Pool Evaporation Calculator Formula

Understand the math behind the pool evaporation calculator. Each variable explained with a worked example.

Use the Pool Evaporation Calculator

Formulas Used

Annual Water Loss

annual_loss = net_evap_gal

Monthly Water Loss

monthly_loss = net_evap_gal / pool_months

Annual Refill Cost

annual_cost = net_evap_gal * 5 / 1000

Cover Would Save

cover_savings_gal = gross_evap_gal * 0.70

Variables

VariableDescriptionDefault
pool_sqftPool Surface Area(sq ft)450
evap_rate_inMonthly Evaporation Rate(inches/month)4
pool_monthsPool Season Months(months)6
cover_reductionCover Reduction(%)0
gross_evap_galDerived value= pool_sqft * evap_rate_in * pool_months * 0.623calculated
net_evap_galDerived value= gross_evap_gal * (1 - cover_reduction / 100)calculated

How It Works

How to Calculate Pool Evaporation

Swimming pools lose significant water to evaporation, especially in hot, dry, or windy climates.

Formula

Evaporation (gal) = Pool Area * Evap Rate * Months * 0.623 * (1 - Cover Reduction)

Typical Evaporation Rates

  • Humid climate: 2-3 inches/month
  • Moderate: 4-5 inches/month
  • Arid/hot: 6-8 inches/month

    • A pool cover reduces evaporation by 70-90%.

    Worked Example

    A 450 sq ft pool evaporating 4 inches/month for 6 months without a cover.

    pool_sqft = 450evap_rate_in = 4pool_months = 6cover_reduction = 0
    1. 01Gross evaporation: 450 * 4 * 6 * 0.623 = 6,729 gallons
    2. 02Net evaporation (no cover): 6,729 gallons
    3. 03Monthly loss: 6,729 / 6 = 1,122 gallons
    4. 04Cover would save: 6,729 * 70% = 4,710 gallons

    Ready to run the numbers?

    Open Pool Evaporation Calculator