid, and a problem with how the left STI road lever interfaces. Because of the way my cables are routed, my triple becomes a double. But serves the purpose for what I needed. The smaller diameter drops are easy to find, but is the smaller width of the choices and pretty affordable especially on Amazon/eBay.

What do you mean by that? Did you lose a chain-ring setting? I have three settings on my chainring, and I need all of them.

Thanks, by the way, for the feedback! I might end up having to get Euro bars or just some better barends.