Area of a Circle Calculator in Square Feet

This area of a circle calculator finds the area in square feet from a circle’s radius in feet. Enter the radius to get the area in square feet, with a square-meter equivalent for metric reference.

Formula

The area of a circle is its radius multiplied by itself, then multiplied by pi:

A = πr²

• A is the circle’s area, in square feet when the radius is entered in feet.
• r is the radius, the distance from the center to the circle’s edge, in feet.
• π (pi) is approximately 3.14159265.

Squaring the radius means multiplying it by itself. For example, if r is 5 ft, then r² is 25 ft². The calculator also converts the result to square meters using 1 ft² = 0.09290304 m². The conversion changes the unit, not the area being measured.

Worked Example

Using the default radius of 5 ft:

1. Square the radius: 5 ft × 5 ft = 25 ft².
2. Multiply by pi: π × 25 ft² = 78.5398… ft².
3. Round to two decimal places: 78.54 ft².
4. Convert that area to square meters: 78.5398… × 0.09290304 = 7.296… m², or 7.30 m².

The calculator’s headline result is 78.54 ft². The metric figure is a rounded equivalent.

Radius, Diameter, and Circumference

The radius and diameter are not interchangeable. The radius runs from the center to the boundary; the diameter crosses the circle through its center and is twice the radius. If you know the diameter, divide it by two before using this calculator. A 10 ft diameter, for instance, means a 5 ft radius and an area of about 78.54 ft².

Circumference is the distance around the circle, not its area. If circumference C is known, the radius is C ÷ (2π), so the area can also be found as C² ÷ (4π). Make sure a measurement described as “across” is actually the full diameter and not a radius.

When to Use the Result

Circle area is useful when estimating the flat surface covered by a circular object or region. Examples include a round tabletop, a circular garden bed, or a marked-out feature on a plan. For a surface measured in feet, square feet are a practical unit; one square foot is the area of a square measuring 1 ft on each side.

Area alone does not tell you how much material to buy. A project may need extra material for overlaps, trimming, uneven edges, or a specific installation pattern. Also, a circular area’s calculated area assumes a true circle and a measurement taken to its boundary. For an irregular or partly obstructed shape, this formula may only provide an approximation.

How to Measure and Enter the Radius

Measure from the approximate center to the outer edge, keeping the tape or measuring line straight. For a physical circle, taking several measurements in different directions can reveal whether the shape is noticeably oval or uneven. If you can only measure across the full circle, record its diameter and halve it.

• Use feet for the calculator input. Convert inches to feet by dividing by 12; for example, 30 inches is 2.5 ft.
• For a measurement in meters, multiply by about 3.28084 to convert meters to feet before entering it.
• Keep the measurement precision you can reasonably support. Extra decimal places in the result cannot compensate for an inaccurately measured radius.

The input accepts a radius from 0 to 100,000 ft. A zero radius gives zero area; a negative radius is not a physical radius and is excluded.

Common Mistakes and Limits

The most common error is entering the diameter as if it were the radius. Because the radius is squared, using a diameter that is twice as large produces an answer four times the correct area. Another frequent mistake is mixing units: entering inches while treating the value as feet makes the result far too large.

Area is expressed in square units, such as ft² or m²—not linear units such as ft or m. The calculator rounds displayed results to two decimal places, but calculations use the unrounded input and intermediate values. For very small circles, rounding may make a result look like 0.00 ft² even though its actual area is positive; use more precise measurements or a different display precision if needed.

This formula describes a flat, two-dimensional circle. It does not calculate the volume of a cylinder, the surface area of a sphere, or the amount of material needed for a three-dimensional object. Those require additional dimensions and different formulas.

Frequently Asked Questions

How do I calculate the area of a circle in square feet?
Measure the radius in feet, square it, and multiply by pi: A = πr². A radius of 5 ft gives about 78.54 ft².

Can I enter the diameter instead of the radius?
Not directly. Divide the diameter by two to get the radius, then enter that value. If the diameter is 12 ft, enter a radius of 6 ft.

What is the area of a circle with a 10-foot diameter?
The radius is 5 ft. Its area is π × 5², or approximately 78.54 ft².

How do I convert square feet to square meters?
Multiply the area in square feet by 0.09290304. For example, 100 ft² is about 9.29 m².

Does this calculator work with inches?
Convert the radius to feet first by dividing inches by 12. For example, a 24-inch radius is 2 ft; the circle’s area is about 12.57 ft².

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in r | Radius | ft | 5 | 0 | 100000
out area | Area | ft² | pi * r ^ 2 | 2
out metric | Area | m² | area * 0.09290304 | 2