Measuring Length & Distance

Every ruler, tape measure, and road sign uses one of two systems: customary units (inches, feet, yards, miles) or metric units (millimeters, centimeters, meters, kilometers). Learn what each unit is good for, how to read the small fraction marks on a ruler, how to convert between units, and practice with the activities below.

Customary Units

Customary units are the system most commonly used for everyday measurement in the United States.

Unit	Abbreviation	About the size of
Inch	in	The width of a quarter, or about the length of a paperclip.
Foot	ft	A standard sheet of paper is about 1 foot long.
Yard	yd	About one big step, or the length of a baseball bat.
Mile	mi	About 4 times around a standard running track.

Fractions of an Inch

Rulers and tape measures don't stop at whole inches — the space between each inch mark is divided into smaller and smaller equal parts: halves, quarters, eighths, and often sixteenths. Reading those small marks correctly trips up a lot of people, but the pattern behind them is simple once you see it.

The pattern: the longer a tick mark is, the more "important" the fraction is. The tallest marks are the whole inches. The next-tallest split each inch into halves. Then quarters. Then eighths. The shortest marks split each eighth in half again, into sixteenths.

To read a mark, count how many small sixteenth-steps it is past the last whole inch, write that as a fraction over 16, then simplify. For example, if a mark is 4 steps past the 2-inch mark:

Example — reading a fraction mark

4 steps past 2 inches = 4/16 of an inch

4/16 simplifies to 1/4

The reading is 2 1/4 inches

Here's what the most common sixteenth-marks look like once they're simplified:

2/16 = 1/8

4/16 = 1/4

6/16 = 3/8

8/16 = 1/2

10/16 = 5/8

12/16 = 3/4

14/16 = 7/8

Metric Units

The metric system is used by scientists everywhere and by most countries in the world. Its biggest advantage: every unit is related to every other unit by a power of 10, so converting is just a matter of shifting a decimal point — no awkward numbers like 12 or 5,280 to remember.

Unit	Abbreviation	About the size of
Millimeter	mm	About the thickness of a dime, or the length of a small paperclip (about 33 mm).
Centimeter	cm	About the width of a fingernail.
Meter	m	A little longer than a yard — about a big stride.
Kilometer	km	About the length of 10 football fields laid end to end.

What the Prefixes Mean

Every metric length unit is built from the same base unit — the meter — with a prefix in front that tells you how it's scaled up or down. Once you know what a prefix means, you know how that unit relates to a meter.

Prefix	Meaning	Relative to a meter
milli-	one thousandth	1 mm = 0.001 m
centi-	one hundredth	1 cm = 0.01 m
(none)	the base unit itself	1 m = 1 m
kilo-	one thousand	1 km = 1,000 m

Think of it like money: a meter is like a dollar. A centimeter is like a penny — it takes 100 of them to make a dollar. A millimeter is smaller still — it takes 1,000 of them to make a dollar. And a kilometer is like a $1,000 bill — it takes 1,000 dollars (meters) to add up to one.

Because each unit is a power of 10 apart, the units also relate to each other directly, not just to the meter:

1 cm = 10 mm

1 m = 100 cm

1 m = 1,000 mm

1 km = 1,000 m

Converting Between Units

Converting within a system just means multiplying or dividing by the relationship between the two units.

1 ft = 12 in

1 yd = 3 ft = 36 in

1 mi = 5,280 ft

1 cm = 10 mm

1 m = 100 cm = 1,000 mm

1 km = 1,000 m

The rule: going from a bigger unit to a smaller unit, multiply. Going from a smaller unit to a bigger unit, divide.

Example — bigger to smaller

How many inches are in 4 feet?

4 ft × 12 = 48

4 feet = 48 inches

Example — smaller to bigger

How many meters are in 350 centimeters?

350 cm ÷ 100 = 3.5

350 centimeters = 3.5 meters

Practice

Four ways to practice what you've learned.

Pick the Best Unit

Choose the unit that makes the most sense for each measurement.

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Conversion Practice

Type your answer as a whole number and check it.

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Which Is Longer?

Two measurements, two different units. Which one is actually bigger — or are they equal?

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Read the Measuring Tape

Where is the arrow pointing? Count the small steps past the last whole inch and simplify the fraction.

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