Pool Fill Time Calculator

How to Calculate Pool Fill Time

The fill-time formula is:

Hours = gallons ÷ sustained GPM ÷ 60

A 20,000-gallon pool filled at a sustained 10 gallons per minute takes:

20,000 ÷ 10 ÷ 60 = 33.3 hours

Real household flow changes with pressure, hose diameter and length, elevation, filters, softeners, other water use, and the number of hoses sharing the service. The calculator’s default 10% allowance avoids treating a short test as a guaranteed flow for an entire day.

Use the percentage field for a partial refill. If you are replacing water to lower a dissolved reading, use the pool dilution calculator first to determine the required gallons.

How to Measure Hose Flow

Use a five-gallon bucket and stopwatch:

1. Open the hose to the setting you will use for the fill.
2. Let trapped air clear and flow stabilize.
3. Time how many seconds it takes to fill a five-gallon bucket.
4. Divide 300 by the seconds.

GPM = 300 ÷ bucket-fill seconds

A 30-second test equals 10 GPM. A 20-second test equals 15 GPM. Repeat the test and use a representative result.

Test at the same time of day you expect to fill if municipal pressure changes during peak use. Remove a restrictive spray nozzle, but keep every hose, backflow device, filter, or treatment component that will remain connected.

How Much Does It Cost to Fill a Pool?

Utility cost is:

Gallons ÷ 1,000 × rate per thousand gallons

Many bills contain separate water and sewer rates. A pool fill sends water into the pool rather than the sewer, so some utilities offer an adjustment if you notify them, document the fill, or meet specific rules. Other utilities do not.

Check for:

• Tiered rates that become more expensive at high usage
• Drought surcharges or seasonal restrictions
• Sewer charges based on water consumption
• Pool-fill permits or notification requirements
• Private-well capacity and treatment limits

Do not enter a national average when your bill shows the actual rate.

Filling with More Than One Hose

Two hoses can shorten the fill, but only if the water supply supports both. Hoses connected to the same restricted branch may each slow down, so multiplying one-hose flow by two can overstate combined performance.

For the best estimate, run all planned hoses and measure their flow separately at the same time. Add the measured GPM values. The calculator assumes each selected hose provides the same flow, then applies the sustained-flow allowance.

Do not run hoses through windows, doors, or walkways where a failed connection can flood the house or create a trip hazard.

New Plaster and Safe Filling

Follow the builder’s written startup instructions. New plaster and other cementitious finishes are particularly sensitive to interrupted fills and waterline marks.

Common precautions include:

• Start only when the fill can continue without interruption.
• Protect the finish from metal hose ends.
• Keep the hose outlet from whipping or concentrating flow on one spot.
• Do not walk on the new surface unless the builder permits it.
• Do not turn the water off midway without approval.
• Do not start circulation equipment until the water reaches the required level.
• Follow the specified brushing and chemistry startup.

Never leave a filling pool unattended. A moved hose, failed fitting, blocked skimmer, or forgotten fill can cause property damage.

Pool Fill Time Calculator FAQ

How long does a 10,000-gallon pool take to fill?

At a sustained 10 GPM, about 16 hours and 40 minutes. At 15 GPM, about 11 hours and 7 minutes. Measure your hose instead of assuming a flow rate.

How long does a 20,000-gallon pool take to fill?

At a sustained 10 GPM, about 33 hours and 20 minutes. Two truly independent 10 GPM hoses would reduce that to about 16 hours and 40 minutes.

Can I fill a pool from a private well?

Possibly, but a large continuous draw can exceed well recovery, damage equipment, stir sediment, or add difficult metals and hardness. Ask a qualified well professional and test the source water before a large fill.

Is delivered water faster than a garden hose?

Often, but access, tanker capacity, discharge method, source-water quality, and price vary. Confirm how the delivery will enter the pool without damaging the finish or yard.

Should I add chemicals while the pool fills?

Follow the builder or finish manufacturer’s startup process. Do not pour concentrated chemicals onto an exposed finish or add products without enough water and circulation to disperse them safely.

How can I calculate a top-up instead of a full fill?

Enter the full pool volume and the approximate percentage being replaced. For a depth-based top-up, each inch over one square foot is about 0.623 gallons.