If you’re in the market for a new inground pool or trying to manage the maintenance of your current one, one of the most frequently asked questions is: How many gallons is a 20×40 pool? This seemingly straightforward question involves several key variables, from pool depth to overall shape. Knowing the volume of your pool is not just a matter of curiosity—it directly impacts everything from chemical balance to heating and maintenance scheduling.
In this article, we’ll explore different types of 20×40 pools, provide accurate formulas for determining their volumes, and explain why understanding your pool’s gallon capacity is essential for optimal pool ownership.
Understanding Pool Dimensions: A Breakdown
A 20×40 pool refers to a rectangular pool that is 20 feet wide and 40 feet long. This is a popular size for residential inground pools, offering a generous space for swimming, lounging, and water activities. However, the third dimension—depth—is where the variation lies.
Why Depth Matters
Most residential pools don’t have uniform depth across the entire floor. They often feature a shallow end (typically around 3–4 feet) and a deep end that can go anywhere from 5 feet to 8 feet or even more. Unless the pool is designed as a “flat bottom” or has a single depth throughout, the calculation changes significantly.
Common Pool Depths
While every pool varies, here are some standard pool depth configurations commonly used in American backyard inground pools:
- Shallow: 3 feet
- Moderate: 4.5 feet (average depth for a 20×40 pool)
- Deep: 6 feet or more
Knowing the depth (or average depth) of your 20×40 pool is key to accurately calculating how many gallons it holds.
Calculating the Volume of a 20×40 Pool in Gallons
To calculate the volume of your pool in gallons, you’ll need to account for:
- Length
- Width
- Depth
- The formula for the shape of your pool (rectangular, oval, etc.)
Basic Formula for Rectangular Pools
Since a 20×40 pool is rectangular in shape, the formula for finding the volume is:
Volume (gallons) = Length (ft) x Width (ft) x Average Depth (ft) x 7.48
The number 7.48 is the conversion factor from cubic feet to gallons. One cubic foot of water equals approximately 7.48 gallons.
Step-by-Step Calculation
Let’s use an average depth of 5.5 feet as an example:
| Length (ft) | Width (ft) | Average Depth (ft) | Multiplication Factor |
|---|---|---|---|
| 40 | 20 | 5.5 | 7.48 |
Putting it all together:
- Multiply length by width: 40 x 20 = 800
- Multiply the result by average depth: 800 x 5.5 = 4,400
- Multiply by 7.48: 4,400 x 7.48 = 32,912
So, a 20×40 pool with an average depth of 5.5 feet holds approximately 32,912 gallons of water.
Variations Based on Depth Changes
Let’s look at how the volume changes depending on different average depths:
| Average Depth (ft) | Total Gallons |
|---|---|
| 4 | 23,936 |
| 5 | 29,920 |
| 6 | 35,904 |
| 7 | 41,888 |
This illustrates why knowing your specific pool’s depth is crucial—it can significantly change the volume you’re dealing with, especially if your pool has a slope from a 3-foot shallow to an 8-foot deep zone.
Understanding Average Depth vs. Total Depth
Sometimes pool owners attempt to calculate volume by simply averaging the shallow and deep ends.
How to Calculate Average Depth
For a typical sloped-end pool, the formula is:
Average Depth = (Shallow End Depth + Deep End Depth) / 2
For example, if your pool is 3 feet at the shallow end and 8 feet at the deep end:
(3 + 8) / 2 = 5.5 feet average depth, which fits into our earlier calculations.
Realistic Examples
Here are a few depth combinations you might find and the corresponding average depth:
- 3’ + 6’ = 4.5’ average
- 4’ + 6’ = 5’ average
- 3’ + 7’ = 5’ average
Each of these would give different gallon totals. Therefore, the exact gallonage must be calculated by measuring your pool’s actual depths rather than assuming generic values.
Why Accurate Pool Volume Matters
Knowing the number of gallons in your pool might seem trivial, but the implications are significant.
