What PoolBoss Says
Multiply length by width by average depth, then by 7.5 for a rectangular pool, or 5.9 for a round or oval one. The number most people get wrong is average depth: on a pool with a deep end, it is not the midpoint between shallow and deep unless the floor slopes evenly the whole way.
Gallonage is the denominator under every dose you pour. Every product label is quoted per 10,000 gallons, so a pool recorded at the wrong volume produces a wrong dose forever - and no amount of careful testing catches it, because the reading is right and the correction is short. It is the one number on a pool record that nobody re-checks, usually because it was copied off the last company's route sheet.
What follows is the formula by shape, the average-depth mistake that produces most bad numbers, the section method that fixes it, and what a volume error actually costs across a season of dosing.
At a glance
Key takeaways
- Length x width x average depth x 7.5 gives gallons for a rectangle; swap 7.5 for 5.9 on a round or oval pool.
- Average depth is not the midpoint between shallow and deep unless the floor slopes evenly the whole way - on a hopper-bottom pool the midpoint runs about 17% high.
- Measure a pool with a deep end in sections: constant-depth sections use their own depth, sloping sections the average of their two ends.
- Measure length and width at the water line, not the deck edge; a foot of coping on each dimension inflates a 16 by 32 pool by nearly 10%.
- A volume error passes into every dose at the same percentage and in the same direction, on every visit, until the number itself is corrected.
- A pool that never holds chlorine while the rest of the route behaves is a recorded-volume suspect before it is a water problem.
- Measure inherited pools during onboarding and write the gallons on the pool's own record - a ten-minute job done once per pool.
How do I figure out how many gallons my pool holds?
Three measurements and one multiplier. Measure the length and width at the water line, work out the average depth, multiply the three together to get cubic feet, then multiply by the gallons-per-cubic-foot figure for the pool's shape. A rectangular or square pool uses 7.5, because a cubic foot of water is about 7.48 gallons. Round and oval pools use 5.9 instead, which folds the curve of the shape into the same multiplication so you are not working out a surface area separately.
Worked through on a common size: a 16 by 32 rectangular pool with a true average depth of 4.9 feet is 16 x 32 x 4.9 = 2,509 cubic feet, and 2,509 x 7.5 puts it at about 18,800 gallons. A 21-foot round pool averaging 4 feet deep is 21 x 21 x 4 x 5.9, or about 10,400 gallons. If you would rather not do the arithmetic on the tailgate, run the numbers in the free pool volume calculator and copy the result onto the pool's record while you are standing there.
- Measure length and width at the water line, not the deck edge. Coping and bond beam can add a foot to each dimension, which inflates a 16 by 32 pool by nearly 10%.
- Work out average depth section by section whenever the floor is not a constant depth or a single even slope. This is the step that decides whether the whole number is right.
- Multiply length x width x average depth to get cubic feet, then apply the multiplier once at the end rather than to each section.
- Use 7.5 for a rectangle or square, and 5.9 for a round or oval pool, where length and width are the two diameters.
- Write the result on the pool's record the day you measure it, with the date, so the next person knows it was measured rather than inherited.
Average depth is where almost every wrong gallonage comes from
Average depth is not the midpoint between the shallow end and the deep end, and treating it as though it were is the single most common source of a bad volume. The midpoint is only correct when the floor slopes evenly from one wall to the other, which describes very few residential pools. Most have a shallow shelf that runs flat for over half the length, a short transition slope, and a small deep hopper at one end. Averaging 3.5 feet and 8 feet gives 5.75, but the pool spends most of its floor at 3.5.
On that 16 by 32 pool, the midpoint method returns 16 x 32 x 5.75 x 7.5 = 22,080 gallons. Measured properly it holds about 18,800. The midpoint runs 17% high, and every dose sized from it overshoots by that same 17% - which is how a technician ends up chasing pH back down on a pool that keeps landing past target.
The fix takes about ten minutes. Split the pool into sections along its length and treat each one on its own: a constant-depth section is simply its own depth, and a sloping section uses the average of its two end depths. Multiply each section's length x width x depth, add the cubic feet together, and convert once at the end.
