Area Calculator Guide for Common Shapes
Find area for rectangles, triangles, circles, and everyday measurement problems.
Quick Answer
rectangle: A = lw; triangle: A = bh/2; circle: A = πr²When to Use This
Area measures a flat region, such as floor coverage or a triangular panel. Select the shape before entering dimensions. A shape with a slanted edge does not necessarily use that edge as its height; height is measured perpendicular to the chosen base.
Step-by-Step Method
- Identify the shape or divide a compound shape into simple non-overlapping pieces.
- Convert all lengths to the same unit.
- Use the formula for that shape with the correct base, perpendicular height or radius.
- Report squared units, and add or subtract component areas when needed.
Calculator Tips and Limits
- Choose the shape in the area tool before entering dimensions.
- Keep π through the calculation and round the final circular area only.
Worked Examples
Same base and height, different area
A rectangle 8 m by 5 m covers 8 × 5 = 40 m². A triangle with base 8 m and perpendicular height 5 m covers half as much: 8 × 5/2 = 20 m².
Use radius, not diameter
A circle with diameter 6 cm has radius 3 cm. Its area is π × 3² = 9π ≈ 28.27 cm². Using 6 as the radius would quadruple the answer.
Common Mistakes
Using a slanted triangle side as height
Unless that side is perpendicular to the base, it is not the height required by A = bh/2.
Converting area like a length
1 m = 100 cm, but 1 m² = 10,000 cm². Square the length conversion factor.
FAQ
How is area different from perimeter?
Area counts the covered surface in square units. Perimeter measures the boundary length in ordinary length units.
Can I subtract a cutout?
Yes. Calculate the outer area and subtract the non-overlapping removed area, keeping units consistent.
References and corrections
OpenStax: Prealgebra 2e — further reading. The worked examples on this page are CalcSolver explanations. Read our calculation and correction method, or report an error with the input and expected result.