There are certain ISO standards for different rulers. But there is no 'standard' size. So it would depend on the one to which you refer.
30 centimeters
Usually 30.
For a small ruler . There are 15 cm in 1 ruler. wheras a big ruler has 30 cm in a ruler.
30.48 cm
100 centimeters.
A ruler is usually divided by: inches, centimeters, and millimeters.
An inch is bigger than a centimeter. The size of the rule depends on how many inches or centimeters you want it to be. One inch is equal to 2.54 centimeters. To convert inches into centimeters, divide the inches by 2.54.
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.
The customary ruler uses both inches and centimeters.
a 30cm ruler
4 inches are 10.16 centimeters.
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.