### Things You'll Need

Calculator

Tape measure

### Tip

An alternative method is to count the number of tiles on the bottom of the pool, then multiply by the surface area of a single tile. This works well for small pools decorated with large tiles.

You will need to know the surface area before you can calculate the pool volume—an essential figure if you want to refill the pond, heat the water or add chemicals.

### Warning

Beware of falling forward when bending to take measurements at the edge of a pool. A forward tumble into an empty pool could result in severe injuries.

Calculating the surface area of rectangular and circular pools is straightforward and uses math we can all remember. Kidney-shaped, or reniform, pools, on the other hand, combine the curves of a circular pool with the elongated shape of a rectangular pool. With a constantly changing width and no straight edges, you might assume the math to find the area of a kidney-shaped object is highly complex, but you'd be wrong. The calculations involve three measurements and one straightforward formula.

## Step 1

Measure the length of the pool along an imaginary line joining the opposite ends of the longest axis. Repeat the measurement until you are satisfied that it is correct. Record the value.

## Step 2

Measure the width of the wider bulge of the kidney shape and record the value as "A." Measure the width of the smaller bulge and record that value as "B." Double-check your measurements.

## Step 3

Calculate the surface area, using the formula: Area = (A + B) x Length x 0.45

For example, a pool is 20 feet long and has widths of 10 and 15 feet. Substituting the dimensions into the formula gives the surface area as (10 + 15) x 20 x 0.45, or 225 square feet.

## Step 4

Check for errors by reversing your calculations. Divide the surface area by 0.45 times the length of the pool. If the answer is not the combined widths of the pool, there was an error in your calculations. Repeat the math until the answer is correct.

### David Robinson

David Robinson has written professionally since 2000. He is a Fellow of the Royal Geographical Society and the Royal Meteorological Society. He has written for the "Telegraph" and "Guardian" newspapers in the U.K., government publications, websites, magazines and school textbooks. He holds an honors Bachelor of Arts in geography and education and a teaching certificate from Durham University, England.