The mistake this page exists to stop
A pool with a shallow end and a deep end is not its deep end. What fills
it is the average of the two, and getting that wrong is expensive in a
quiet way — every chemical you add is dosed per 10,000 gallons.
gallons = surface area × average depth × 7.48
A 32 × 16 ft pool sloping 3 ft → 8 ft:
average depth (3 + 8) ÷ 2 = 5.5 ft
correct 21,065 gallons
using 8 ft 30,640 gallons ← 45% too high
Same pool. Half again the chlorine, every time.
Shape matters more than people expect
A round pool is not a square one. A 24 foot round pool has 452 square
feet of surface; the 24 by 24 foot square around it has 576 — so treating
it as a square over-states the water by 21%. An oval comes out at about
78.5% of its enclosing rectangle. Pick the shape and the geometry is done
properly.
Common pools
From this calculator. Rectangles use length × width, round uses πr², oval uses π × half-length × half-width.
Where the conversion comes from
A US gallon is 231 cubic inches, which makes 7.48052 gallons to the cubic
foot. That is a definition rather than a measurement, so it is the one
number on this page with no uncertainty attached to it at all — unlike
most of what this site calculates, where the constants come from
published data sheets.
Common mistakes
- Using the deep end. Over-doses by up to half on a sloped pool.
- Treating a round pool as square. 21% too much water.
- Mixing US and UK gallons. A 20% difference, in the under-dosing direction.
- Averaging a hopper pool. A sudden deep section is not an even slope.
The rest of the job
For the surface area of an awkwardly shaped pool or deck, the
square footage calculator
handles L-shapes and circles and hands the figure over. Building the
surround? The paver calculator
counts the patio and the
slab calculator covers a poured
deck. Backfilling round a liner? The
sand calculator sizes that.
Common questions
How many gallons is my pool?
Surface area times the AVERAGE depth, times 7.48. A 32 by 16 foot pool sloping from 3 to 8 feet averages 5.5 feet, which is 21,065 US gallons. The average is the number that matters: measure only the deep end and you get 30,640 gallons, which is 45% too high and will have you over-dosing every chemical you add.
Do I use the deep end or the shallow end?
Neither — the average of the two. Enter both ends above and it is worked out for you. This is the single commonest error in pool volume, and it costs money every time you shock or chlorinate: chemicals are dosed per 10,000 gallons, so a volume 45% too high means 45% more product than the water needs, every single time.
How do I work out a round pool?
Pick "Round" and give the diameter — the calculator uses πr² rather than treating it as a square. That distinction matters more than it sounds: a 24 foot round pool has 452 square feet of surface, not the 576 a 24 by 24 foot square would, so treating it as a square over-states the volume by 21%.
What about an oval pool?
Oval uses π × (length ÷ 2) × (width ÷ 2), which comes out at about 78.5% of the rectangle that would enclose it. A 32 by 16 foot oval holds roughly a fifth less water than a 32 by 16 foot rectangle of the same depth.
Is this US gallons or imperial?
US gallons, at 7.48052 per cubic foot, because that is what pool chemical labels in North America are written against. A UK gallon is about 1.2 times larger, so using this figure against a UK-labelled product would under-dose. Switch the calculator to metric and it answers in litres instead.
My pool has a shallow shelf and a sudden deep hopper. Is the average still right?
Not exactly, and the ticket says so. Averaging two ends assumes the floor slopes evenly between them. A pool with a long flat shallow area and a short deep hopper holds less than the average of its extremes suggests — for those, measure the depth at several points and average those instead, or treat it as two sections and add them.
Sources
This calculator uses no material constants — only geometry and unit
conversions, both exact by definition. Nothing here depends on a
supplier's published figure.
How we choose and cite these figures
