Figuring out how many gallons of water are in your swimming pool is an important part of maintaining it properly. By calculating the volume, you’ll know how much water to add when filling it up or refilling it after cleaning. The volume will also tell you how much pool chemicals to use. Fortunately, with some basic measurements of your pool, calculating gallons is straightforward.
Quick Answers
Here are some quick answers to common questions about calculating gallons of water in a pool:
How do I measure my pool to figure out gallons?
You’ll need to measure the length, width, and average depth of your pool in feet to calculate its volume in gallons. Length and width are measured at the water line. For depth, measure at the deepest point and shallowest point then average them.
What shape pool do gallon calculations work for?
The pool gallon formula works for any regular shape – rectangle, circle, oval, etc. Irregularly shaped pools may need to be broken into sections for which you’ll calculate the gallons separately.
Do I calculate using inside pool dimensions or outside?
Always use the inner dimensions – length, width, and depth measurements should be taken from the inside walls of the pool. This gives the volume of water the pool holds.
What’s the formula for pool gallons?
Length x Width x Average Depth x 7.5 = Gallons
What does the 7.5 represent in the formula?
The 7.5 converts the pool’s volume in cubic feet to gallons. It’s the number of gallons in one cubic foot.
Step 1: Measure Length and Width
Determining the length and width of your pool is straightforward. You’ll want to measure the maximum length and width at the water line when the pool is full. Use a tape measure and measure in feet if possible for easiest calculations.
For rectangular pools, length and width are simple. Just measure the two opposing sides. For oval or irregularly shaped pools, you may need to measure a few different spots and average them. Write down the length and width measurements to use in your calculations.
Step 2: Find Average Depth
Depth can vary in different areas of the pool, so you’ll need to find the average. To do this:
- Measure the deepest spot of the pool and note the depth.
- Find the shallowest spot and measure depth there.
- Add the deep end and shallow end measurements together.
- Divide the sum by 2 to get the average.
For example, if the deep end is 6 feet and shallow end is 4 feet:
6 + 4 = 10
10 / 2 = 5
The average depth would be 5 feet.
Step 3: Insert Measurements into Formula
Now you’re ready to calculate the volume. Use this formula:
Length x Width x Average Depth x 7.5 = Gallons
Insert your measurements in feet into the appropriate spots in the formula and multiply it out to find the number of gallons.
For example, for a pool that is 20 feet long, 10 feet wide, and averages 5 feet deep:
20 x 10 x 5 = 1000 cubic feet
1000 cubic feet x 7.5 = 7500 gallons
Gallons Formula as a Table
| Length | 20 feet |
| Width | 10 feet |
| Average Depth | 5 feet |
| Volume in Cubic Feet | Length x Width x Depth = 1000 ft3 |
| Gallons | Cubic Feet x 7.5 = 7500 gallons |
Double Check Using an Online Calculator
To confirm your calculations are correct, plug your numbers into an online pool gallons calculator. There are many free calculators available that will do the math for you.
Match the online calculator result to yours to verify accuracy. If they’re different, re-measure your pool and go through the steps again to recalculate.
Factors that Change Pool Volume
While measurements may give the original volume of a pool, there are some factors that can change the actual amount of water over time:
- Splashing/Spilling – Water splashed or spilled out of the pool will reduce the volume. During busy pool seasons with lots of users, the level can drop noticeably.
- Rain – In outdoor pools, rainfall directly onto the water surface increases volume.
- Drainage – Small amounts of water are lost periodically due to drainage from skimmers, main drains, and auto-fill valves if equipped.
- Evaporation – Over time, water evaporates from the pool surface into the air, decreasing volume.
- Objects – Large floating objects or pool toys displace water, taking up space and reducing the volume.
For these reasons, periodically re-measuring and recalculating is a good idea get an updated gallons figure. It’s also important when refilling the pool after cleaning or adding chemicals to ensure correct water levels.