Chemical Treatment Accuracy is Critical
Pool chemicals such as chlorine, algaecides, and pH balancers work based on the volume of water. A miscalculated volume can lead to chemical imbalance, which can result in:
- Green or cloudy water
- Eye and skin irritation for swimmers
- Damage to pool equipment and surfaces
Whether you’re balancing alkalinity, adding shocking treatments, or stabilizing pH, the dosage information on pool chemical products is usually based on per 10,000 gallons—so knowing your pool’s capacity is step one.
Time and Energy for Filling and Heating
When filling your pool initially, or refilling after a winterization process or repair, you’ll want to estimate how long it will take and understand the cost involved. Residential water flow varies, but many estimates place it at:
50 gallons per minute for a typical outdoor hose spigot
Using that figure:
- 32,912 g / 50 gpm = 658 minutes = about 11 hours
That’s a full day of continuous water flow, and a large water bill. Similarly, if you’re installing a pool heater, the BTU requirement depends on water volume, so you’ll need the exact gallonage to select the proper equipment.
Pool Maintenance and Equipment Sizing
Pump and filter efficiency is tied to pool volume. Proper filtration cycles require knowing how many gallons the system must turnover each day. Most guidelines suggest:
Running the pump until the water turns over at least once in a 24-hour period
Without knowing your pool’s volume, you can’t calculate turnover rate or determine how long you should run the pump.
Common Pool Shapes and Volume Differences
While 20×40 pools are usually rectangular, it’s worth mentioning variations from oval and free-form shapes. Technically, “20×40” can describe any pool where the dimensions of width and length measure 20 and 40 feet, regardless of the perimeter edge shape.
Rectangular vs. Oval Comparisons
A 20×40 oval pool has rounded sides and will hold fewer gallons than a rectangular pool of the same outer dimensions because it uses less surface area.
To get a clear sense:
- A round pool uses this formula:
π x radius² x average depth x 7.48
whereas the volume of an oval includes the width and length for surface area and uses the same average depth formula as a rectangle, but divides by 4.
In short, unless your 20×40 pool is oval, you can safely use the rectangular formula.
Tools to Help Calculate Pool Volume
Luckily, you don’t have to do all this math manually.
Online Pool Volume Calculators
Many websites offer easy-to-use pool volume calculators where you can input:
- Length
- Width
- Minimum and maximum depths
- Pool shape
The calculator estimates your pool’s total water volume. However, it’s still best to know the math behind it for accuracy and troubleshooting.
Smart Pool Gauges and Sensors
New smart technology includes pool sensors that not only monitor chemical levels but can also use pre-programmed volume data to make automated dosages or filtration recommendations. These systems usually require manual input of total gallons, so it’s essential to be precise on initial setup.
Impact on Water Costs and Conservation
In regions experiencing drought or water scarcity, pool owners are under pressure to be responsible stewards. Knowing how many gallons are in your pool can help you make better water usage decisions.
Cost Estimation per Fill
The cost of filling a 32,912 gallon pool depends on your local water utility rate. For example, in some U.S. cities, water costs might be:
- $.003 per gallon → $98.74
- $.005 per gallon → $164.56
- $.007 per gallon → $230.38
If you’re considering adding a hot tub, installing water-saving pool covers, or looking into drought-resistant landscaping, knowing the gallon load of your pool gives you useful data to factor into those decisions.
Regular Top-Offs, Evaporation, and Water Loss
Even after a pool is filled, water evaporates, especially in warm climates. The U.S. Department of Energy estimates:
Outdoors, pools can lose more than 0.25 inches of water per day due to evaporation
For a 20×40 pool, this translates into roughly 150–250 gallons per day, depending on exposure, temperature, and wind. By knowing your volume, you can better estimate these ongoing costs and how they impact your water bill and sustainability goals.
