How to Find the Radius of a Cylinder

Objects with cylindrical shape

Microsoft Encarta defines a cylinder as "an object or shape with straight sides and circular ends of equal size." A radius of a cylinder is the distance from the center of one of the circular ends to its outer edge. You might have to figure the radius of a cylinder in a high school or college geometry course. And you also may have to deal with cylinder measurements in career fields such as physics and engineering.

Things You'll Need

• Tape measure or ruler
• Calculator

Instructions

Measurement

1. Use a tape measure or ruler to measure from the center point of one of the ends of the cylinder to its outer edge to find a radius (if the center point is marked).

2. Measure across one of the bases of the cylinder to find the diameter, as an alternative (the width of the circle that comprises the base).

3. Divide the diameter measurement by two to find the radius.

Formulas

1. Divide the volume of the cylinder by pi (3.14). For instance, if you're given a volume of 706.5 cubic cm, divide 706.5 by 3.14 to get 225.

2. Divide your answer from Step 1 by the length of the cylinder. For instance, if you're given a length of 9 cm, divide 225 by 9 to get 25.

3. Take the square root of your answer from Step 2 to find the radius. For instance, the square root of 25 is 5. Therefore, the radius of a cylinder with a volume of 706.5 cubic cm is 5 cm.

References

• MSN Encarta: Cylinder
• Maths Teacher: Volume of a Cylinder