Tips for Measuring Pool Volume
Getting accurate measurements is crucial for figuring out pool gallons correctly. Here are some tips:
- Use a steel or fiberglass tape measure for best accuracy and to avoid issues with pool chemicals degrading the tape.
- For length and width, measure from the water line on opposing finished surfaces. Don’t measure from the coping.
- For depth, tie a weight to the end of the tape so it hangs vertically for a precise depth reading.
- Measure depth in several spots – take 3-5 readings and average for most accurate results.
- Recalculate periodically or after refilling to account for changes in volume over time.
- Have someone double check your measurements for a second opinion.
Common Pool Sizes and Gallon Amounts
Here are some typical measurements for common residential pool sizes along with the approximate volume in gallons:
| Pool Size | Length x Width | Average Depth | Gallons |
|---|---|---|---|
| Small | 16′ x 32′ | 4′ | 24,000 |
| Medium | 20′ x 40′ | 5′ | 45,000 |
| Large | 24′ x 40′ | 6′ | 72,000 |
These figures give a rough estimate for comparison. The measurements of your specific pool may vary.
Calculating Partially Filled Pool Volumes
The pool gallon formula gives the total volume when filled. But what about a partially filled pool with a lower water level? You can still find the volume by adjusting the depth measurement:
- Measure the water depth at the deep end and shallow end as usual.
- Find the difference between the full depth and current water depth at each end.
- Subtract the depth differences from the original average depth.
- Use the adjusted average depth in the gallon formula.
For example, for a pool that is normally 4 feet deep at shallow end and 8 feet deep at deep end. The average full depth is 6 feet. If the water is now 3 feet deep in shallow end and 7 feet deep at the deep end:
- Full shallow end depth: 4 feet
- Current shallow end depth: 3 feet
- Difference: 4 – 3 = 1 foot
- Full deep end depth: 8 feet
- Current deep end depth: 7 feet
- Difference: 8 – 7 = 1 foot
Average full depth was 6 feet.
The difference at each end was 1 foot.
So the adjusted average depth for calculating partial volume is: 6 – 1 = 5 feet
Use 5 feet as the depth in the gallon formula along with the full length and width to find volume of the partially filled pool.
Calculating Volume Lost from Leaks
If your pool has developed a leak, calculating the volume of water lost over time can give a sense of the severity of the leak to guide repairs. Here’s how to estimate volume lost:
- Determine the current volume using the partially filled pool method.
- Determine the last time the pool was completely filled and the volume at that time.
- Subtract the current volume from the full volume.
For example:
- Volume when last filled to the top: 30,000 gallons
- Current volume with lower water level: 27,500 gallons
- Volume lost: 30,000 – 27,500 = 2,500 gallons
In this case, 2,500 gallons have leaked out since the last complete fill. Knowing the time period over which this leak occurred will tell you the rate of loss to assess the leak damage.
Importance of Measuring Pool Volume
Getting an accurate pool volume in gallons is important for several reasons:
- Chemical Dosing – Most pool chemical directions are based on volume. Wrong volume means incorrect chemical amounts.
- Water Balance – Proper filtration, circulation, and water replacement is facilitated by accurate water volume.
- Safety – Ensuring correct water levels for safe use requires maintaining volume.
- Heating Efficiency – Proper heating depends on having the expected amount of water to heat.
- Measuring Water Loss – You need to know starting volume to determine leakage amounts.
Take the time to carefully measure and recalculate your pool’s gallons regularly. Your pool maintenance and swimmer safety will benefit from getting it right.
Conclusion
Figuring out your pool’s volume in gallons is easy with some carefully taken measurements and a simple formula. Measure length, width, and average depth in feet, then multiply them together and by 7.5 to convert to gallons. Recalculating periodically accounts for changes in water level over time. An accurate pool volume supports proper maintenance, chemical dosing, and safety. Take a few minutes to measure and calculate – knowing your pool’s gallons makes a big difference.
