Area and perimeter calculator

A free area and perimeter calculator for kids: pick a shape, type the sides and see a labelled picture, the formula and every step.

🧒 Kids: pick a shape, type the numbers and tap Work it out. Try to work it out first, then check!

How to use the area and perimeter calculator

1. Pick a shape with the buttons at the top.
2. Type the measurements. For triangles, parallelograms and trapezoids, the slanted sides are optional: they are only needed for the perimeter.
3. Choose the units, then tap Work it out.
4. Check the picture. It is drawn from your numbers, so a strange shape means a measurement may be wrong.

Ready-made examples

Each formula in the table opens the calculator on that shape, with example numbers already filled in.

Area and perimeter formulas

Each shape name opens the calculator on that shape. For a printed copy of these formulas, see the area and perimeter anchor chart.

Area and perimeter: what is the difference?

Perimeter is the distance all the way around a shape, like a fence around a garden. It is a length, so it is measured in cm, m, inches or feet.

Area is the space inside the shape, like the grass inside the fence. It counts squares, so it is measured in square units: cm², m², square inches or square feet.

Two shapes can have the same perimeter but different areas. A 6 by 4 rectangle and a 5 by 5 square both have a perimeter of 20, but their areas are 24 and 25.

Worked examples

Why the height matters

For triangles, parallelograms and trapezoids, area uses the height. That is the straight-up distance, at a right angle to the base, drawn as a dashed line.

The slanted sides are longer than the height. Using one of them instead of the height is the most common mistake in grade 6 area questions. The slanted sides only matter for the perimeter.

Units and square units

Always write the units with the answer. A perimeter of 26 cm is a length, and an area of 40 cm² is a number of 1 cm squares.

Never mix units. If one side is in meters and another in centimeters, change them to the same unit first. The calculator assumes every measurement uses the units you choose.

Common mistakes

• Adding instead of multiplying for area: 8 + 5 is not the area of an 8 by 5 rectangle. Area counts squares in rows: 8 × 5.
• Only adding two sides for perimeter: a rectangle has four sides, so add both lengths and both widths.
• Forgetting the ½ for triangles: a triangle is half of a rectangle with the same base and height.
• Using the diameter as the radius: the radius is half the diameter. Check which one the question gives.
• Leaving out the square units: area answers are in cm², m², in² or ft².

Area and perimeter by grade

• Grade 3 (3.MD.C.7, 3.MD.D.8): area of rectangles by counting squares and multiplying, and perimeter of polygons.
• Grade 4 (4.MD.A.3): area and perimeter formulas for rectangles, and finding a missing side.
• Grade 6 (6.G.A.1): area of triangles, parallelograms, trapezoids and composite shapes.
• Grade 7 (7.G.B.4): area and circumference of a circle.

Lessons: how to find area and how to find perimeter. Practice: area and perimeter practice, triangles, parallelograms and trapezoids and circles.

For teachers

Put the calculator on the projector and ask the class to predict each answer first. Then change one measurement and ask what happens to the area: doubling one side doubles the area, but doubling both makes it four times bigger.

For printed practice, use the area of rectangles worksheet, the perimeter worksheet and the area of triangles worksheet.

More geometry help

• Geometry practice: 10 picture questions on any of 18 skills, from naming shapes to the Pythagorean theorem.
• Area and perimeter calculator: six shapes, a labelled picture and every step.
• Lessons: shapes for kids · measuring angles · how to find area · how to find perimeter · how to find volume · the coordinate plane
• Worksheets: 2D shapes · types of angles · perimeter · area of rectangles · area of triangles · volume
• Charts to print: 2D shapes · 3D shapes · types of angles · area and perimeter

Back to geometry for kids · 3rd grade math · 4th grade math · 6th grade math · 8th grade math