A 15,000 gallon pool filled from one garden hose that delivers 10 gallons a minute takes 25 hours, a day and an hour, and uses about $116 of water at the 2024 US average price of $7.74 per 1,000 gallons. Two hoses on separate taps halve that to 12.5 hours. The only inputs are how much water the pool takes and how fast your hose runs, and the second one is easy to measure yourself.
How fill time is calculated
Divide the gallons by the flow in gallons per minute to get minutes, then divide by 60.
Hours = gallons ÷ (GPM per hose × number of hoses) ÷ 60
For the example: 15,000 ÷ 10 = 1,500 minutes, ÷ 60 = 25 hours. The cost is gallons ÷ 1,000 × the price: 15 × $7.74 = $116.10. If your bill charges sewer as well on water that never goes down a drain, the combined national average of $16.83 gives $252.45, but many utilities do not bill outdoor use for sewer, so read your bill.
Find your hose flow: the bucket test
The most reliable number is your own. Washington State University's irrigation calculator says it is more accurate to measure flow with a known volume and a stopwatch than to compute it. Open the tap fully, put the hose in a 5 gallon bucket and time it. A bucket that fills in 30 seconds means 10 GPM, in 20 seconds 15 GPM, in 60 seconds 5 GPM. The calculator does this conversion in the Bucket test tab.
If you cannot test, the Hose size tab uses typical ranges from a hose flow guide at 30 to 80 psi on a 50 ft hose: about 6 to 10 GPM for a 1/2 inch hose, 9 to 17 GPM for 5/8 inch, and 15 to 25 GPM for 3/4 inch. The tool uses the middle of each range (8, 13 and 20 GPM), and long hoses, kinks and nozzles lower it.
How long to fill a pool by size
At 10 GPM: a 5,000 gallon pool takes 8.3 hours, a 10,000 gallon pool 16.7 hours, a 20,000 gallon pool 33.3 hours. At 13 GPM the 15,000 gallon pool takes 19 hours and 14 minutes. Enter your gallons or use the pool volume calculator first if you do not know them; above-ground owners can read theirs from the pool gallons calculator.
Filling faster
Use every outdoor tap, each with its own hose, since two hoses on one tap share the supply. Bigger hoses help more than higher pressure.
Top-up: how much water to add
The tool also prices topping up. Every square foot of pool holds 0.6234 gallons per inch of depth. For a 32 by 16 ft pool (512 sq ft) adding 2 inches takes 638 gallons, about 49 minutes at 13 GPM, and $4.94 of water. That is a good size for a hot week; the pool evaporation calculator estimates how often you will have to do it.
After the fill
Test the water once it has circulated. New water usually needs stabilizer and alkalinity: the pool chemical calculator turns your test results into doses, and the pool stabilizer calculator sets cyanuric acid for a new fill. A partial refill dilutes what is in the pool, so retest before dosing. If you run a salt system, add salt after the pool is full, with the pool salt calculator.
Water cost
The 2024 EPA WaterSense estimate of national average residential rates is $7.74 per 1,000 gallons for water and $9.09 for wastewater. Real bills vary a lot by city and by tier, so type your own rate. Tiered rates make the last thousand gallons of a big fill cost more than the first.
Every pool tool, with your saved gallons, is on the pool calculators page.