To calculate pool gallons, find area × average depth × 7.48 for water volume in US gallons.
Knowing the water volume keeps dosing, heating, and turnover on target. You’ll measure the pool, find the area by shape, use average depth, and convert cubic feet to gallons using 7.48. This walkthrough gives plain formulas, quick checks, and a few tested tricks so you can stop guessing and set chemicals with confidence.
Quick Formulas By Pool Shape
Pick the row that matches your pool. Use feet for every measurement. For irregular layouts, split the shell into simple shapes and add the results.
| Shape | Area Or Setup | Gallons Formula |
|---|---|---|
| Rectangle / Square | Area = length × width | length × width × average depth × 7.48 |
| Circle | Area = π × radius² | π × radius² × average depth × 7.48 |
| Oval | Area = π × (major/2) × (minor/2) | π × (A/2) × (B/2) × average depth × 7.48 |
| Kidney | Area ≈ π × (large+small)/4 × length/2 | approx. area × average depth × 7.48 |
| Freeform | Split into rectangles/arcs | sum of each shape × 7.48 |
| Lap Lane (Trapezoid) | Area = length × (width1+width2)/2 | length × (w1+w2)/2 × avg depth × 7.48 |
| Spa / Round Hot Tub | Area = π × radius² | π × radius² × water depth × 7.48 |
| Shallow Ledge | Treat ledge as a thin rectangle | ledge area × ledge depth × 7.48 |
How To Calculate Pool Gallons Step By Step
1) Measure Length, Width, And Depths
Run a tape along the longest length and the widest width at waterline. Note the shallow end depth and the deepest water depth. Skip coping thickness. For kidney or freeform shells, take two widths: large and small. Write everything in feet to avoid mix-ups.
2) Find Average Depth
Average depth is the midpoint between shallow and deep. Use (shallow + deep) ÷ 2. A pool that ranges from 3 ft to 8 ft has an average depth of 5.5 ft. Benches and tanning ledges count too; include them if they hold water.
3) Compute Surface Area
Use the right formula for the shape. Rectangle uses length × width. Circle uses π × radius². Oval uses π × (A/2) × (B/2). For a freeform shell, split into two or three simple zones and add the areas. Close counts; you’re estimating volume for chemistry and equipment sizing.
4) Convert Cubic Feet To Gallons
Multiply area × average depth to get cubic feet. Then multiply by 7.48 to convert to US gallons. That constant comes from the relationship between gallons and cubic feet published by the U.S. Geological Survey. If you work in meters, convert to feet or switch the constant to liters (1 cubic meter = 1,000 liters).
5) Sanity-Check With A Second Method
If the shell has odd curves, cross-check with a calculator from a leading equipment brand. When two methods land within a few percent, you’re set. If not, remeasure widths and the deepest spot.
Worked Examples You Can Copy
Rectangle, Gradual Slope
Size: 30 × 15 ft, depths 3.5 ft to 6.5 ft. Average depth = (3.5 + 6.5)/2 = 5.0 ft. Area = 30 × 15 = 450 ft². Cubic feet = 450 × 5.0 = 2,250. Gallons = 2,250 × 7.48 ≈ 16,830.
Round Above-Ground Pool
Size: 18 ft diameter, water depth 4 ft. Radius = 9 ft. Area = π × 9² ≈ 254.5 ft². Cubic feet ≈ 254.5 × 4 = 1,018. Gallons ≈ 1,018 × 7.48 ≈ 7,620.
Oval In-Ground With Hump
Size: 34 × 18 ft, depths 3 ft to 7 ft. Average depth = 5 ft. Area = π × (34/2) × (18/2) ≈ π × 17 × 9 ≈ 480.7 ft². Cubic feet ≈ 480.7 × 5 = 2,403.5. Gallons ≈ 2,403.5 × 7.48 ≈ 17,970.
Kidney Shape, Two Widths
Length 36 ft. Large width 18 ft. Small width 12 ft. Average depth 5 ft. Approximate area ≈ π × (18 + 12)/4 × 36/2 ≈ π × 7.5 × 18 ≈ 424.1 ft². Cubic feet ≈ 424.1 × 5 = 2,120.5. Gallons ≈ 2,120.5 × 7.48 ≈ 15,850.
Calculating Pool Gallons For Any Shape
Every pool can be reduced to rectangles, triangles, and circles. That’s the trick for ledges, benches, and angled walls. Add a line sketch on paper, label each zone, and write its area and depth. Add the cubic feet numbers, then apply 7.48. It’s quick, repeatable, and tidy in a logbook.
