Calculate how much water your pool loses to evaporation. Understand the impact of wind, humidity, and temperature on your pool's water level. Monitoring evaporation is key to identifying potential leaks and optimizing your water conservation efforts.
Start by gathering four environmental inputs the tool needs: average air temperature, water temperature, relative humidity, and wind speed at the pool surface. Measure water temperature with a floating thermometer placed away from return jets, and read humidity from a local weather source or a simple hygrometer. You also enter your pool's surface area in square feet or square meters, since evaporation scales directly with exposed surface, not total volume. If you only know length and width, multiply them for a rectangular surface; for free-form pools, estimate the wetted area. Note whether the pool is heated, because warmer water and a higher water-to-air temperature gap dramatically raise the evaporation rate. Finally, record any water-moving features like fountains, waterfalls, or spillways and whether a cover is used, since both swing results significantly. Accurate surface area and a recent water temperature reading are the two inputs that most influence the estimate, so prioritize measuring those rather than guessing from memory.
Evaporation is commonly estimated with a vapor-pressure model: Rate = (95 + 0.425 × W) × (Pw − Pa) / Y, where W is wind speed (mph), Pw is the saturation vapor pressure at water temperature, Pa is the vapor pressure of the air, and Y is the latent heat of vaporization. The result is pounds of water per square foot per hour, which converts to depth and gallons. As a worked example: a 600 sq ft pool surface losing 0.25 inch of depth per day loses 600 × (0.25/12) = 12.5 cubic feet, and 12.5 × 7.48 = about 93 gallons (≈352 liters) per day. Over a 30-day summer month that is roughly 2,800 gallons (≈10,600 liters). A solar or safety cover typically cuts that loss by 50 to 95 percent, so the same pool covered overnight might lose only 30 to 45 gallons (≈115 to 170 liters) daily. Multiply gallons lost by your water rate to estimate the monthly cost of refilling.
Accuracy matters here because evaporation is the baseline you compare against when hunting a suspected leak; overestimating normal loss can hide a real plumbing crack, while underestimating it can send you chasing a leak that is just July heat. A frequent mistake is blaming evaporation for water that is actually splashed out by heavy bather load or an aggressive waterfall, so log usage alongside level drops. Another error is ignoring that every gallon of fresh fill water dilutes your chemistry: heavy summer top-offs lower stabilizer (CYA), salt, and calcium hardness, so retest those after large refills and re-dose if needed. Refilling also shifts your LSI balance, so recheck pH and alkalinity weekly during peak season. Conserve by covering the pool whenever it is unused, lowering water temperature a few degrees, and running fountains only when you want the cooling. There is no chemical hazard in this calculation itself, but the downstream re-dosing it triggers should always follow add-slowly, retest-then-adjust discipline.
A typical pool loses about 1/4 to 1/2 inch of water per day, depending on the local temperature, wind speed, and humidity levels.
Perform a 'Bucket Test.' Place a bucket of water on the pool step and mark the water level in both the bucket and the pool. If the pool level drops more than the bucket, you likely have a leak.
Yes. Splashing water and spray increase the surface area exposed to the air, which can significantly accelerate the rate of evaporation and cooling.