Mathematics · Grade 3 · Data and Measurement Stories · Term 3

Measuring Length and Mass

Students use standard units to measure length (cm, m) and mass (g, kg).

• Ontario Curriculum Expectations
• 3.MD.A.2

About This Topic

Measuring physical attributes is a practical application of number sense and spatial reasoning. In Grade 3, the Ontario curriculum shifts focus toward standard units of measurement, including centimeters and meters for length, grams and kilograms for mass, and milliliters and liters for capacity. Students learn to select the most appropriate unit for a task and use tools like rulers and scales with precision.

This topic is essential for everyday life, from cooking and construction to science and sports. It also provides an opportunity to discuss how different cultures have historically measured things (e.g., using body parts or natural cycles) before the adoption of the metric system in Canada. Students grasp this concept faster through hands-on investigations where they must estimate, measure, and then compare their results with others.

Key Questions

1. Explain why we need standard units like centimeters or grams instead of using our hands or feet.
2. Analyze how we choose the most appropriate unit for a specific measurement task.
3. Compare the relationship between the size of the unit and the number of units needed.

Learning Objectives

• Compare the number of units needed to measure the same length using centimeters versus meters.
• Analyze the relationship between the size of a unit of measurement and the quantity of units required to measure an object.
• Explain why standard units of measurement are necessary for consistent and reliable communication.
• Calculate the mass of objects using grams and kilograms, selecting the appropriate unit for each item.
• Demonstrate how to use a ruler to measure length to the nearest centimeter and a scale to measure mass to the nearest gram.

Before You Start

Introduction to Measurement

Why: Students need a foundational understanding of what measurement is and why it is useful before learning about specific standard units.

Comparing Lengths and Masses

Why: Students should have prior experience comparing objects based on their length and mass using non-standard units or simple comparative language (longer, shorter, heavier, lighter).

Key Vocabulary

centimeter (cm)	A standard metric unit used to measure length. There are 100 centimeters in 1 meter.
meter (m)	A standard metric unit used to measure length. It is longer than a centimeter and often used for larger distances.
gram (g)	A standard metric unit used to measure mass. It is a small unit, often used for lightweight objects.
kilogram (kg)	A standard metric unit used to measure mass. It is equal to 1000 grams and used for heavier objects.
standard unit	A measurement unit that is agreed upon and used consistently by everyone, like centimeters or grams, to ensure accurate communication.

Watch Out for These Misconceptions

Common Misconception

Students often start measuring from the '1' mark on a ruler instead of the '0' or the edge.

What to Teach Instead

Use 'broken rulers' (rulers that start at 3 or 4) to force students to count the actual units. Peer checking during measurement tasks helps students catch this common error and understand that measurement is about the space between the marks.

Common Misconception

Confusing mass (how heavy) with capacity (how much it holds).

What to Teach Instead

Compare a large, light box (high capacity, low mass) with a small, heavy rock (low capacity, high mass). Hands-on 'heavier or bigger' challenges help students distinguish between these two physical attributes.

Active Learning Ideas

Inquiry Circle: The Measurement Olympics

Students compete in events like the 'Cotton Ball Shot Put' (length) or the 'Water Transfer' (capacity). They must estimate their result, measure it using the correct tool, and record their data in a class leaderboard.

50 min·Small Groups

Stations Rotation: Unit Explorers

Stations are set up for different attributes: one for weighing classroom objects in grams, one for measuring the perimeter of desks in cm, and one for finding the capacity of various containers. Students must justify why they chose a specific unit at each station.

45 min·Small Groups

Think-Pair-Share: The Best Tool for the Job

Present three items: a ladybug, a student, and a car. Students think about which unit (mm, cm, m, km) and tool they would use for each. They share their reasoning with a partner, focusing on why a ruler wouldn't work for a car.

20 min·Pairs

Real-World Connections

• When bakers measure ingredients for a cake, they use grams for flour and sugar to ensure the recipe turns out correctly. Using a standard unit like grams ensures that anyone following the recipe, anywhere in the world, will get the same result.
• Construction workers use meters to measure the length of lumber or the height of walls. Using standard units prevents costly errors that could occur if everyone used different, non-standard measurements like hand spans.

Assessment Ideas

Quick Check

Provide students with a collection of objects (e.g., a pencil, a book, a shoe, a bag of sugar, a small toy). Ask them to choose the most appropriate unit (cm or m for length; g or kg for mass) for each object and record their choices. Then, have them measure the length or mass of two items using the chosen standard unit.

Discussion Prompt

Present students with two scenarios: measuring the length of a classroom using only centimeters, and measuring the length of a pencil using only meters. Ask: 'Which measurement would require a larger number of units? Why? Explain why using centimeters for the classroom and meters for the pencil makes more sense.'

Exit Ticket

Give each student a card with a picture of an object (e.g., a door, a feather, a watermelon, a paperclip). Ask them to write down the most appropriate standard unit (cm, m, g, or kg) to measure its length or mass, and briefly explain their choice.

Frequently Asked Questions

What units of measurement are taught in Ontario Grade 3?

Students focus on metric units: millimeters, centimeters, and meters for length; grams and kilograms for mass; and milliliters and liters for capacity. They also learn to measure temperature in degrees Celsius and time to the nearest minute.

How can I help students choose the right unit?

Use 'referents.' If a student knows a paperclip is about 1 gram and a liter of milk is 1000 milliliters, they can use those mental images to choose the right unit for a new object. Active 'referent hunts' in the classroom are great for this.

How can active learning help students understand measurement?

Measurement is inherently active. By participating in a 'Measurement Olympics,' students are not just reading about meters; they are seeing how far a meter actually is. This physical experience builds a 'sense' of measurement that allows them to make much more accurate estimates in the future.

How can I include Indigenous perspectives in measurement?

Discuss traditional measurement units used by Indigenous peoples, such as the 'finger-width' or 'arm-span.' Compare these to standard metric units to discuss why standardization became important for global trade, while acknowledging the utility of personal benchmarks.