- Shallow shelf: 16 ft wide x 18 ft long at a constant 3.5 ft = 1,008 cubic feet.
- Transition slope: 16 ft x 8 ft, running from 3.5 ft down to 8 ft, so 5.75 ft average = 736 cubic feet.
- Deep hopper: 16 ft x 6 ft at 8 ft = 768 cubic feet.
- Total: 2,512 cubic feet x 7.5 = about 18,800 gallons, roughly 3,200 less than the midpoint shortcut claimed.
Shapes and their multipliers
Each shape has a multiplier that converts length x width x average depth straight into gallons, and how far you can trust the answer depends on how regular the shape is. A rectangle measured carefully lands within a few percent of true. A freeform lagoon with a tanning ledge and a grotto is a guess until somebody measures it in sections.
For a kidney, measure the widest width and the narrowest width across the waist, add the two together, and use 3.38: (width A + width B) x length x average depth x 3.38. It is the standard approximation and it holds up on a normal kidney, but it degrades quickly on anything kidney-shaped with extra curves added, where sectioning is the only honest method.
The accuracy column below is worth reading as a working tolerance rather than a promise. It assumes the depths were actually measured. A regular shape with guessed depths is less accurate than an irregular shape with measured ones, which is the whole point of the previous section.
| Shape | Formula | Typical accuracy |
|---|---|---|
| Rectangle or square | Length x width x average depth x 7.5 | Within about 3% with measured depths |
| Round | Diameter x diameter x average depth x 5.9 | Within about 5% |
| Oval | Length x width x average depth x 5.9 | Within about 5% |
| Kidney | (Width A + width B) x length x average depth x 3.38 | Within about 10% |
| Freeform or lagoon | Measure in sections, add the cubic feet, then x 7.5 | Within about 10% only if sectioned |
A 20% volume error is a 20% dosing error on every visit forever
Dose rates are quoted per 10,000 gallons, so an error in the volume passes straight through to the amount poured - unchanged, in the same direction, on every visit, for as long as the number stays wrong. A pool recorded 20% low gets 20% less of everything: chlorine, acid, stabilizer, salt. The readings that follow look like a water problem, so they get treated as one, and the actual fault sits in a field nobody has looked at in years.
A technician takes over 26 pools in Goodyear and Litchfield Park, Arizona from a retiring operator. The route sheet lists one of them at 15,000 gallons. It is the 16 by 32 with the shelf and the hopper, and measured section by section it holds closer to 18,800. Three years of acid and chlorine had been sized 20% short, which is exactly why the file note says the pool 'always runs low', and why the previous owner had been adding a second stop most Augusts to hold its chlorine. Re-measuring took eleven minutes.
The tell is a pool that never quite holds on a route where the same technician's other pools behave. Before blaming the water, the fill line or the customer's swim load, check the number the doses are being sized from: sizing a dose to the pool's real volume is the first thing to rule out, not the last. It shows up most clearly on alkalinity, where a worked acid dose per 10,000 gallons is a large enough pour that a fifth of it going missing is visible in the jug.
Measure it once, write it on the pool record, and stop guessing
Volume is a measure-once number. A pool's gallonage does not change unless it is resurfaced or rebuilt, so the eleven minutes are spent a single time and repaid on every dose after it. The place it belongs is the pool's own record, beside the surface type and the sanitizer type, where the next person to service it will actually see it - not on a route sheet that gets retyped each season, and not in one technician's head.
Whatever you run the route on should keep each pool's gallons on its own record alongside the readings you log at every visit, so the volume and the chemistry that depends on it sit in one place. In PoolBoss, gallons is a field on the pool itself, next to shape, surface type and sanitizer type, and it stays with the pool as technicians change. The software does not work the volume out for you - that is a measurement someone takes and records - but once it is recorded it stops being a number anyone re-guesses.
