Area Calculator
Calculate the area of rectangles, squares, circles, triangles, trapezoids and parallelograms.
Area
80 square units
Shape
Rectangle
Area
80
Formula
Area = width × height
The standard area equation
The calculation
Area = width × height
10 × 8 = 80
Area is reported in square units of whatever you entered — square metres if you used metres, square feet if you used feet. For a circle's radius, diameter, area and circumference at once, use the Circle Calculator. For triangles where you only know the three sides, the Triangle Calculator uses Heron's formula.
How to use this calculator
- Pick a shape from the toggle at the top — rectangle, square, circle, triangle, trapezoid or parallelogram.
- Enter the measurements the shape needs. For triangles and parallelograms, the height is the perpendicular distance, not a slanted side.
- Read the area in square units of whatever you measured in.
How it works
Each shape has its own formula:
- Rectangle —
width × height - Square —
side² - Circle —
π × r² - Triangle —
½ × base × height - Trapezoid —
½ × (a + b) × height, where a and b are the two parallel sides - Parallelogram —
base × perpendicular height
The calculator picks the right formula based on the shape you select and shows the working underneath the result. For triangles where you only know the three side lengths, use the Triangle Calculator (which applies Heron’s formula). For a circle’s radius, diameter and circumference at once, use the Circle Calculator.
Frequently Asked Questions
What is area? ▾
Area is the amount of 2D space a shape covers. It's measured in square units of whatever length you measured the sides in — square metres if you used metres, square feet if you used feet. Doubling every length quadruples the area, which is why a 20-foot square room has four times the floor space of a 10-foot square room, not twice.
What's the formula for each shape? ▾
Rectangle: width × height. Square: side². Circle: π × r². Triangle: ½ × base × height. Trapezoid: ½ × (a + b) × height, where a and b are the two parallel sides. Parallelogram: base × perpendicular height. The calculator shows whichever formula matches the shape you picked, with your numbers plugged in.
Why does a parallelogram have the same area as a rectangle with the same base and height? ▾
Because you can mentally slice a parallelogram into pieces and slide them around to form a rectangle of the same base and height — none of the area is created or destroyed. The slanted side doesn't matter for area; only the perpendicular height does. This is a common gotcha — the side length is NOT the height.
What's Heron's formula and when do I use it? ▾
Heron's formula gives a triangle's area from its three side lengths alone, without needing a perpendicular height. It's useful when you can measure the sides but not the height — like surveying a triangular plot of land. The Triangle Calculator handles this; this Area Calculator uses the simpler ½ × base × height when you have the height.
Does the calculator handle units? ▾
No — the calculator is unit-agnostic. Whatever unit you enter for length, the area comes out in the matching square unit. If you mix units (one side in feet, another in inches) you'll get a meaningless number, so convert everything to the same unit before entering.