Geometry Guide
Area Calculator Guide: Rectangles, Triangles, Circles, and Trapezoids
Calculate flat shape area and understand which measurements each formula needs.
Quick answer
Area measures the amount of flat space inside a shape. A rectangle uses length x width. A triangle uses base x height / 2. A circle uses pi x radius squared. A trapezoid uses (base 1 + base 2) x height / 2.
Why this matters
Area is used for flooring, paint coverage, garden planning, screens, land, school geometry, and packaging. Choosing the wrong formula is more common than arithmetic mistakes. A circle needs radius, not diameter, unless you divide diameter by two first.
Example
A rectangular room that is 12 feet long and 8 feet wide has an area of 96 square feet. If flooring is sold in boxes covering 20 square feet, the raw math suggests 4.8 boxes. In real planning, you would round up and include extra material for cuts and waste.
How to use the calculator
Choose the shape first, then enter the measurements that apply to that shape. Ignore fields that do not apply. Keep all measurements in the same unit. If length is in feet and width is in inches, convert before calculating.
Common mistakes
One mistake is mixing units. Another is using slanted side length as triangle height. Height must be perpendicular to the base. A third is reporting area in ordinary units instead of square units. Ten feet by ten feet is 100 square feet, not 100 feet.
When not to rely on this estimate
For construction, land measurement, permits, or material orders, verify dimensions and add appropriate waste or tolerance. This calculator gives formula-based estimates.
FAQ
What is area measured in?
Square units, such as square feet or square meters.
Can I mix feet and inches?
Convert to one unit first.