BB.2 Which customary unit should you use?
What are customary units?
Customary units are the standard units of measurement used in the United States. They measure length, weight, and capacity (liquid volume). Unlike the metric system, customary units do not use powers of ten. You must choose the unit that matches the size of the object you are measuring.
- The length of a pencil is about 7 inches.
- The weight of a slice of bread is about 1 ounce.
- The capacity of a small milk carton is about 1 cup.
Choosing the wrong customary unit can make a measurement sound silly. You would not say a pencil is 7 feet long or a car weighs 2 ounces.
Customary units for length
Length measures how long, wide, tall, or far away something is. The customary units for length from smallest to largest are inch (in.), foot (ft), yard (yd), and mile (mi). Use the size of the object to decide which unit fits best.
- A paper clip is about 1 inch long.
- A ruler is about 1 foot long.
- A football field is about 100 yards long.
- The distance between two towns is measured in miles.
Remember: 12 inches = 1 foot, 3 feet = 1 yard, and 5,280 feet = 1 mile. When measuring a small object, use inches. When measuring a long distance, use miles.
Customary units for weight
Weight measures how heavy something is. The customary units for weight from smallest to largest are ounce (oz), pound (lb), and ton (T). Use ounces for light objects, pounds for medium objects, and tons for very heavy objects.
- A slice of cheese weighs about 1 ounce.
- A loaf of bread weighs about 1 pound.
- A small car weighs about 1 ton.
- A pencil weighs less than 1 ounce.
Remember: 16 ounces = 1 pound, and 2,000 pounds = 1 ton. If an object is too heavy to lift, it is probably measured in tons.
Customary units for capacity
Capacity measures how much liquid a container can hold. The customary units for capacity from smallest to largest are cup (c), pint (pt), quart (qt), and gallon (gal). Use cups for small amounts and gallons for large amounts.
- A small juice glass holds about 1 cup.
- A small ice cream container holds about 1 pint.
- A large milk jug holds about 1 gallon.
- A soup pot holds about 1 quart.
Remember: 2 cups = 1 pint, 2 pints = 1 quart, and 4 quarts = 1 gallon. A gallon is much larger than a cup, so use gallons for big containers.
How to choose the right customary unit
To choose the right customary unit, ask yourself three questions: What am I measuring? (length, weight, or capacity), How big is the object?, and What unit makes sense? Compare the object to the real-world examples you know.
- A spoon is small, so measure its length in inches.
- A dog is heavy, so measure its weight in pounds.
- A bathtub holds a lot of water, so measure its capacity in gallons.
- A book is light, so measure its weight in ounces.
If you are not sure, think of a familiar object that is about the same size. For example, a gallon of milk is about the same as a large water bottle.
Comparing customary units
Customary units are related to each other. Knowing the comparisons helps you convert between units. For example, you can change feet to inches or quarts to cups. Always multiply when going from a larger unit to a smaller unit, and divide when going from a smaller unit to a larger unit.
- 2 feet = 24 inches (2 × 12)
- 3 pounds = 48 ounces (3 × 16)
- 4 quarts = 1 gallon (4 ÷ 4 = 1)
- 6 cups = 3 pints (6 ÷ 2 = 3)
Memorize the basic equivalencies: 12 inches = 1 foot, 3 feet = 1 yard, 16 ounces = 1 pound, 2 cups = 1 pint, 2 pints = 1 quart, and 4 quarts = 1 gallon.
Estimating with customary units
Estimating means making a reasonable guess about a measurement. When you estimate, you choose the unit that is closest to the real size. Estimation helps you check if your exact measurement makes sense.
- A classroom door is about 7 feet tall.
- A textbook weighs about 2 pounds.
- A swimming pool holds about 10,000 gallons of water.
- A school hallway is about 30 yards long.
Estimation is not guessing wildly. Use benchmarks you already know, such as the length of your foot (about 1 foot) or the weight of a loaf of bread (about 1 pound).
Real-world problems with customary units
In real life, you often need to choose the right unit and then solve a problem. Read the problem carefully to decide if it is about length, weight, or capacity. Then pick the unit that fits the situation.
- Maria is baking a cake. She needs 2 cups of flour. (capacity)
- Tom is mailing a letter. He needs to know its weight in ounces. (weight)
- The track team runs 1 mile each day. (length)
- The soup recipe calls for 1 quart of broth. (capacity)
Always label your answer with the correct unit. A number without a unit does not tell you what was measured.
Why customary units matter
Customary units are used every day in the United States. They help you measure ingredients for cooking, distance for travel, weight for shipping, and capacity for filling containers. Knowing which unit to use makes you a better problem solver.
- A recipe might say "add 1 cup of milk."
- A road sign might say "Next exit 2 miles."
- A package might be labeled "weight: 3 pounds."
- A gas tank might hold 15 gallons.
When you see a measurement in a book or on a sign, ask yourself: Does this unit make sense for this object? If not, the unit may be wrong.
Common Core alignment: CCSS.MATH.CONTENT.4.MD.A.1 – Know relative sizes of measurement units within one system of units including km, m, cm; kg, g; lb, oz; l, ml; hr, min, sec. Within a single system of measurement, express measurements in a larger unit in terms of a smaller unit. Record measurement equivalents in a two-column table.
Notes for teachers
All content is 100% free, student-safe, aligned with CCSS.MATH.CONTENT.4.MD.A.1. Use it for whole-class instruction, independent practice, or homework.