I Filled My Pool With a Garden Hose — It Took 35 Hours and Cost More Than I Guessed
I topped off our above-ground pool with the garden hose on a Saturday morning, figured it'd be full by lunch, and went back to mowing the lawn. It was still filling at bedtime. By the time it finally topped out — a day and a half later — I'd used enough water to notice on the next bill, and I still had no idea how much chlorine to add to it safely.
That's the part nobody tells you about pool ownership: everything downstream of "how much water is in there" depends on a number most people never actually calculate. Fill time, water cost, and chemical dosing are all just that one volume number multiplied by something else. Get the gallons wrong and you either under-treat the water (algae, cloudy water, bacteria risk) or waste money over-dosing it. This walks through the exact formulas — the same ones behind the Pool Volume Calculator — so you can stop guessing.
⚠️ Disclaimer
This article is for general informational purposes only and does not constitute chemical safety or engineering advice. Always follow the manufacturer's instructions on your specific pool chemicals and check local water restrictions before filling a pool.
How Much Water Does Your Pool Actually Hold?
Pool volume is the total amount of water a pool holds, measured in gallons or litres, calculated by multiplying its surface area by its average depth. It's the single number every other pool calculation — chemical dosing, fill time, heating cost, pump turnover rate — is built on top of.
Circular: π × (Diameter ÷ 2)² × Average Depth × 7.48
Average Depth = (Shallow Depth + Deep Depth) ÷ 2
The 7.48 is the number of gallons in a cubic foot — skip it entirely if you're working in metres, since a cubic metre already equals 1,000 litres.
Worked example — rectangular in-ground pool (US, imperial): A 32 ft × 16 ft pool with a shallow end of 3 ft and a deep end of 7 ft:
- Average depth: (3 + 7) ÷ 2 = 5 ft
- Volume: 32 × 16 × 5 = 2,560 cubic feet
- Gallons: 2,560 × 7.48 = ~19,150 gallons
Worked example — round above-ground pool (metric): A common 24 ft (7.3 m) diameter above-ground pool with a uniform 1.2 m depth:
- Volume: π × (7.3 ÷ 2)² × 1.2 = π × 13.32 × 1.2 = ≈50,200 litres
That's the size difference that catches people out — a backyard above-ground pool and an in-ground pool that "look about the same size" can differ by tens of thousands of litres once you actually run the formula, because depth and shape both compound the error if you eyeball them.
| Pool Type | Typical Size | Avg Depth | Volume |
|---|---|---|---|
| Small above-ground (round) | 15 ft dia. | 4 ft | ~5,300 gal |
| Large above-ground (round) | 24 ft dia. | 4 ft | ~13,500 gal |
| Medium in-ground (rectangular) | 32 × 16 ft | 5 ft | ~19,150 gal |
| Large in-ground (rectangular) | 40 × 20 ft | 5.5 ft | ~32,900 gal |
How Long Will It Actually Take to Fill?
Once you know the gallons, fill time is just division — but the answer surprises almost everyone the first time they run it.
A standard garden hose (5/8-inch, average household water pressure) delivers roughly 9–10 gallons per minute. Run that against the 19,150-gallon rectangular pool from above:
- 19,150 gallons ÷ 9.5 GPM = 2,016 minutes
- 2,016 ÷ 60 = ~33.6 hours
That's the maths behind my own weekend — a mid-sized pool at typical garden hose pressure takes well over a full day of continuous flow, not the "few hours" most people assume. A larger 32,900-gallon pool at the same flow rate stretches past 57 hours — the better part of three days.
⚠️ Note
Many US municipalities restrict or meter high-volume residential water use during drought conditions, and some charge a higher tiered rate once usage crosses a threshold in a billing period. Check your local utility's rules before filling a large pool in one go — splitting the fill across two billing cycles can sometimes avoid a tier jump.
What Does It Cost to Fill and Run?
US municipal water rates vary enormously by city — from roughly $2 to $10 per 1,000 gallons depending on the utility and any tiered pricing once you cross a usage threshold. UK and Australian water is typically metered similarly, at roughly £2–£4 or AU$2–$5 per 1,000 litres.
Worked example (US): at a mid-range $6 per 1,000 gallons, filling the 19,150-gallon pool from the earlier example costs:
- 19,150 ÷ 1,000 × $6 = ~$115
That's before sewer surcharges some utilities add for outdoor water use (a cost that doesn't apply if your utility exempts irrigation/pool meters — worth checking, since it can cut the bill noticeably).
