Pool Fill Time Calculator
Estimate how long it takes to fill (or top up) your pool from a hose or supply line. Enter the volume in gallons and the flow in gallons per minute — a typical garden hose delivers around 9 GPM.
Calculator
Filling 20,000 gallons at 9.0 GPM takes about 37.0 hours.
Filling a pool always takes longer than people expect, because a garden hose moves a surprisingly small amount of water per minute against the enormous volume of a pool. This calculator turns your pool’s gallons and your hose’s flow rate into a realistic number of hours — and days — so you can plan the fill around your schedule and avoid leaving a hose running unattended overnight.
The single most important input is the flow rate in gallons per minute (GPM). A standard ⅝-inch garden hose delivers roughly 9 GPM, but real flow depends on your home’s water pressure, the hose diameter and length, and how many other fixtures are drawing at the same time. The most accurate way to find your actual flow is a quick bucket test: time how many seconds it takes to fill a five-gallon bucket, then divide 300 by that number of seconds to get GPM. Feeding a measured rate into the calculator gives a far better estimate than a rule of thumb.
Formula
Fill time is the volume divided by the volume delivered per hour. Since flow is given per minute, multiply by 60 to convert minutes to hours:
hours = gallons ÷ (GPM × 60)
For example, one gallon per minute equals 60 gallons per hour, so the pool volume divided by (GPM × 60) gives the fill time directly in hours. Divide by 24 for days.
Worked example
A 20,000-gallon pool filled from a garden hose flowing at 9 GPM:
20,000 ÷ (9 × 60) = 20,000 ÷ 540 ≈ 37.0 hours
That is about a day and a half of continuous running. Running two hoses at once roughly halves the time; a larger supply line or a pressure boost raises the GPM and shortens it further.
Getting the flow rate right
Because fill time depends so heavily on flow, small changes in GPM matter a lot. Doubling from 9 to 18 GPM — for instance by running two hoses, or one short, wide hose straight off a spigot — cuts the estimate in half. Conversely, a long, thin, kinked hose on low pressure can drop well below 9 GPM and stretch the fill past two full days.
Never leave a filling pool completely unattended for long stretches, especially overnight. It is easy to overshoot and waste water, and a running hose is easy to forget. If you are filling a new pool with a plaster or vinyl surface, follow the builder’s instructions on continuous filling to avoid staining or a floating liner. Once you know the hours, the fill-cost tool tells you what that water will cost at your own rate.