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.