Area Calculator

Calculate the area of common building shapes including squares, rectangles, triangles, and circles. See the formula for each shape.

Mason ReidFounder and Lead Estimator at MeasureThen

Last updated 2026-08-21

What Is a Area Calculator?

The area calculator finds the area of common building shapes including rectangles, triangles, circles, and more. It is the first step in any project that needs flooring, tiling, paint, or land measurement. Homeowners and DIYers use it to get a square footage number they can carry into a materials or cost estimate.

How It Works

Rectangle: D1 × D2. Triangle: 0.5 × D1 × D2. Circle: π × Radius².

Example

A 10 ft × 12 ft rectangle is 120 sq ft; a circle with a 5 ft radius is about 78.5 sq ft.

How to Calculate

Pick the shape that matches your space, such as a rectangle for a room or a triangle for a gable.
Measure the dimensions in feet. For a rectangle enter the length and width, and for a circle enter the radius.
Read the area in square feet. A 10 ft by 12 ft room returns 120 sq ft, and a circle with a 5 ft radius returns about 78.5 sq ft.
For irregular spaces, split them into rectangles and triangles, run each shape, and add the results.
Use the final number with a materials or cost calculator to order flooring, tile, or paint.

Area formulas for common shapes

Shape	Formula	Example
Rectangle	Length × Width	10 ft × 12 ft = 120 sq ft
Triangle	0.5 × Base × Height	0.5 × 8 ft × 6 ft = 24 sq ft
Circle	π × Radius²	π × 5 ft × 5 ft = 78.5 sq ft
Trapezoid	0.5 × (Base A + Base B) × Height	0.5 × (10 + 14) × 6 = 72 sq ft
Parallelogram	Base × Height	10 ft × 7 ft = 70 sq ft

Sources

Frequently Asked Questions

Which shape should I pick?

Rectangle for rooms and lots, triangle for gables and odd lots, circle for round patios or tanks.

What units does Dimension 1 use?

Feet — enter both dimensions in feet and the area comes out in square feet.

How do I handle irregular shapes?

Split them into rectangles and triangles and add the areas together.

Mason ReidFounder and Lead Estimator at MeasureThen

Mason Reid founded MeasureThen after two decades of hands-on work in construction and estimating. Across residential and light commercial projects, he sized concrete pours, framed walls, hung drywall, and priced out roofing, fencing, and deck work — learning the exact math that every job depends on. At MeasureThen he writes the formulas, checks every calculator against real-world assumptions, and makes sure each one is simple enough for a homeowner to trust and a contractor to use. When he is not measuring twice, he is helping projects stay on budget.

Every MeasureThen calculator uses simple, transparent formulas and standard industry assumptions drawn from common construction practice. We show the math behind each result and include worked examples so you can follow along and double-check the numbers. Results are estimates: materials, labor, and pricing vary by region and market, so always confirm important figures with a licensed professional and your local building codes.