Area calculator for rectangles, triangles, circles and more
Choose a shape, enter its measurements in one unit and read the area in square units, with square feet, square meters, square yards and acres. A 5 × 3 rectangle is 15 square units; a circle with a 2 ft radius covers about 12.57 square feet.
Inputs used by this calculator
Area Calculator uses the fields below. The starting values reproduce the page example; replace them with values from the same units and time period before using the result.
| Input | Starting example | Accepted value | How it is used |
|---|---|---|---|
| Shape | Rectangle or square | Rectangle or square · Triangle · Circle · 2 more choices | Supplies the shape quantity used by the published equation or result breakdown. |
| Length, base or first axis | 5 | 0 or greater | Supplies the length, base or first axis quantity used by the published equation or result breakdown. |
| Width, height or second axis | 3 | 0 or greater | Supplies the width, height or second axis quantity used by the published equation or result breakdown. |
| Second base (top) | 2 | 0 or greater | Selects the unit or conversion path used by Area Calculator. |
| Radius | 2 | 0 or greater | Supplies the radius quantity used by the published equation or result breakdown. |
| Unit | Feet | Feet · Inches · Yards · 2 more choices | Selects the unit or conversion path used by Area Calculator. |
What area measures
Area is the amount of flat surface inside a shape, counted in square units: square feet for a room, square yards for carpet, acres for land. Every length must use the same unit, and the answer is in that unit squared.
Rectangles and triangles
A rectangle's area is length times width. A triangle is half of a rectangle with the same base and height, so its area is one-half times base times height, where the height is measured at a right angle to the base.
rectangle = length × widthtriangle = ½ × base × heightCircles and ellipses
A circle's area is π times the radius squared; the circumference is 2 × π × radius. An ellipse uses its two semi-axes, half of the full width and half of the full height: area = π × a × b. The ellipse perimeter has no exact simple formula, so the calculator uses Ramanujan's close approximation.
circle = π × r²ellipse = π × a × bTrapezoids
A trapezoid has two parallel sides of different lengths. Average them and multiply by the height between them: ½ × (base₁ + base₂) × height. Many garden beds and roof sections are trapezoids.
Irregular shapes
Split an irregular floor plan or lot into rectangles, triangles and circles, calculate each piece and add them. Subtract the area of any cut-out, such as a fireplace or pool. For rooms measured in feet, the Square Footage Calculator adds material allowances.
Converting square units
Square units convert by the square of the length factor: one square yard is 9 square feet and one square meter is about 10.764 square feet. The result lists square feet, square meters, square yards and acres for the same area.
Area formulas with examples
| Shape | Formula | Example |
|---|---|---|
| Rectangle | length × width | 5 × 3 = 15 |
| Triangle | ½ × base × height | ½ × 5 × 3 = 7.5 |
| Circle | π × radius² | π × 2² = 12.57 |
| Trapezoid | ½ × (base₁ + base₂) × height | ½ × (5 + 2) × 3 = 10.5 |
| Ellipse | π × a × b (semi-axes) | π × 5 × 3 = 47.12 |
Frequently asked questions
How do I find the area of a rectangle?
Multiply length by width.
How do I find the area of a circle?
Multiply π by the radius squared: a 2 ft radius gives about 12.57 square feet.
What is the area of a triangle?
One-half times base times height.
How do I calculate the area of an irregular room?
Split it into rectangles and triangles, calculate each and add them.
How many square feet are in a square yard?
Nine.
What is the difference between area and perimeter?
Area is the surface inside; perimeter is the distance around the edge.
What are semi-axes of an ellipse?
Half of the full width and half of the full height.
What to keep in mind
Use the same unit for every length. For an ellipse, enter the two semi-axes (half of each full width). Irregular shapes can be split into these pieces and added.
Results are rounded for display in US format, with money in dollars and cents; CSV exports keep full calculation precision. This is a calculation summary, not an official certificate.