For a circle of diameter d and radius r:

The circumference is πd or 2πr.

The area is πd²/4 or πr².

An arc of a circle is part of the circumference.

A tangent to a circle is a straight line which meets the circle at one point only. A radius drawn from the point where a tangent meets a circle is at right angles to the tangent.

A sector of a circle is the area between an arc of the circle and two radii. The area of a sector is 1/2r²θ, where θ is the angle in radians between the radii.

A chord is a straight line joining two points on the circumference of a circle. When two chords intersect, the product of the parts of one chord is equal to the products of the parts of the other chord. In the following figure, AE×BE=CE×ED.

A segment of a circle is the area bounded by a chord and an arc. Angles in the same segment of a circle are equal: in the following figure, angle A =angle B.

Circles: areas and circumferences

Diameter	Area	Circumference	Diameter	Area	Circumference
1	0.7854	3.142	51	2042.8	160.2
2	3.1416	6.283	52	2123.7	163.4
3	7.0686	9.425	53	2206.2	166.5
4	12.566	12.57	54	2290.2	169.6
5	19.635	15.71	55	2375.8	172.8
6	28.274	18.85	56	2463.0	175.9
7	38.485	21.99	57	2551.8	179.1
8	50.265	25.13	58	2642.1	182.2
9	63.617	28.27	59	2734.0	185.4
10	78.540	31.42	60	2827.4	188.4
11	95.033	34.56	61	2922.5	191.6
12	113.10	37.70	62	3019.1	194.8
13	132.73	40.84	63	3117.2	197.9
14	153.94	43.98	64	3217.0	201.1
15	176.71	47.12	65	3318.3	204.2
16	201.06	50.27	66	3421.2	207.3
17	226.98	53.41	67	3525.7	210.5
18	254.47	56.55	68	3631.7	213.6
19	283.53	59.69	69	3739.3	216.8
20	314.16	62.83	70	3848.5	219.9
21	346.36	65.97	71	3959.2	223.1
22	380.13	69.11	72	4071.5	226.2
23	415.48	72.26	73	4185.4	229.3
24	452.39	75.40	74	4300.8	232.5
25	490.87	78.54	75	4417.9	235.6
26	530.93	81.68	76	4536.5	238.8
27	572.56	84.82	77	4656.6	241.9
28	616.75	87.96	78	4778.4	245.0
29	660.52	91.11	79	4901.7	248.2
30	706.86	94.25	80	5026.5	251.3
31	754.77	97.39	81	5153.0	254.5
32	804.25	100.5	82	5381.0	257.6
33	855.30	103.7	83	5410.6	260.8
34	907.92	106.8	84	5541.8	263.9
35	962.11	110.0	85	5674.5	267.0
36	1017.9	113.1	86	5808.8	270.2
37	1075.2	116.2	87	5944.7	273.3
38	1134.1	119.4	88	6082.1	276.5
39	1194.6	122.5	89	6221.1	279.6
40	1256.6	125.7	90	6361.7	282.7
41	1320.3	128.8	91	6503.9	285.9
42	1385.4	131.9	92	6647.6	289.0
43	1452.2	135.1	93	6792.9	292.2
44	1520.5	138.2	94	6939.8	295.3
45	1590.4	141.4	95	7088.2	298.5
46	1661.9	144.5	96	7238.2	301.6
47	1734.9	147.7	97	7389.8	304.7
48	1809.6	150.8	98	7543.0	307.9
49	1885.7	153.9	99	7697.7	311.0
50	1963.5	157.1