Whether you’re filling a brand new pool, refilling after a drain, or just topping off after evaporation, knowing how long the process will take helps you plan ahead. Let’s break down the math and give you practical guidance.
The Fill Time Formula
Fill Time (hours) = Pool Volume (gallons) ÷ Flow Rate (GPM) ÷ 60It’s that simple — divide your total pool volume by your water flow rate to get total minutes, then convert to hours.
Common Water Source Flow Rates
| Water Source | Typical GPM | 15,000 gal Pool | 25,000 gal Pool |
|---|---|---|---|
| Standard Hose (½”) | 9 GPM | 27.8 hours | 46.3 hours |
| Large Hose (¾”) | 15 GPM | 16.7 hours | 27.8 hours |
| Two Garden Hoses | 18 GPM | 13.9 hours | 23.1 hours |
| Well Pump | 10 GPM | 25.0 hours | 41.7 hours |
| Water Delivery Truck | 100 GPM | 2.5 hours | 4.2 hours |
| Fire Hydrant | 250 GPM | 1.0 hour | 1.7 hours |
Tips for Faster Filling
- Use the largest hose you have — a ¾” hose delivers nearly twice the water of a ½” hose
- Run multiple hoses — two hoses from different spigots roughly doubles your flow rate
- Check your well pump capacity — well pumps vary widely from 5 to 25+ GPM
- Consider a water delivery truck — for pools over 20,000 gallons, this can save a full day
- Monitor the fill — never leave a filling pool unattended for extended periods
Estimating Water Cost
Most municipal water is billed per 1,000 gallons. The national average is around $4.00 per 1,000 gallons, but rates vary significantly by region:
| Region | Avg. Rate per 1,000 gal | 20,000 gal Pool Cost |
|---|---|---|
| Northeast | $5.50 | $110.00 |
| Southeast | $3.50 | $70.00 |
| Midwest | $3.00 | $60.00 |
| Southwest | $6.00 | $120.00 |
| West Coast | $7.50 | $150.00 |
Important: Some water utilities offer a “pool fill adjustment” — a one-time credit on your sewer bill since filled pool water doesn’t enter their system. Call your utility company before filling to ask about this credit.
How Evaporation Affects Your Pool
On average, pools lose ¼ to ½ inch of water per day to evaporation, which translates to:
- A 16×32 ft pool loses about 100–200 gallons per day
- That’s 3,000–6,000 gallons per month in hot, dry climates
- A pool cover can reduce evaporation by 90–95%
When to Drain vs. Top Off
- Top off: Normal evaporation, minor water level drops, routine maintenance
- Partial drain (25–50%): High CYA levels, total dissolved solids, calcium hardness issues
- Full drain: Replastering, structural repairs, complete chemistry reset
Warning: Never drain a pool without professional guidance. An empty pool can “float” out of the ground due to hydrostatic pressure, especially during rainy seasons. Many pool builders recommend never draining a fiberglass pool and only partially draining a vinyl liner pool.
Calculate your exact pool volume with our pool volume calculator to get precise fill time and cost estimates for your specific pool.