Swimming Pool Water Calculator
Find out how many gallons your swimming pool holds and how much water a year of evaporation adds. Use it when filling a new pool, budgeting for water costs, or planning seasonal top-offs.
Last updated: September 2026
Formula below · 2 sources (epa.gov, Wikipedia) · Updated Sep 2026
Compare with similar
About this calculator
Pool volume depends on shape. For a rectangular pool: Volume (gal) = length × width × depth × 7.48, where 7.48 converts cubic feet to gallons. For a round pool: Volume = π × (diameter/2)² × depth × 7.48. For an oval pool: Volume = π × (length/2) × (width/2) × depth × 7.48. Monthly evaporation loss is calculated the same way but substitutes the evaporation depth (in feet, converted from inches by dividing by 12) for the pool depth, and the annual figure is 12 times that. Total annual water = initial fill volume + 12 × monthly evaporation volume. The settings use year-round averages of about 1, 4 and 8 inches per month; an uncovered pool commonly loses about 1/4 inch a day in summer, and a cover cuts evaporation by up to 95%. Evaporation rates vary by climate — hot, dry, and windy locations can lose 1–2 inches per week. The 7.48 factor is exact: one cubic foot holds 7.48052 U.S. gallons.
How to use
Say you have a rectangular pool 20 ft long, 10 ft wide, and 5 ft deep, with moderate evaporation of about 4 inches per month. Fill volume: 20 × 10 × 5 × 7.48 = 7,480 gallons. Evaporation loss: 20 × 10 × (4/12) × 7.48 ≈ 498.67 gallons per month, or 5,984 gallons over 12 months. Total annual water: 7,480 + 5,984 = 13,464 gallons. At a typical residential water rate of $0.005 per gallon, that's about $67 for the first fill plus a year of top-offs.
Frequently asked questions
How many gallons of water does an average residential swimming pool hold?
Most backyard rectangular pools range from 10,000 to 30,000 gallons depending on size and depth. A common 16 × 32 ft pool with an average depth of 5 feet holds approximately 19,200 gallons. Above-ground round pools are typically smaller, commonly 5,000 to 15,000 gallons. Knowing your pool's volume is essential for correctly dosing chemicals.
How much water does a swimming pool lose to evaporation each month?
Evaporation losses vary widely based on climate, temperature, humidity, wind, and whether the pool is covered. In hot, arid regions pools can lose 1–2 inches of water per week, translating to roughly 1,300–2,700 gallons monthly for a mid-sized 16 × 32 ft pool. A solar or safety cover can cut evaporation by up to 95%, dramatically reducing refill costs and chemical consumption.
How long does it take to fill a swimming pool with a garden hose?
A standard garden hose delivers about 8–10 gallons per minute. To fill a 20,000-gallon pool, you'd need approximately 2,000–2,500 minutes, or 33–42 hours of continuous flow. Many homeowners use two hoses or arrange a bulk water delivery to reduce fill time. Always check with your local utility first, as some municipalities restrict large single-day draws.