How to calculate swimming pool evaporation

Updated February 21, 2017

Water evaporation can lead to low water levels in your pool. Left unattended, this can burn out the pool pump. To avoid costly repair and maintain the proper chemical balance in your pool, measure the water evaporation rate over a week. This will help you counter the effects of excessive evaporation during hot, dry months and keep your pool equipment humming efficiently.

Use a ruler or measuring stick to measure the distance from the top edge of the pool to the waterline. Record this measurement and date the entry.

Wait one week and repeat the measurement. Do not add water to the pool during this time. Record the second measurement. Subtract the second measurement from the first. Divide the result by 12. Record the result.

Determine the surface area of a rectangular pool by multiplying the width by the length. For a round pool, multiply the diameter--the distance across the middle of the pool--by 3.14. Multiply the result by the evaporation result recorded in Step 2. The result is the evaporation rate of your pool expressed in cubic feet.

Multiply the evaporation rate result by 7.48 to determine how many gallons of water the pool loses in one week. This is the amount of water you need to add to your pool each week to maintain the water level.


The water should never be so low that the intake at the top of the pool sucks air. Keeping the water level halfway up the top tile strip in a traditional pool is usually sufficient. Knowing how many gallons you add to your pool each week can help you determine whether your water bill is correct or if you have a leak elsewhere.


If the pool loses an excessive amount of water, the pool filtration system make have a leak. Check the seals and gaskets to ensure they are not leaking. If no leaks are visible in the above ground equipment, hire a professional to find underground leaks.

Things You'll Need

  • Ruler or measuring stick
  • Calculator
  • Paper
  • Pen or pencil
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About the Author

Sherry Gray started her writing career in 2010 when the company she worked for as a web developer began to fail. In college she majored in English, taking every writing and literature course available plus advertising and business. Gray feels finally putting her education to work was a great career choice.