Practical Applications: Pool Contractors and Chemical Retailers
When talking to professionals—whether pool technicians, chemical dealers, or service technicians—having an accurate gallonage ready can speed up purchases, repairs, and maintenance services.
Purchasing Pool Chemicals
If you’re buying pool chemicals online or at a hardware store, products will often ask for the square footage of your pool. However, gallonage is more relevant for effectiveness, especially when it comes to:
- pH adjusters
- Stabilizers (Cyanuric Acid)
- Clarifiers
- Algaecides
Bringing the specific gallon capacity helps ensure you’re not under-dosing or over-dosing, making your chemical budget more efficient and your pool more swimmer-friendly.
Professional Pool Maintenance Services
Companies offering maintenance services from weekly tech visits to seasonal opening or closing will often require you to provide pool volume. Their treatments and time estimates are based on these numbers, which help them:
- Estimate labor hours
- Deliver accurate chemical blending
- Track chemical consumption during months of operation
If you provide incorrect pool gallonage, both you and the technician may end up surprised by the results from the pool water tests.
FAQ: Common Misconceptions About Pool Volume
Sometimes pool owners assume their pool’s capacity without doing the math or measuring, which can lead to errors.
- “I know someone with the same size pool and we just copied what they do” – This might work initially, but it risks long-term problems like overchlorination or corrosion.
- “I figured based on square footage” – Square footage tells you about tiling or cover costs, but volume requires depth.
- “I measured with a tape at the edge” – While useful for perimeter or equipment placement, it cannot help with depth for volume.
It’s best to measure all dimensions or consult documentation provided at the time of installation.
Conclusion: Knowing the True Volume for Safety and Efficiency
Ultimately, knowing how many gallons a 20×40 pool contains is more than just numbers on a page. It’s an essential figure for effective pool ownership, influencing everything from your safety and enjoyment to the financial and environmental cost.
Whether you’re maintaining an existing pool, filling one for the first time, or planning for construction, performing an accurate volume calculation ensures:
- Your chemical treatments will be effective
- Your heating and filtration systems will run efficiently
- Your water usage and associated costs are better understood
Taking time to understand and record your exact volume pays dividends for years of safe, clean, and enjoyable swimming. And for pool owners, it’s as simple as getting your tape measure out, plugging a few key measurements into the volume formula, and getting back a precise number.
And for a 20×40 pool with an average depth of 5 feet? You’re looking at around 29,920 gallons. Add a little slope, and you’ll easily surpass 32,000 gallons.
So next time someone asks: How many gallons is a 20×40 pool?—don’t just throw out a guess. You’re now equipped to answer with the precision it deserves.
How many gallons does a 20×40 pool hold?
A 20×40 pool’s total water volume depends heavily on its average depth. For a typical rectangular pool of these dimensions with an average depth of 5 feet, the total volume can be calculated by multiplying length × width × average depth × 7.5 (the number of gallons in a cubic foot of water). In this case, the calculation would be 20 × 40 × 5 × 7.5, which equals 30,000 gallons.
However, if the pool has a variable depth—such as a shallow end of 3 feet and a deep end of 8 feet—the average depth would be calculated differently, which affects the overall volume. Pool shapes that aren’t perfectly rectangular, such as L-shaped or irregular designs, may also influence the gallon count. It’s important to precisely determine your pool’s dimensions before adding chemicals or planning maintenance.
How do I calculate the volume of my 20×40 pool?
To calculate the volume of your 20×40 pool, first measure the length and width, which are typically constant for this type of pool. Next, determine the average depth, especially if your pool has a slope from shallow to deep end. To find the average, add the shallow depth and the deep depth together, then divide by two. Once you have these numbers, multiply length × width × average depth × 7.5.
For example, a 20×40 pool with a shallow end of 3 feet and a deep end of 7 feet has an average depth of (3 + 7) ÷ 2 = 5 feet. Multiplying 20 × 40 × 5 × 7.5 would yield 30,000 gallons. If your pool is rounded or oval-shaped, you’ll need to adjust the formula accordingly, using π (pi) for accurate calculations. Accuracy in measurement is key to ensuring proper chemical levels and maintenance.
