What is the formula for the area of a circle?

The area of a circle is A = pi x r^2. For r = 7 and pi = 22/7, A = (22/7) x 49 = 154 square units.

What is the formula for the circumference of a circle?

The circumference is C = 2 x pi x r, or C = pi x d. For r = 7, C = 2 x (22/7) x 7 = 44 units.

How do you convert radius to diameter?

Double the radius: d = 2r. For r = 7, d = 14. To go back, halve the diameter: r = d / 2.

How do you find the area of a semicircle?

Take half of the circle area: A = (pi x r^2) / 2. For r = 7 that is 154 / 2 = 77 square units.

What is the perimeter of a semicircle?

It is the curved arc plus the diameter: P = pi x r + 2r. For r = 7, P = 22 + 14 = 36 units.

Which value of pi should I use: 22/7, 3.14 or 3.14159?

Use 22/7 when r is a multiple of 7 for neat answers. Use 3.14 for quick homework estimates, and 3.14159 or the pi button on a calculator when you need more precision.

Circle Formulas Table: Area, Circumference & Solver

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A circle needs only its radius to find everything else. This reference table lists the formulas for diameter, circumference, area and semicircles with a solved example at r = 7, plus an instant radius solver.

Diameterd = 2r2 × 7 = 14

CircumferenceC = 2πr44 at r = 7

AreaA = πr²154 at r = 7

Radiusr = d / 214 / 2 = 7

Semicircle Area(πr²) / 2154 / 2 = 77

Semicircle Perimeterπr + 2r22 + 14 = 36

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Radius (r)

Value of pi

Tip: open this page with ?radius=7 (any positive number) to start with that radius. Whole radii from 1 to 20 are highlighted in the chart.

Circle Formulas with Solved Examples (r = 7, π = 22/7)
Property Name	Symbol	Mathematical Formula	Solved Example (r=7)
Diameter	d	d = 2r	2 × 7 = 14
Circumference	C	C = 2 × π × r	2 × (22/7) × 7 = 44
Area	A	A = π × r²	(22/7) × 7² = 154
Semicircle Area	A_semi	A = (π × r²) / 2	154 / 2 = 77
Semicircle Perimeter	P	P = π × r + 2r	22 + 14 = 36
Radius (from diameter)	r	r = d / 2	14 / 2 = 7
Radius (from area)	r	r = √(A / π)	√(154 / (22/7)) = √49 = 7

Radius to Diameter, Circumference and Area Chart (r = 1 to 20, π = 3.14159)
Radius (r)	Diameter (d)	Circumference (C)	Area (A)
1	2	6.28	3.14
2	4	12.57	12.57
3	6	18.85	28.27
4	8	25.13	50.27
5	10	31.42	78.54
6	12	37.70	113.10
7	14	43.98	153.94
8	16	50.27	201.06
9	18	56.55	254.47
10	20	62.83	314.16
11	22	69.12	380.13
12	24	75.40	452.39
13	26	81.68	530.93
14	28	87.96	615.75
15	30	94.25	706.86
16	32	100.53	804.25
17	34	106.81	907.92
18	36	113.10	1017.88
19	38	119.38	1134.11
20	40	125.66	1256.64

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Frequently Asked Questions

Question	Answer
What is the formula for the area of a circle?	The area of a circle is A = &π; × r². For r = 7 and π = 22/7, A = (22/7) × 49 = 154 square units.
What is the formula for the circumference of a circle?	The circumference is C = 2 × &π; × r, or C = &π; × d. For r = 7, C = 2 × (22/7) × 7 = 44 units.
How do you convert radius to diameter?	Double the radius: d = 2r. For r = 7, d = 14. To go back, halve the diameter: r = d / 2.
How do you find the area of a semicircle?	Take half of the circle area: A = (&π; × r²) / 2. For r = 7 that is 154 / 2 = 77 square units.
What is the perimeter of a semicircle?	It is the curved arc plus the diameter: P = &π; × r + 2r. For r = 7, P = 22 + 14 = 36 units.
Which value of pi should I use: 22/7, 3.14 or 3.14159?	Use 22/7 when r is a multiple of 7 for neat answers. Use 3.14 for quick homework estimates, and 3.14159 or the π button on a calculator when you need more precision.

Circle Formulas Table: Area, Circumference & Solver

Instant Circle Solver

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Frequently Asked Questions

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