3rd Grade Classifying Triangles by Side Lengths Worksheet: 6 Triangles

A triangle has three sides, and their lengths tell us its name. This 3rd grade classifying triangles by side lengths worksheet gives your child six triangles to sort, labelled a to f. Each side carries a number and a unit, such as 4 mm, 5 cm, 7 ft or 4 yd. When all three sides are equal, the triangle is equilateral. When only two sides match, it is an isosceles triangle. If no sides match, the triangle is scalene. Your child reads the three numbers, compares them and then writes the right word under the drawing. The units change from one triangle to the next, so the page also gives practice in reading measures. Page two is the answer key, and it explains each answer in two short lines.

Six answers to check

Triangle a (4 mm, 3 mm, 5 mm) is scalene. Triangle b (5 cm, 7 cm, 5 cm) is isosceles. Then c is equilateral, with three sides of 7 ft. So is d, with three sides of 5 in. Next, e is isosceles (4 yd, 4 yd, 6 yd), while f is scalene (3 m, 5 m, 7 m). All six answers in the key are right.

The page has a few small slips. The title says "sides lengths" instead of "side lengths," and the key writes "trangle" twice. The instruction says "scalene, isosceles, and equilateral," but each triangle gets only one name, so read it as "or." Naming triangles by their sides usually comes in grade 4 or 5, so this page is a gentle stretch.

At the table: circle the equal sides first

Before your child writes a word, ask them to circle the numbers that match in each triangle. No circle means scalene. Two circles mean isosceles, and three circles mean equilateral. This simple step turns the three long words into a quick counting job.

Also point out triangle e. It has a square corner, yet it is still isosceles, because two of its sides are both 4 yd. As a result, your child learns that a square corner does not change the name by sides.

After the page: build triangles with paper strips

Cut paper strips in three colours: red ones 10 cm long, blue ones 7 cm long and green ones 5 cm long. First, ask your child to build a triangle from three strips of one colour. Then they build one with two strips of the same colour, and one with three colours. Each time, they name the triangle and say why. Finally, look for triangles at home, such as a slice of pizza or a road sign, and guess their type.