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Geometry & Measurement

Area Calculator

Choose a shape, enter the sizes shown, and get its area (plus perimeter where it has a simple formula). Each result shows the formula and the substitution.

Updated October 3, 2026Written and reviewed by Mateuss M.Method: how we calculate

Try an example:

AnswerArea 20

Perimeter 18

Step-by-step working 4 steps

1
Area
```
A = length × width = 4 × 5 = 20
```
2
Perimeter
```
P = 2(l + w) = 2(4 + 5) = 18
```
3
Diagonal
```
d = √(l² + w²) = 6.403124
```
4
Units
Area is in square units of whatever length unit you used.

How to use the area calculator

Choose the shape.
Enter the sizes; unused boxes disappear.
Read the area (and perimeter where shown).
Keep all lengths in the same unit.

Area formulas

Rectangle: A = l × w
Triangle: A = ½ × b × h
Parallelogram: A = b × h
Trapezoid: A = ½(a + b) × h
Ellipse: A = π × a × b

Always use the perpendicular height, not a slanted side.

Worked examples

Each example below is generated by the same calculator you used above, so the working always matches the answer.

Rectangle 4 × 5

AnswerArea 20

Perimeter 18

Step-by-step working 4 steps

1
Area
```
A = length × width = 4 × 5 = 20
```
2
Perimeter
```
P = 2(l + w) = 2(4 + 5) = 18
```
3
Diagonal
```
d = √(l² + w²) = 6.403124
```
4
Units
Area is in square units of whatever length unit you used.

Triangle base 6, height 5

AnswerArea 15

Step-by-step working 2 steps

1
Area
```
A = ½ × base × height = ½ × 6 × 5 = 15
```
The height is measured at a right angle to the base.
2
Units
Area is in square units of whatever length unit you used.

Trapezoid 3, 5, height 4

AnswerArea 16

Step-by-step working 2 steps

1

Area

A = ½(a + b) × h = ½(3 + 5) × 4 = 16

a and b are the two parallel sides.

2
Units
Area is in square units of whatever length unit you used.

Ellipse 4 × 2

AnswerArea 25.1327

Perimeter 19.3769

Step-by-step working 3 steps

1
Area
```
A = π × a × b = π × 4 × 2 = 25.132741
```
a and b are the semi-axes (half the full width and height).
2
Perimeter (Ramanujan approximation)
```
P ≈ 19.376896
```
Ellipses have no exact simple perimeter formula.
3
Units
Area is in square units of whatever length unit you used.

Common mistakes to avoid

Using a slanted side as the height

Height is measured at right angles to the base.

Forgetting the ½ for triangles and trapezoids

A triangle is half of a matching parallelogram.

Mixing units

Convert everything to the same unit first; area is in square units.

Frequently asked questions

How do I find the area of an irregular shape?

Split it into rectangles, triangles and trapezoids, find each area, and add them.