Regardless of what you happen to be measuring,
10 mm = 1 centimeter
18 mm = (18/10) = 1.8 centimeter
Assuming you mean a quarter US dollar, it is 2.426 cm.
Meter.... or more likely millimeters
the quarter
That depends what it is a quarter of.
The diameter of a quarter is 24.26 mm (0.955 in).
The diameter of a United States 25 cent (quarter) coin is 2.426 centimetres, or 24.26 millimetres.
Assuming you mean a quarter US dollar, it is 2.426 cm.
The measurement of 4 mm in diameter is equal to 0. 1575 inches in diameter. It is also equal to 0. 4 centimeters in diameter.
Meter.... or more likely millimeters
the quarter
That depends what it is a quarter of.
You could use a simple ruler, or a micrometer for greater accuracy.
The diameter of a quarter is 24.26 mm (0.955 in).
2 and a half centimeters, which is about the diameter of a US quarter coin.
An investigator would find that scientists use the meteras a measure of length. This standard can be applied with a prefix to denote multiples or fractions of the standard. Let's look at a couple.We'd see the term kilometer applied to mean 103 or 1,000 meters. You might measure the distance from, say, Denver, Colorado to Omaha, Nebraska using kilometers. It'd be about 870 kilometers (km).A meter divided into a hundred equal parts is made up of 100 centimeters, as a centimeter is 10-2 or 1/100th of a meter. If you wished to express the diameter of, say, a United States quarter using this system, you'd say it was approximately 2.4 centimeters (cm) in diameter. Using 1/1000th of a meter (10-3 or a millimeter) to measure that quarter, you'd say it's diameter is about 24 millimeters, or 24 mm.
A US quarter has a diameter equal to 0.00002426 km
Inside diameter, it is usually considered nominal. Anything is considered to be the size it is called, if you use fine enough measure almost anything is going to vary or be off a few thousands of an inch.