How can I measure my hose's flow rate?

Bucket-test it. Time a 5-gallon bucket filling from the hose. Your flow rate in gpm is 300 ÷ those seconds. A bucket that fills in 25 seconds is 12 gpm, 33 seconds is 9 gpm, 50 seconds is 6 gpm. The calculator has the helper inline.

Why does the estimate vary so much?

Because hose flow varies enormously — by water pressure, hose diameter, hose length. A 5/8" residential hose typically gives somewhere between 6 and 12 gpm; that range alone doubles or halves your fill time. The calculator surfaces both ends as a range; the bucket test pins your specific number.

How can I fill my pool faster?

Two levers. (1) Run more hoses in parallel from separate spigots — two hoses ≈ halve it, three cut it again. (2) Use a wider hose — a 3/4" hose moves about 2.5× the water of a 5/8". Pulling both levers together can take a 1.5-day fill down to half a day.

How long to fill by pool size?

At a typical 9 gpm: 10,000 gal ≈ 19 hr (~0.8 days), 15,000 ≈ 28 hr (~1.2 days), 20,000 ≈ 37 hr (~1.5 days), 30,000 ≈ 56 hr (~2.3 days). Linear in volume.

Can I leave the hose running overnight?

You'll have to — multi-day fills run overnight by definition. Set a reminder so the pool doesn't overflow when full. On well water, a long continuous draw can outpace the well's recovery and damage the pump — check your well's sustained yield, or fill in stages.

Will filling it cost a lot?

Less than most people fear — usually around a hundred dollars for an average pool on city water — but possibly double that if your bill charges sewer per gallon (which it likely shouldn't on fill water; many utilities will waive). See the cost-to-fill calculator for the dollars and the sewer-exemption wedge.

Can I swim once it's full?

Not until startup chemistry is balanced — pH, alkalinity, calcium, chlorine, CYA all need to land in safe ranges first. That's the chemistry cluster's job (volume → chlorine → CYA → pH → alkalinity → calcium → LSI). The fill is the start of the project, not the end.

Pool Fill Time Calculator

From your gallons and your hose's flow rate — with the bucket test to measure your real hose, and the honest range because flow varies.

Hook

Longer than you think.

Filling a pool takes far longer than most people expect — a typical backyard pool on a single garden hose runs about a day and a half of continuous flow, hose running through the night.

The exact time depends on something you probably don't know offhand: how many gallons a minute your hose actually delivers, which varies more than you'd guess with pressure, diameter, and length. The good news is you can measure it in thirty seconds with a bucket.

Promise

This calculator tells you how long your fill will take — from your pool's volume and your fill rate — and shows how much faster a second hose or a wider one gets you there. It gives you the bucket test to measure your hose's real flow rate (so the estimate is yours, not a generic guess), and it's honest that the answer is a range, because hose flow isn't a fixed number. Plan the fill before you start it.

Here's the deal: fill time is just your gallons divided by how fast water comes out — simple, except “how fast” is the part you don't know. A garden hose might give you six gallons a minute or twelve depending on your pressure and the hose, and that doubles or halves the time. Measure your actual rate, divide your gallons by it, and you'll know whether you're looking at a day or three.

What you'll give us

Your pool's volume (jump to the volume calculator if you don't know it), your hose's flow rate (or 30 seconds with a 5-gallon bucket to measure it), and how many hoses you're running. We show the time as a range — flow varies — and surface a caution for multi-day fills.

Three inputs → one engine → time as a range with a caution flag for multi-day fills. The wedge — fillTimeRange() — surfaces all three outputs by construction.

The calculator

Fill the fields — the bucket-test helper is right under the rate input — and hit Calculate. The result panel shows the point estimate, the range across the typical flow band, the multi-day caution when applicable, and the speedup options.

It takes longer than you think

Here's the number that surprises people: filling an average backyard pool from a single garden hose takes about a day and a half of continuous running. Not a few hours — a day and a half, hose going through the night.

A bigger pool, or lower water pressure, and you're looking at two or three days. That's worth knowing before you start, because it means planning: you can't fill on Saturday morning for a Saturday afternoon swim, the hose runs unattended overnight, and if you're on well water you need to make sure you don't run the well dry. Filling a pool is a multi-day project, not an afternoon chore.

Fill time falls steeply as flow rises. The shaded band is the typical variance for a single 5/8-inch garden hose at residential pressure — the same pool can take a day or three depending on your spigot.

