Area Calculator
Area formulas depend on shape. Rectangle uses length x width. Circle uses pi x radius squared.
Choose the shape first
Choose the shape, then enter the measurements the formula needs. A rectangle needs length and width. A triangle needs base and perpendicular height. A circle needs radius or diameter.
Use the same unit for every length in the same calculation. If one value is in feet and another is in inches, convert first.
This tool is useful for homework, rooms, material estimates and quick geometry checks. For irregular shapes, split the shape into simpler pieces and add their areas.
Area means surface size
Area measures the amount of flat surface inside a shape. The answer is written in square units, such as square feet, square meters or square centimeters.
A square unit is not the same as a normal length unit. A square that is 1 foot by 1 foot has an area of 1 square foot.
If the number seems large, check the unit. Small units create bigger area numbers because many small squares fit inside the same surface.
Shape formulas used here
A rectangle uses area = length x width. A triangle uses area = base x height / 2. A circle uses area = pi x radius squared.
A parallelogram uses base x height. A trapezoid uses (base 1 + base 2) x height / 2.
The height in these formulas is perpendicular height. It is not always the slanted side. This is one of the most common geometry mistakes.
Rectangle and circle examples
For a rectangle that is 8 feet long and 6 feet wide, multiply 8 x 6. The area is 48 square feet.
For a triangle with base 10 inches and height 7 inches, calculate 10 x 7 / 2. The area is 35 square inches.
For a circle with radius 4 centimeters, calculate pi x 4^2. That is about 50.27 square centimeters.
Using diameter as radius
The biggest mistake is using the wrong height. Triangle, parallelogram and trapezoid formulas need perpendicular height, not a tilted side unless that side is perpendicular.
Another mistake is forgetting square units. If the inputs are feet, the area is square feet.
Also avoid applying a length conversion after finding area. Area unit conversion uses squared factors, so convert dimensions first or use an area converter.
Area Calculator FAQ
How do I choose the right area formula?
Match the formula to the shape and the measurements you actually have. If you have length and width, a rectangle formula may fit. If you have base and perpendicular height, a triangle or parallelogram formula may fit.
If the shape is not standard, split it into rectangles, triangles or trapezoids.
Then add the pieces together. This is usually easier than looking for one complicated formula.
Why is area measured in square units?
Area counts how many unit squares cover a surface. If the unit is feet, the area unit is square feet.
A 3 foot by 4 foot rectangle contains 12 one foot by one foot squares.
That is why the answer is 12 square feet, not 12 feet.
What is the difference between perimeter and area?
Perimeter measures the distance around a shape. Area measures the surface inside the shape.
A fence problem often uses perimeter. A flooring, painting or surface coverage problem often uses area.
The units help you tell them apart. Perimeter uses length units. Area uses square units.
Do I use radius or diameter for circle area?
Circle area uses radius. If you have diameter, divide it by 2 first.
For example, a circle with diameter 10 has radius 5. The area is pi x 5^2.
Using diameter as radius makes the answer four times too large.
How do I find area of an irregular shape?
Break the shape into smaller shapes you can calculate. Rectangles and triangles are usually easiest.
Find the area of each piece, then add them together. If part of a large rectangle is missing, subtract the missing piece.
Draw the split lines on the shape first. That makes it easier to avoid double counting.