i inch
Grab a ruler. Draw a 3 cm line. There you go!
On a standard ruler, 3.9 inches would be equivalent to 9.906 centimeters. This conversion is based on the fact that 1 inch is equal to 2.54 centimeters. Therefore, to convert inches to centimeters, you would multiply the number of inches by 2.54.
Scroll down to related links and look at "Inches and centimeters in comparison".
4 inches are 10.16 centimeters.
A ruler
The uncertainty of a ruler in centimeters refers to the smallest measurement that can be reliably determined using the ruler. This uncertainty is typically around 0.1 centimeters, meaning that measurements taken with the ruler may have a margin of error of up to 0.1 centimeters.
You should get a ruler and start measuring things in your home that look like they'd be two centimeters.
The customary ruler uses both inches and centimeters.
a 30cm ruler
It look like 1.93
where is .39 on a ruler
You use a ruler or similar object that is marked with centimeters. If your ruler only has inches, measure the inches, then multiply with 2.54 to get centimeters.