Measure your hose's real flow (the bucket test)

The calculator needs one thing you probably don't know: how many gallons a minute your hose delivers. Don't guess — measure it, it takes thirty seconds.

Grab a five-gallon bucket, put the hose in, and time how long it takes to fill. Then your flow rate in gallons per minute is just three hundred divided by those seconds: a bucket that fills in thirty seconds is ten gallons a minute, fifty seconds is six. Now your fill-time estimate is built on your actual hose, your actual pressure — not a generic average that could be off by half.

Time a 5-gallon bucket filling from your hose, divide 300 by the seconds. That's your gpm — measured, not guessed.

The calculator has a bucket-test helper inline — type the seconds, hit Apply, and it sets the rate field. You can fine-tune from there if you want.

Speeding it up: more hoses, wider hoses

If a day and a half is too long, you have two levers. The first is more hoses: a second hose running in parallel roughly halves the time, a third cuts it again — fill from every spigot you've got.

The second is a wider hose: stepping from a standard five-eighths-inch hose to a three-quarter-inch one moves noticeably more water, because flow rises faster than width. Between the two, you can take a day-and-a-half fill down to half a day. Just remember each hose is its own flow rate — measure them, don't assume they're equal.

Adding a second hose roughly halves the time. Stepping to a 3/4" hose moves more water per minute again. Both levers can be pulled together.

Time is one axis; cost and chemistry are the others

Knowing how long isn't the whole picture. The same fill has a cost — what the water runs you, and whether your bill double-charges sewer on it (our cost-to-fill calculator covers that, and it's worth a look before you fill).

And once it's full, a fresh fill needs its startup chemistry balanced before anyone swims — that's its own job, in the chemistry calculators. So plan all three: how long, how much, and how to balance it. The fill is the start of the project, not the end.

Time tells you when. Cost tells you what. Chemistry tells you whether anyone can swim. The fill calculator answers one axis; the others have their own calculators.

Where the numbers come from

Five short steps from gallons + flow rate to a fill time. EXACT math throughout — what varies is your hose's flow rate, which is why the answer is a range and we give you the bucket test.

Step 1 · gallons from the volume calculator
Volume in your unit converts to gallons via the shipped volumeToGal function — same NIST-exact L_PER_GAL the volume calculator uses. Cross-cluster reuse, single source.
Step 2 · the bucket test (the wedge, if you don't know your rate)
Time a 5-gallon bucket filling from the hose. Your gpm is gpm = 5 × 60 ÷ seconds = 300 ÷ seconds. EXACT — 25 s → 12 gpm, 33 s → 9 gpm, 50 s → 6 gpm. Turns the unknown flow rate into a 30-second measurement.
Step 3 · time = volume ÷ (gpm × 60)
For the standard 20,000-gallon pool at 9 gpm typical: 20000 / (9 × 60) = 37.04 hr ≈ 1.5 days. EXACT division.
Step 4 · the range, by construction
Same 20,000-gallon pool, flow band 6–12 gpm. fastestHours uses 12 gpm → 27.78 hr (faster flow → shorter fill, hence the name). slowestHours uses 6 gpm → 55.56 hr (slower flow → longer fill). Both returned by fillTimeRange() — the result panel cannot show a single false-precise number.
Step 5 · the multi-day caution (by construction)
When the point estimate > 24 hours, the engine sets hasMultiDayCaution = true and the result panel surfaces the F9 risks: overnight unattended running (overflow risk), well-water sustained-draw (pump damage), drought-restriction localities. The flag fires at 37hr on the standard system — every typical fill triggers it. That's the point.

Eight worked examples

Engine values, never hand-typed. Flow rates are labeled typicals; your actual rate (use the bucket test) is what makes your estimate yours.

E1 — How long does it take to fill a pool? (the headline)

20,000 gallons at 9 gpm (typical single 5/8" hose): 37.0 hr ≈ 1.5 days.

Takeaway: An average pool on one garden hose takes about a day and a half — plan for the hose running overnight, not an afternoon job.

E2 — Two hoses fill pool

20,000 gallons at 18 gpm (two hoses in parallel): 18.5 hr. About half the single-hose time.

Takeaway:A second hose roughly halves it — under a day. Fill from every spigot if you're in a hurry.

E3 — 3/4-inch hose pool fill

20,000 gallons at 23 gpm (single 3/4" hose): 14.5 hr.

