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Circle

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Square

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Triangle

Rectangle

Rectangle

Pentagon

Pentagon

Hexagon

Hexagon

Oval

Oval

Star

Star

Rhombus

Rhombus

Parallelogram

Parallelogram

Trapezoid

Trapezoid

Octagon

Octagon

Grade 3+ mini lesson

Find the Base & Height

Give learners a quick routine for locating the base and height before solving area or volume questions. Model the three-step process below, then explore how it looks on common shapes.

Step 1

Choose a stable side as your base.

Step 2

Drop a line that meets the base at 90°.

Step 3

Measure and label both with units.

Triangle

Any side can be the base. The height must touch the base at a right angle, even if it lands inside the triangle.

Base + Height
Triangle base and heightbaseheight
  1. Lay the triangle on the desk. The side touching the desk is your base.
  2. Find the point that is not on the base (the “top” vertex).
  3. Drop an imaginary elevator line straight down to the base.
  4. Stop when the line hits the base at 90°. That segment is the height.

Teacher Tip: Use the corner of an index card to check that your height line forms a square corner with the base.

Classroom Idea: Give students straws and sticky notes so they can build and label bases/heights before drawing them.

Rectangle or Square

Right angles everywhere means any side works as a base and the opposite side is automatically the height.

Base + Height
Rectangle base and heightbaseheight
  1. Choose the side you want to sit on the bottom. That is your base.
  2. The side directly across from it becomes the height.
  3. Count grid squares or units to record both numbers.
  4. Swap the base and height to show that the area stays the same.

Teacher Tip: Label the base with “b” and the height with “h” so students see how it connects to the area formula A = b × h.

Classroom Idea: Use centimeter grid paper so learners can trace a rectangle, highlight the base, then count the vertical squares for the height.

Parallelogram or Rhombus

Sides lean, so the height is not the slanted edge—drop a straight line that makes a right angle with the base.

Base + Height
Parallelogram base and heightbaseheight
  1. Pick one of the long sides to act as your base.
  2. Find a corner that is not on that base.
  3. Draw a straight path from that corner to the base that meets it at 90°.
  4. Measure that perpendicular distance; that is your height.

Teacher Tip: Slide a sticky note so its edge lines up with the base—the fold gives students a guide for the perpendicular height.

Classroom Idea: Cut out a parallelogram, rearrange it into a rectangle to prove why the perpendicular height is the one that matters for area.

Trapezoid

Trapezoids have two parallel bases. The height is the distance between them.

Base + Height
Trapezoid base and heightbaseheight
  1. Identify the two parallel sides—they are both called bases.
  2. Choose either base as your “bottom” for the problem.
  3. From a point on the top base, drop a straight segment down to the bottom base.
  4. Make sure the segment forms a right angle. That measurement is the height.

Teacher Tip: Have students mark both bases in the same color so they focus on the perpendicular distance between them.

Classroom Idea: Use two rulers held parallel while a third ruler slides straight down to show what “height between bases” looks like.