Measure angles in whole-number degrees using a protractor. Sketch angles of specified measure.
Official wording from the Common Core State Standards for Mathematics (© 2010 National Governors Association Center for Best Practices and Council of Chief State School Officers). View on thecorestandards.org
A protractor is a half circle marked in degrees from 0 to 180, usually with two scales running in opposite directions. To measure an angle, students place the protractor's center point exactly on the vertex, line up the zero line with one ray, and read where the other ray crosses the scale. The tricky part is choosing the right scale: they should use the one that starts at 0 on the ray they lined up.
Fourth graders measure angles in whole-number degrees and also sketch angles of a specified measure, such as 35° or 140°. Before measuring, a quick estimate helps: is the angle smaller than a right angle (acute, less than 90°), larger (obtuse, between 90° and 180°), or exactly 90°? If the protractor reading disagrees with the estimate, the student probably read the wrong scale. Sketching angles also reinforces the idea from 4.MD.C.5 that an angle is a turn from one ray to another.
A 50° angle read as 130° is the most common protractor error. Estimating first, and starting from the 0 on the lined-up ray, prevents it.
Students line up the bottom edge of the protractor instead of its center mark. The center must sit exactly on the vertex.
When rays are too short to reach the scale, students give up. Extending the rays with a ruler does not change the angle.
A protractor's outer scale reads 130 and its inner scale reads 50 where the second ray crosses. The first ray is lined up with the 0 on the inner scale. The angle looks smaller than a right angle. What is its measure?
Answer: The angle measures 50°.
Give students angles drawn in many orientations, not only with one ray horizontal, so they learn to rotate the protractor. Partner checks, where one student measures and the other estimates, catch scale errors quickly.
Assessments sometimes show a picture of a protractor and ask for the reading, including cases where the ray is on the right-hand 0. Practice reading both scales from images as well as with real tools.
Original questions written for this standard. Choose an option or type your answer, then press Check. Every question has a worked explanation.
Answer: C) On the vertex of the angle
The center mark must sit exactly on the vertex, where the two rays meet.
Answer: B) 65°
The angle is acute, so it must be less than 90°. The reading that matches is 65°.
Answer: 140
The inner and outer readings at any point add to 180, so the other reading is 180 - 40 = 140.
Answer: D) 125°
An obtuse angle is greater than 90° and less than 180°. Only 125° fits.
Answer: C) Draw a ray, then place the protractor's center on its endpoint with the zero line on the ray
Start with one ray, center the protractor on its endpoint, align 0 with the ray, mark 70 on that same scale, and then draw the second ray.
A full lesson with slides, activities and an exit ticket on measuring and sketching angles with a protractor, pitched to grade 4 and editable in PowerPoint or Google Slides.
Make a lesson →A printable, differentiated worksheet on 4.MD.C.6 with an answer key, ready in about a minute.
Make a worksheet →Turn measuring and sketching angles with a protractor into a quiz students answer online that marks itself, with a class summary for you.
Build a test →So you can measure an angle whichever way it opens. Use the scale that reads 0 on the ray you lined up with the protractor's base.
A standard half-circle protractor reads up to 180°. Larger angles can be found by measuring the smaller angle and subtracting from 360°, which goes beyond this fourth grade standard.