Takeaway:A wider hose moves a lot more water — a 3/4" hose nearly halves a 5/8" hose's time.

E4 — Pool fill time varies (the range honesty)

Same 20,000-gallon pool, flow band: fastestHours = 27.8 hr at 12 gpm (good pressure), slowestHours = 55.6 hr at 6 gpm (low pressure). Field names describe the TIME bound, not the flow.

Takeaway:Your pressure and hose make a huge difference — the same pool can take one day or three. That's why you measure your rate.

E5 — Fill time by pool size (linear)

Gallons	Hours at 9 gpm
10,000	18.5 hr (≈ 0.8 days)
15,000	27.8 hr (≈ 1.2 days)
20,000	37.0 hr (≈ 1.5 days)
30,000	55.6 hr (≈ 2.3 days)

Takeaway: Time scales straight with gallons — a 30k pool on one hose is over two days.

E6 — The bucket test (the wedge — 300 ÷ seconds EXACT)

Seconds (5-gal bucket)	gpm
25 s	12.00 gpm
33 s	9.09 gpm
50 s	6.00 gpm

Takeaway: Thirty seconds with a five-gallon bucket gives you your real flow rate — and a fill estimate built on your hose, not a guess.

E7 — Fill pool well water (caution)

A 20,000-gallon pool over ~37hours of continuous draw can outpace a typical residential well's recovery — risk of running it dry and damaging the pump.

Takeaway:On well water, fill in stages or check your well's sustained yield first — don't run it dry.

E8 — Metric (75 m³ via the shipped volume engine)

75 m³ → 19,812.9 gallons via the same NIST-exact L_PER_GAL the volume + fill-cost calcs use. At 9 gpm: 36.7 hr.

Takeaway: Same math in metric — volume over flow rate — and the gallon conversion reconciles across three water/chemistry calculators by construction.

Reference tables

T1 · Fill time by pool volume × flow rate

EXACT math · hours = gallons ÷ (gpm × 60). Flow-rate columns span low pressure (6 gpm) to a 3/4" hose (23 gpm).

Gallons \ gpm	6	9	12	18	23
5,000	13.9 hr	9.3 hr	6.9 hr	4.6 hr	3.6 hr
10,000	27.8 hr	18.5 hr	13.9 hr	9.3 hr	7.2 hr
15,000	41.7 hr	27.8 hr	20.8 hr	13.9 hr	10.9 hr
20,000	55.6 hr	37.0 hr	27.8 hr	18.5 hr	14.5 hr
25,000	69.4 hr	46.3 hr	34.7 hr	23.1 hr	18.1 hr
30,000	83.3 hr	55.6 hr	41.7 hr	27.8 hr	21.7 hr

T2 · Bucket-test conversion (the wedge dataset)

EXACT math · gpm = 300 ÷ seconds for a 5-gallon bucket. Time a fill, look up the rate.

Bucket fills in (seconds)	gpm
15 s	20.00 gpm
20 s	15.00 gpm
25 s	12.00 gpm
30 s	10.00 gpm
33 s	9.09 gpm
40 s	7.50 gpm
50 s	6.00 gpm
60 s	5.00 gpm

T3 · Fill time by hose setup

ESTIMATE · uses typical hose rates. Your actual hose will vary; bucket-test it for tighter numbers. Multi-hose setups assume each hose runs at its full rate (shared pressure can reduce each).

Setup \ pool	10k gal	20k gal	30k gal
Single 5/8" hose (9 gpm)	18.5 hr	37.0 hr	55.6 hr
Single 3/4" hose (23 gpm)	7.2 hr	14.5 hr	21.7 hr
Two 5/8" hoses (18 gpm)	9.3 hr	18.5 hr	27.8 hr
Three 5/8" hoses (27 gpm)	6.2 hr	12.3 hr	18.5 hr

Tables released CC BY 4.0. T1 + T2 are exact math; T3 uses typical hose rates (labeled).

Methodology & sources

Fill time = volume ÷ flow rate. EXACT math. Volume comes from the shipped volume calculator's engine — the same volumeToGal function, the same NIST-exact L_PER_GAL constant. The fill-time and fill-cost calcs (the cluster siblings) both reconcile to the volume calc on metric inputs by construction.

