I've got a new Jamis Renegade Exploit (renegadeexploit). I want to put a slightly longer stem on and I am confused as to what size stem I need to fit my bike. I am not referring to the length of the stem, but the size where it attaches to the bike and the size at the handlebars. Both the fork and handlebars are stock.

The purpose for the switch is to go from a 90 to a 100mm stem. I would like to stick with a Ritchey - either a longer one that is the same as the stock one or an upgraded stem.

Thanks in advance for any advice.

You need a 1-1/8" stem (hard not to get this with threadless stems) and probably a 31.8mm bar clamp diameter. If the bars have a big bulge where the stem attaches, it's 31.8. If not, it's 26.0. A few road bars (mostly older or boutique models) are 25.4. Almost all new bikes use 31.8.

Why is the steerer tube measured in inches, and the handle bar diameter in mm?

By the way, get a 31.8 stem, even if your bars are 26 you can get adapters so they fit. You'll have more flexibility to change bars with the 31.8.

Almost all modern road stems are the same:

1 1/8th inch at the steerer tube, 31.8mm at the clamp.

Honestly you'd be hard pressed to buy the wrong stem these days, given than smaller handlebars are rare and niche, and 1" steerer tubes are uncommon/retro.

So something like this:

Amazon.com : Ritchey Comp 4Axis Road/Mountain Bicycle Stem - 31.8 x +/- 84 Degree - BB Black : Sports & Outdoors

is what you want. Of course you may want to go up a few grades from the "Comp". One word of warning, if you want easy installation, beware the Ritchey C-260 stem, it's kind of hard to get the handlebars on it without removing the bar tape in some cases. I have one and it's a great stem, just the installation can be finicky.

Shopping in person will let you try before You Buy.

It Just Is , get used to it! .. Its a Tube and tube standards are Inches and Fractions ..

but metric equivalencies seem to be how the Bike Biz speaks 31.8 is almost 1.25" ( 1.25196850394 )

There was a French Metric tube spec for a while , but they Lost out in the market place
and now buying a french frame creates multiple Problems .. 25 vs 25.4 and so forth.

They wanted the 0 Prime Meridian to be Paris , but the British Navy was Dominant even 300 years ago

so it has been Greewnwich Mean Time .. England where the Prime Meridian is for Time and Navigation.

The Jamis Renegade Exploit's spec sheet confirms the bike has a 1 1/8" steerer and 31.8 handlebar like the guys said above. It would be somewhat surprising if it had something different. Along with the clamp sizes and length, the remaining variable for a stem is the angle. The stock stem is a six degree stem if you want the new one to keep the same angle. Or 84°, depending on how it's listed. (Either way, it's 6° from a right angle.)

The actual diameter is 31.75 mm or 1.250" The numbers are rounded off for simplicity.

Thanks to everyone for your input. Got a new stem on the way.

One measurement is worth 1,000 assumptions. Measure both your fork's steerer tube and your handlebar clamp surface yourself. I've been messing with bikes for a long time. I can't tell you how many times I skipped measuring something because I was sure that I knew the answer and was wrong.