Pool & spa
Pool Water Calculator
Works out how much water a pool holds and what filling or topping it up involves.
Result
Evaporation varies enormously with climate, wind, whether the pool is heated and whether it is covered. A cover cuts it by half or more. Losing noticeably more than this consistently usually means a leak rather than weather.
Get a Free Business AuditHow the calculation works
Volume in cubic feet times 7.48 for gallons, from the same geometry as any volume calculation. The practical use here is water rather than chemistry: how long a fill takes at a given flow rate, what it costs at your water rate, and how much evaporation and splash-out replaces over a season. Fill time is gallons divided by the supply flow in gallons per minute, which for a garden hose is considerably slower than people expect on a large pool.
What moves the number
- Volume, which sets both fill time and cost.
- Supply flow rate, since a hose fill on a large pool takes many hours.
- Water rate locally, which varies substantially and may include sewer charges.
- Evaporation rate, which depends on climate, wind and whether a cover is used.
Where this one goes wrong
Check whether your water utility charges sewer on water that never enters the sewer, because many bill sewer volumetrically against metered water, and filling a pool can produce a large sewer charge for water that went into the ground and the air. Some utilities offer a pool fill adjustment or a separate meter, and asking before filling is worth real money on a large pool. On evaporation, a cover reduces it substantially and is the main lever an owner has, both on water and on heating cost, since evaporation is also the largest heat loss from a heated pool.
Before you quote
- Ask the utility about sewer charges on pool fill water before filling.
- Calculate fill time from actual supply flow, not an assumption.
- Raise covers, since they cut both evaporation and heat loss.
Where the rules come from
Questions
About this calculator
Will filling my pool show up on the sewer bill?
It can, because many utilities bill sewer volumetrically against metered water, and pool fill water never reaches the sewer. Some offer a pool fill adjustment or a separate meter. Asking before filling is worth doing, since on a large pool the sewer portion can be substantial.
How long does it take to fill a pool?
Gallons divided by your supply flow in gallons per minute. On a large pool with a garden hose that is many hours and sometimes more than a day, which is why bulk water delivery is sometimes used. Measuring your actual hose flow into a bucket gives a far better estimate than guessing.
How much water does a pool lose to evaporation?
It depends on climate, wind, humidity and surface temperature, and it is more than most owners expect, especially on a heated pool. A cover is the main control and reduces it substantially. Evaporation is also the largest heat loss from a heated pool, so a cover saves water and energy together.
The formula
hours = gallons / GPM / 60
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How to use this calculator
- Pool volume (gallons). Enter your own figure; the page starts at 20,000.
- Fill rate (GPM). A garden hose is roughly 8 to 12 GPM.
- Water cost per 1,000 gallons ($). Enter your own figure; the page starts at $4.5.
- Evaporation and splash-out per month (%). Enter your own figure; the page starts at 4%.
The result updates as you type, so you can try a figure, see what it does to time to fill, and change it back. Nothing you enter is sent anywhere or saved.
A worked example
These are the figures the calculator opens with. Every number in the results column below is produced by the same calculation the page runs, so you can follow it through before putting your own figures in.
What goes in
| Input | Value |
|---|---|
| Pool volume (gallons) | 20,000 |
| Fill rate (GPM) | 9 |
| Water cost per 1,000 gallons | $4.5 |
| Evaporation and splash-out per month | 4% |
What comes out
| Result | Value |
|---|---|
| Time to fill main result | 37.0 |
| Cost to fill | $90 |
| Top-up water per month | 800 |
| Top-up cost per month | $3.60 |
About 37.0 hours to fill, costing $90. Evaporation and splash-out then take roughly 800 gallons a month.
How sensitive the answer is
Holding everything else steady and changing only water cost per 1,000 gallons, this is what happens to time to fill. It is worth knowing which of your inputs the answer actually hinges on, and which barely move it.
| Water cost per 1,000 gallons ($) | Time to fill |
|---|---|
| $2.25 | 37.0 |
| $3.38 | 37.0 |
| $4.5 example above | 37.0 |
| $6.75 | 37.0 |
| $9 | 37.0 |
Every other input is held at the example value while this one changes, which is not how a real job behaves. Treat it as a guide to which number matters most, not as five separate quotes.
What each figure means
- Pool volume (gallons)
- A number, between 0 and 2000000, starting at 20,000.
- Fill rate (GPM)
- A garden hose is roughly 8 to 12 GPM.
- Water cost per 1,000 gallons ($)
- A money amount, starting at $4.5.
- Evaporation and splash-out per month (%)
- A percentage, between 0 and 40, starting at 4%.
What the results tell you
| Result | What it is |
|---|---|
| Time to fill | The headline result. A decimal figure. |
| Cost to fill | A money figure. |
| Top-up water per month | A number. |
| Top-up cost per month | A money figure. |
Estimating is the easy part
The jobs you never hear about cost more than the ones you mis-measure. BDEVY builds the marketing, systems and automation that bring them in.