What factors affect the gallon count of a 20×40 pool?
Several factors affect the total gallon count of a 20×40 pool. The most significant is the overall depth of the pool; pools with a shallow end and a graduated slope to a deeper end will hold more water than those with a uniform depth. Variations in design, such as the presence of steps, benches, or built-in spas, also impact the total volume since these structures displace water.
Additionally, the exact dimensions of the pool can vary depending on the builder or installation method. While 20 feet by 40 feet gives a standard square footage, small differences in rounding or construction can lead to volume variances. Also, a kidney-shaped or curved pool of similar footprint will hold fewer gallons than a rectangular one. All these factors should be considered when estimating chemical use or determining the capacity needed for replacement or repairs.
How does the pool’s shape influence the total water volume?
The shape of a pool plays a key role in determining its water-holding capacity. A rectangular 20×40 pool is straightforward to calculate using standard formula techniques and offers the maximum volume for its footprint. In contrast, a 20×40 pool with an irregular shape, such as a kidney or L-shape, will have a lower volume, as corners and curves reduce usable water space.
Additionally, unique or custom-built shapes—like freeform, oval, or figure-eight designs—can further change the cubic feet and therefore the total gallons. These pools tend to have less predictable volumes, and manual or approximate measurement methods may be required. If your pool has an unusual shape, professional assessment or the use of water volume calculators specific to those designs may be necessary for accurate volume estimation.
What is the importance of knowing the exact gallon count in a pool?
Knowing the exact gallon count of your pool is crucial for maintaining a safe and clean environment. Proper chemical balance depends on accurate water volume; adding too much or too little can lead to unsafe swim conditions, equipment damage, or discomfort for swimmers. The effectiveness of sanitizing agents like chlorine and pH adjusters depends on being dosed correctly according to volume.
Additionally, understanding your pool’s capacity is important when selecting equipment like pumps, filters, heaters, and saltwater chlorinators, all of which are rated for specific flow rates and volumes. Miscalculating the pool’s size can lead to improper equipment installation or system overwork, which can shorten its lifespan and increase energy costs. Accurate volume knowledge supports both maintenance efficiency and optimal swimming enjoyment.
How does pool depth variation affect the required amount of chemicals?
Pool depth variation affects chemical dosage because more water volume requires a proportional adjustment in the amount of chemicals used for treatment. For example, a 20×40 pool with an average depth of 6 feet holds significantly more water—about 36,000 gallons—than the same pool with an average depth of 4 feet, which contains around 24,000 gallons. The deeper pool will need about 50% more chlorine or algaecide to achieve the same chemical balance as the shallower version.
This becomes especially important when shocking the pool or adjusting pH, alkalinity, or calcium hardness levels. Inaccurate estimates can lead to ineffective treatments or chemical imbalances that cause issues like cloudy water, equipment corrosion, or swimmer discomfort. Using the correct volume ensures precise dosing, which in turn leads to better water quality and more effective long-term pool maintenance.
Can a pool’s water volume affect heating and maintenance costs?
Yes, a pool’s water volume directly impacts both heating and maintenance costs. Larger volumes require more energy to heat, and the time it takes to reach a desired temperature increases accordingly. For example, heating a 30,000-gallon 20×40 pool will cost more than heating a similar pool with 20,000 gallons. The type of heater—gas, electric heat pump, or solar—will influence the rate of heat provided, but efficiency always varies with pool size.
Additionally, a higher water volume affects circulation and filtration efficiency. Larger volumes may require stronger pumps and longer run times to ensure proper water turnover and debris removal. This increases both wear on the equipment and energy use. Understanding your pool’s exact volume helps in selecting appropriate heating and filtration solutions, optimizing efficiency, and reducing overall operating costs over time.