caliber, caliper

Caliber or caliber means the measure of the inside of a tube, or something else that is a section of a cylinder--
You can't use a bullet of that caliber in this gun. It's the wrong size.
You'll need a length of 2" caliber pipe to finish this job.

Caliper or caliper ( almost rhymes with caliber, usually "calipers" ) means a tool to measure the thickness of something, or its inside or outside diameter, or the distance between its parts. A caliper has two sides hooked together like a pair of tongs or forceps--
You will need an inside caliper to measure the tube.
Caliper also means the thickness of paper, for example, or a tree--
If we use 1/8" caliper cardboard, the cartons will be less expensive.

Now that you know that, you can say--
"We can measure the caliper of the tree before we cut it down, and the caliber of the hollow log after it is cut."
"We will need a caliper to measure the caliber of the tube."

Note--you may encounter "caliber" used to mean the quality or measure of worth of a person. This seems trite, as well as nonsensical, as people can't be measured like tubes. It's an expression that annoys a lot of people.
Note # 2--"calibre" is a British or old-fashioned spelling of "caliber".