How to find circle perimeter

How to find circle perimeter

Perimeter of a geometrical figure is called length of the line limiting it. If this figure – a circle, then for finding of its perimeter is enough to determine length of the corresponding circle. It can be done directly, having measured length of this circle, or having calculated it by mathematical formulas.

It is required to you

  • - calculator;
  • - ruler;
  • - compasses;
  • - cord.


1. If a circle material (i.e. it is not drawn on paper, and represents a physical object), take a piece of a cord (ropes, a cord, a thread) and arrange it on circle border. Note on a point cord, corresponding to the beginning and the end of measurements (for reliability it is possible to tie small knots). Then measure length of this site of a cord by means of a ruler or a construction roulette. The received number will also be circle perimeter.

2. If the circle (wheel, the lying barrel) can be rolled, then just give a ride to it on one turn. Then measure by a ruler or a roulette length of the mark left by a circle. If the trace when swing does not remain, note points of the beginning and the end of the movement of a round subject. The cord in this case is not required. If a wheel very small (for example, the roller glass-cutter), then for best accuracy measurements give a ride to it on several turns, and then divide length of the passable way into quantity of turns.

3. If to measure perimeter of a circle or it is difficult to give a ride to it, then measure its diameter. It is better to make it by means of a cord. Fix one end of a cord in any point of a circle and find the most remote point on the opposite side. If a circle very big, then having fixed one end of a cord, just come over to the opposite side. Then measure length of a cord and increase this number by 3.14 (p).

4. If on a circle its center is somehow noted, then measure its radius. For this purpose just measure distance between the center and any point on circle border. Then increase this value by 6.28 (2πи). Also the perimeter of a nacherchenny circle is the same way calculated.

5. If the circle is entered in a square (in practice it can be some packing, for example, a box from a bulb), then measure length of the party of this square. It will also be diameter of the circle entered in a square. If, on the contrary, the square is inscribed in a circle, then measure square diagonal length. This number will also be diameter (but already described) circles.

Author: «MirrorInfo» Dream Team