Key Takeaway
A one-time full fill is rarely the expensive part of pool ownership — evaporation replacement (typically ¼–½ inch of water lost per week in hot, dry, or windy climates) and heating cost add up far more over a season. Filling top-ups back in with the same hose-flow math keeps that ongoing cost visible instead of invisible.
Running the pump and filter also adds to the electric bill — the Electricity Cost Calculator breaks down exactly what a pump running 8 hours a day costs at your local kWh rate, which is usually a bigger annual expense than the water itself.
How Much Chlorine Does Your Pool Actually Need?
This is where getting the volume wrong actually matters for safety, not just cost. Pool chemicals are dosed per unit of water volume — every instruction on a bag of shock or a jug of liquid chlorine assumes you know the gallons.
Shock treatment is a large one-time dose of chlorine used to rapidly raise free chlorine and break down chloramines (the compound that causes the "chlorine smell" and eye irritation, which is actually a sign of too little free chlorine, not too much). The widely used rule of thumb from pool industry guidance is:
For the 19,150-gallon pool: 19,150 ÷ 10,000 × 1 = ~1.9 lbs of shock, rounded up to 2 lbs since most shock is sold in pre-measured 1 lb bags.
For routine maintenance rather than shocking, a commonly used dosing rule is that about 13 fluid ounces of 12.5%-strength liquid chlorine raises free chlorine by 1 ppm per 10,000 gallons of water. To raise the same 19,150-gallon pool's free chlorine by 2 ppm (a typical weekly top-up target in a range of 1–3 ppm recommended by the CDC's Model Aquatic Health Code):
- (19,150 ÷ 10,000) × 13 oz × 2 ppm = ~49.8 oz, or just under 1.5 gallons of liquid chlorine
| Task | Formula | 19,150-gal Example |
|---|---|---|
| Shock treatment | Volume ÷ 10,000 × 1 lb | ~1.9 lbs |
| Raise FC by 1 ppm | Volume ÷ 10,000 × 13 oz (12.5%) | ~24.9 oz |
| Raise FC by 2 ppm | Volume ÷ 10,000 × 13 oz × 2 | ~49.8 oz |
💡 Pro Tip
Test your water first — a $10 liquid test kit or digital reader tells you your actual free chlorine level before you dose, so you're topping up to the target range instead of guessing blind. Dosing math only works if you know your starting point, not just your pool's volume.
The World Health Organization recommends a free chlorine residual of at least 1 mg/L (ppm) in swimming pools, with most public health guidance capping routine levels around 3–5 ppm to balance disinfection against skin and eye irritation. Under-dosing a pool because you underestimated its volume is a real water-safety issue, not just a maintenance inconvenience — which is exactly why getting the gallons right comes before every other number.
Frequently Asked Questions
How do I find my pool's average depth if it slopes?
Measure the shallow end and the deep end separately, add them together, and divide by two. If your pool has a uniform depth throughout — common in most above-ground pools — just use that single measurement for both fields.
How many gallons is a typical backyard pool?
A medium rectangular in-ground pool around 32 by 16 feet with an average depth of about 5 feet holds roughly 19,000 gallons. Smaller above-ground pools typically hold 5,000 to 15,000 gallons, while large in-ground pools can exceed 30,000 gallons.
Is it cheaper to fill a pool with a hose or have it delivered by truck?
A garden hose using municipal water is usually cheaper per gallon than a water delivery truck, but the truck fills far faster (often under an hour versus over a day) and can be worth the premium during drought restrictions or if your municipal pressure is too low to fill in a reasonable time.
Why does correct pool volume matter so much for chemicals?
Every chemical dosing instruction — shock, liquid chlorine, pH balancers, algaecide — is written per gallon or per 10,000 gallons of water. Underestimating your pool's volume leads to under-treated water that can grow algae or harbor bacteria; overestimating wastes money on chemicals you didn't need to add.
Does pool shape (oval vs rectangular vs round) change how I calculate volume?
Yes — each shape uses a different area formula before multiplying by average depth. Rectangular pools use length × width, round pools use π × radius², and oval pools use π × half-length × half-width. Using the wrong formula for your pool's actual shape is one of the most common estimation mistakes.
Try It Yourself
Every downstream number — fill time, water cost, chemical dosing — starts with knowing your pool's actual volume, not a guess based on how big it looks.
Use the Pool Volume Calculator to get your exact gallons or litres for rectangular, circular, or oval pools, plus fill-time and chlorine shock estimates.
Also worth checking:
- Electricity Cost Calculator — see what running your pool pump and filter actually adds to your bill
- Water Footprint Calculator — see how a pool fill compares to your household's broader water use
- Volume Converter — convert between gallons, litres, and cubic metres for any product's dosing instructions