For inherited pools, make it part of onboarding rather than a project you schedule and never run. Measure the volume on the first or second visit to each new account, on the same trip you photograph the equipment pad and note the gate code. On a 26-pool takeover that is roughly five hours spread across two weeks of normal visits, and it retires the most consequential wrong number on the route.
FAQ
Frequently asked questions
Does an attached spa or water feature count toward my pool's volume?
If the spa shares water with the pool - a spillover spa on the same equipment, circulating as one body of water - then yes, add it to the total and dose the combined volume. A typical attached spa holds 400-800 gallons, so on an 18,800-gallon pool it is a 2-4% adjustment: inside the noise for a routine chlorine dose, but worth folding in once you are writing the number down anyway. If the spa runs on its own equipment with its own water, it is a separate body of water with its own volume and its own readings, and combining the two leaves both wrong. Waterfalls, sheer descents and bubblers that draw from the pool and return to it add no volume at all - that water is pool water in transit, and it has already been counted once.
How accurate does my pool volume need to be for dosing?
Within about 10% is fine for routine work, and getting closer than 5% is rarely worth the extra effort. The reason is scale: on an 18,800-gallon pool, raising free chlorine by 1 ppm takes roughly 19 fluid ounces of 12.5% liquid chlorine, and being 1,000 gallons out changes that by about an ounce - less than the variation in how the jug actually gets poured. What breaks a pool is not a 5% error but a 20% one, because that is large enough to hold the water consistently under or over target and to survive every retest looking like a chemistry problem. The precision that matters is in the direction and size of the error, not the decimal places. A number that is roughly right and written on the record beats a precise number that lives in one person's memory.
Can I work out my pool's gallons from how long it takes to fill?
You can, but read the water meter rather than the clock. A garden hose delivers somewhere between 8 and 17 gallons a minute depending on hose diameter, length and household pressure, and that spread is wide enough to put a fill-time estimate 30-40% out, which is worse than simply measuring the pool. If the property has a water meter, take a reading before the fill and another after it: the difference is the actual gallons delivered, and it is the most accurate figure you will ever get on that pool. The catch is that it only works on a new build or a full drain-and-refill. For the ordinary case of a pool already full of water, measuring the dimensions is faster, needs nothing drained, and can be done while the filter runs.
How do I measure the deep end without getting in the water?
Use a weighted line from the deck. Tie a small weight to a tape or a marked cord, lower it until it rests on the floor, and read the depth at the water line - it is good to within a couple of inches, which is plenty for this. Take one reading at the deepest point, one where the shallow shelf ends and the slope begins, and one in the middle of the shallow section, and you have everything the section method needs. Treat the depth markers on the coping as a starting point rather than a source: they are set at build time, they describe the wall rather than the floor, and on a resurfaced pool they can be several inches out because the plaster thickness changed. Where a pool light is visible it usually sits about 18 inches below the water line, which is a quick sanity check on a reading that looks wrong.
Does a low water level change the volume I should dose to?
Yes, and by more than most people expect, because the missing water comes off the widest part of the pool. Every inch of water level on a 16 by 32 pool is about 320 gallons, so a pool sitting 6 inches low is down roughly 1,900 gallons - around 10% of an 18,800-gallon pool, which is precisely the size of error that puts doses visibly off target. Dose to the water actually in the pool, then top it up, rather than dosing to the recorded volume and wondering why the retest overshoots. It matters most in late summer on uncovered pools in dry climates, where evaporation can pull an inch a week, and immediately after a filter backwash, which can take several hundred gallons out in a few minutes.
Should I re-measure a pool I inherited from another company?
Yes, and treat it as part of taking the account on rather than a job for later. An inherited gallonage is usually a copied number with no traceable origin: it came off the previous company's route sheet, which came off the one before that, and nobody remembers whether it was ever measured or estimated from the deck. Measuring takes about ten minutes a pool, so a 26-pool takeover is roughly five hours spread across the first fortnight of visits. Prioritise the pools whose notes complain about chemistry - an account described as always running low, or one that has been getting an extra summer visit to hold its chlorine, is the likeliest to be carrying a volume error, and it is the one where correcting the number changes what the route costs to service.
