"Concentric" means to have a same centre. Concentric figures or objects share a common centre, axis or origin with one inside the other. Concentric circles are circles of different sizes which have a common centre. In other words we can say that, two or more circles which share the same centre are called concentric circles. Concentric circles have same centre but different radii, for it the radius is same they become equal. Since the concentric circles have different radii and same centre, they are at the same distance apart all the way around and they fit inside each other.
Concentric Circle
The picture above shows the concentric circles (5 circles) with centre “C”.

The best example for concentric circles is the evenly spaced circles of a target used in target archery or firearms. The centre of these concentric circles is called the “bulls eye” in target archery or the dart game.

To find the area of circle we use the formula πr2, where π is the mathematical constant which is approximately equal to 3.142 and “r” is the radius of the circle. Concentric circles have two or more circles involved. So this formula can be used even for concentric circles. The area between any two concentric circles is called the Annulus which is like a ring.

The area of this ring or annulus can be calculated by subtracting the area of the smaller circle from the area of the bigger circle.

Area of Concentric Circles

In this figure above we have two concentric circles each of radius “r” and “R”.

The area of each circle is given by πr2 and πR2.

Thus the area of Annulus is given by πR2 - πr2 = π (R2 – r2)