Measure it with a measuring tape.
A cloth tape measure.
Take a tape measure or a measuring wheel and run it around the border of the pool.
You can place a string around the head, and then measure the string. Or measure directly with a measuring tape, if you have one that is flexible enough (basically, a measuring tape made up of cloth or something similar).
The answer depends on what characteristic you want to measure: its temperature, refractive index, circumference, depth, volume, etc.
Measure it with a measuring tape.
A tape measure.
measure liquids or the area/circumference of the cylinder.
You measure the girth of something by using a flexible or cloth measuring tape. Use this measuring tape to wrap around the object to measure the circumference.
A cloth tape measure.
A flexible measuring tape can be used to measure the circumference of a pendulum bob. Wrap the measuring tape snugly around the pendulum bob to determine its circumference. Alternatively, a string can be used to measure the circumference and then measured against a fixed ruler or measuring tape.
Take a tape measure or a measuring wheel and run it around the border of the pool.
A measuring tape is flexible - a metre rule isn't !
To measure circumference the measuring instrument must follow the curved path exactly which will be difficult without special instruments. It is easier to calculate a circumference by measuring a straight line diameter and then multiply that dimension with Pi (3.141593....).
You can place a string around the head, and then measure the string. Or measure directly with a measuring tape, if you have one that is flexible enough (basically, a measuring tape made up of cloth or something similar).
The answer depends on what characteristic you want to measure: its temperature, refractive index, circumference, depth, volume, etc.
To accurately measure a round table, use a measuring tape to find the diameter by measuring straight across the center of the table. Multiply the diameter by 3.14 (pi) to find the circumference.