The bucket-test wedge. For a 5-gallon bucket, gpm = 300 ÷ seconds EXACTLY. The math is first-principles: 5 gallons over N seconds = 5/N gallons per second = 300/N gallons per minute. The value is in the measurement: typical 5/8" residential hose flow ranges 6–12 gpm by pressure/diameter/length, and most owners don't know which end they're on. The bucket test pins it.

Range honesty.Hose flow varies — same pool, same owner, different day, different number. Rather than print a false-precise time, the engine's fillTimeRange() returns a point estimate AND the times at the typical band's edges (6 gpm slowest, 12 gpm fastest). Field names describe the TIME bound (fastestHours holds the SHORTEST fill, slowestHours the LONGEST) — not the flow input that produced them.

Multi-day caution by construction.When the point estimate exceeds 24 hours, the engine sets a flag the result panel renders as the F9 cautions: overnight unattended running (overflow risk), well-water sustained-draw (pump damage), drought-restriction jurisdictions. We can't know your well's yield or your city's rules, but we know multi-day fills mean unattended overnight — and we surface the risk so you can plan.

Typical-hose rates are labeled typicals: 5/8" residential ≈ 9 gpm, 3/4" ≈ 23 gpm. Both span wide bands in practice — your bucket-test rate is the source of truth.

Scope. This page is the TIME axis only. Cost is the cost-to-fill calculator (with the sewer-exemption wedge). Startup chemistry is the chemistry cluster. Ongoing evaporation top-off — the inverse question, water leaving rather than entering — is the evaporation calculator. Honest cross-links, not scope creep.

2nd spoke of the Pool Water & Filling cluster. Its arrival brings the cluster hub live: the cluster hub explains the four-questions arc (how much / how long / what does it cost / how big can it get).

Reference tables T1/T2/T3 released under CC BY 4.0. T1 + T2 are exact math; T3 uses typical hose rates (labeled).

Frequently asked questions

How long does it take to fill a pool?: For an average 20,000-gallon pool on a typical garden hose (~9 gpm), about 37 hours — a day and a half of continuous running. The hose runs overnight; plan for the unattended part. A wider hose or a second one roughly halves it.
How can I measure my hose's flow rate?: Bucket-test it. Time a 5-gallon bucket filling from the hose. Your flow rate in gpm is 300 ÷ those seconds. A bucket that fills in 25 seconds is 12 gpm, 33 seconds is 9 gpm, 50 seconds is 6 gpm. The calculator has the helper inline.
Why does the estimate vary so much?: Because hose flow varies enormously — by water pressure, hose diameter, hose length. A 5/8" residential hose typically gives somewhere between 6 and 12 gpm; that range alone doubles or halves your fill time. The calculator surfaces both ends as a range; the bucket test pins your specific number.
How can I fill my pool faster?: Two levers. (1) Run more hoses in parallel from separate spigots — two hoses ≈ halve it, three cut it again. (2)Use a wider hose — a 3/4" hose moves about 2.5× the water of a 5/8". Pulling both levers together can take a 1.5-day fill down to half a day.
How long to fill by pool size?: At a typical 9 gpm: 10,000 gal ≈ 19 hr (~0.8 days), 15,000 ≈ 28 hr (~1.2 days), 20,000 ≈ 37 hr (~1.5 days), 30,000 ≈ 56 hr (~2.3 days). Linear in volume.
Can I leave the hose running overnight?: You'll have to — multi-day fills run overnight by definition. Set a reminder so the pool doesn't overflow when full. On well water, a long continuous draw can outpace the well's recovery and damage the pump — check your well's sustained yield, or fill in stages.
Will filling it cost a lot?: Less than most people fear — usually around a hundred dollars for an average pool on city water — but possibly double that if your bill charges sewer per gallon (which it likely shouldn't on fill water; many utilities will waive). See the cost-to-fill calculator for the dollars and the sewer-exemption wedge.
Can I swim once it's full?: Not until startup chemistry is balanced — pH, alkalinity, calcium, chlorine, CYA all need to land in safe ranges first. That's the chemistry cluster's job (volume → chlorine → CYA → pH → alkalinity → calcium → LSI). The fill is the start of the project, not the end.

Related calculators

Next in Pool Water & Filling: Round Pool Gallons.

More in Pool Water & Filling: Cost to Fill a Pool.

Related across clusters: Pool Volume Calculator, Pool Evaporation Calculator.

All Pool Water & Filling calculators: browse the hub.