Area Calculator
Calculate area for 13 shapes with formulas, worked examples, unit guidance and adjustable precision. Processing takes place locally in your browser.
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Area formulas for 13 shapes
Choose a shape, enter its required dimensions and get the area with adjustable decimal precision. The calculator supports a square, rectangle, triangle from base and height, triangle from three sides, parallelogram, trapezoid, circle, sector from angle, sector from arc length, annulus, circular segment, ellipse and rhombus.
View the formulas · See worked examples · Check the units
Formula reference
Square: A = s²
Rectangle: A = width × height
Triangle, base and height: A = base × height ÷ 2
Triangle, three sides: A = √(p(p − a)(p − b)(p − c)), where p = (a + b + c) ÷ 2
Parallelogram: A = base × perpendicular height
Trapezoid: A = (base 1 + base 2) × height ÷ 2
Circle: A = πr²
Sector, central angle: A = angle ÷ 360 × πr²
Sector, arc length: A = radius × arc length ÷ 2
Annulus: A = π(outer radius² − inner radius²)
Circular segment: the sector area minus or plus the corresponding triangle area, depending on the central angle
Ellipse: A = π × semi-major axis × semi-minor axis
Rhombus: A = diagonal 1 × diagonal 2 ÷ 2
Worked area examples
8 ft × 5 ft = 40 ft²
Radius 3 cm: π × 3² ≈ 28.27 cm²
Heron’s formula gives 6 square units
Bases 4 m and 8 m, height 5 m: 30 m²
Units and precision
The calculator does not convert dimensions automatically. Enter every length in the same unit; the answer uses that unit squared. For example, feet produce ft² and meters produce m². Decimal-place selection changes the displayed rounding, not the underlying formula.
Input checks and limitations
- Dimensions must be positive, finite numbers.
- Three side lengths must satisfy the triangle inequality.
- An annulus needs an outer radius greater than its inner radius.
- A sector or segment angle must be greater than 0° and no more than 360°.
- For an irregular floor plan, divide the outline into regular shapes and add or subtract their areas.
Frequently asked questions
How do I calculate area?
Identify the shape, measure the dimensions required by its formula, use one unit throughout and report the result in square units.
How do I calculate the area of a circle?
Square the radius and multiply by π. A circle with radius 3 cm has an area of about 28.27 cm².
Can I mix feet and inches?
Convert all dimensions to feet or all to inches before calculating. Convert the final square-unit result only after the calculation.
For a sector entered by radius and arc length, the arc length must not exceed the full circumference 2πr.
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