Why Gallons Matter For Daily Care
Everything in pool care scales with water volume. Doses for chlorine, acid, and stabilizer are labeled in ounces or pounds per 10,000 gallons. Heater sizing, pump run time, turnover rate, even winterizing plugs all tie back to the number you just computed. With the right volume, you can hit targets without wasting chemicals.
Common Pitfalls And Easy Fixes
Mixing Units
Feet for length and width, inches for depth—this mismatch is the top source of errors. Keep a single unit system from start to end.
Wrong Average Depth
Don’t use the deep end alone. A shallow-to-deep slope needs the midpoint. If the pool has a hopper, divide the floor into two zones and average each zone, then add the volumes.
Freeform Guesswork
Curvy shells can be simple. Break them into two rectangles and a half circle. Measure each, compute gallons, then add them. If a tanning ledge holds only six inches of water, include 0.5 ft as depth for that area.
Units And Conversions That Matter
The conversion 1 cubic foot = 7.48 gallons is standard in water math and documented by the USGS conversion factors. If you measure in yards, multiply cubic yards by 201.97 to get gallons. If you measure in inches, convert to feet before any area or depth math. Skip metric-to-imperial midstream; sticking to one system keeps the final number consistent.
From Fill Time To Gallons
Hose timing can back-solve volume without geometry. First, time how long it takes to fill a 5-gallon bucket. Say it takes 40 seconds: that’s 7.5 gallons per minute. Next, time a fill from the tile line at skimmer level to an inch above. If that takes 65 minutes, add 65 × 7.5 = 487.5 gallons. Note the waterline rise—say it’s 1 inch—so the surface area equals 487.5 gallons ÷ (7.48 × 1/12). That gives you a surface area estimate, which you can multiply by average depth to confirm total gallons.
Turnover And Equipment Checks
Once you know the gallons, you can check turnover. Divide total gallons by pump flow (gallons per minute) and you’ll get minutes per full turnover. A common residential target is one to two turnovers per day. If the number looks off, either the flow estimate is high or the volume estimate needs a tweak. A simple clamp-on flow meter or a pump curve chart can refine the math.
How to Calculate Pool Gallons With Confidence
If you’re learning how to calculate pool gallons for the first time, run one hand calculation and one brand calculator. Keep both results in your logbook. Next spring, measure waterline depth, repeat the math, and compare. Small differences come from splash-out, backwashing, or minor settling.
Gallons Per Square Foot Quick Chart
This chart lets you skip a step. Multiply surface area by the number in the middle column to jump straight to gallons.
| Average Depth (ft) | Gallons Per Sq Ft | When It Helps |
|---|---|---|
| 1.0 | 7.48 | Sun shelves, shallow ledges |
| 1.5 | 11.22 | Play ledges, baja steps |
| 2.0 | 14.96 | Spas and kiddie zones |
| 3.0 | 22.44 | Sports pools, shallow halves |
| 4.0 | 29.92 | Round above-ground pools |
| 5.0 | 37.40 | Typical deep-end average |
| 6.0 | 44.88 | Diving wells and hoppers |
| 8.0 | 59.84 | Competition or deep training |
Verification And Small Tweaks
Bucket Test Cross-Check
After a refill or a big drain, time how long it takes your hose to fill a known bucket. Then time a top-off from tile line A to B. Convert minutes to gallons. If the top-off adds 500 gallons and raised the water 1 inch, you can back-solve pool surface area to confirm your sizing.
Backyard Reality Adjustments
Vinyl liners bow a bit. Plaster shells have rounded coves. These details shave or add a few percent. Most dosing charts tolerate that spread. If metal stains or algae keep returning, remeasure and adjust the base volume by 5% up or down and watch results for a week.
References And Helpful Tools
The constant 7.48 gallons per cubic foot comes from the USGS conversion factors. For a fast double-check, try the Pentair pool volume calculator. Both align with the math in this guide.
FAQ-Free Tips That Save Time
Keep A Volume Note In Your Test Kit
Write the final gallon figure on a weatherproof label. Stick it inside the lid of your kit so doses are quick during storms or parties.
Store Two Numbers
Record total gallons and gallons per inch of water height. The second number helps during backwashing or rain dilution after a big storm.
Use The Same Tape Every Season
Different tapes can disagree by a hair. Reusing one tape keeps repeat checks consistent, useful when tracking heater performance or run time changes.
With sound measurements and the simple 7.48 constant, you now own the number that anchors dosing, heating, and turnover. That’s the heart of how to calculate pool gallons and keep the water dialed in.