Pool Volume and Chlorine Dosing: Gallons, ppm and Salt

Working out pool volume for every common shape and a sloping floor, what 1 ppm means, chlorine doses for liquid, cal-hypo, dichlor and trichlor, and salt for a salt cell.

2026-10-29 · 11 min read

Illustration for the Pool Volume Calculator guide

Every chemical you put in a pool is dosed by volume. A bottle that says "one gallon treats 10,000 gallons" is useless if you do not know whether your pool holds 14,000 or 24,000, and guessing low on a shock dose is how algae survives. This guide works out pool volume for the common shapes, explains what a part per million really is, and turns that into doses of chlorine and salt you can trust. The Pool Volume Calculator does the same arithmetic.

Volume is area times average depth

Any pool with vertical walls holds its surface area times its average water depth. Measure depth to the waterline, not to the top of the wall; water usually sits about halfway up the skimmer opening, several inches below the coping.

For a floor that slopes evenly from a shallow end to a deep end, the average depth is the mean of the two:

average depth = (shallow + deep) / 2

A pool 3.5 feet deep at one end and 8 feet at the other averages 5.75 feet. Pools with a sudden drop into a small deep "hopper" hold less than this average suggests, because the deep part covers only part of the floor. For those, split the pool into a shallow rectangle and a deep rectangle and add them, or accept that the result is a slight overestimate.

Then convert. One cubic foot is 7.4805 US gallons. One cubic metre is 1,000 litres. One US gallon is 3.785 litres and one imperial gallon 4.546 litres.

Surface area by shape

Shape	Area	Notes
Rectangle	length x width	Most in-ground pools
Round	pi x diameter² / 4	Above-ground rounds
Oval (true ellipse)	pi / 4 x length x width	Source of the "x 5.9" shortcut
Oval with straight sides	(length - width) x width + pi x width² / 4	Many above-ground "ovals"
Kidney	0.45 x (width A + width B) x length	Trade rule of thumb
Free-form	from plans	Ask the builder or measure in pieces

The well-known shortcut "length x width x average depth x 5.9 = gallons" for oval pools is just pi / 4 x 7.48 = 5.875, rounded. It is right for a true ellipse. Many above-ground pools sold as ovals are really a rectangle with half circles on each end, and those hold noticeably more: a 15 by 30 foot straight-sided oval has 402 square feet of surface, against 353 for an ellipse of the same size.

Five pool outlines, rectangle, round, ellipse, straight-sided oval and kidney, each labelled with its area formula, and a side section of a sloping pool floor showing average depth as the mean of the shallow and deep ends

Worked volumes

Pool	Area	Average depth	Volume
16 x 32 ft rectangle, 3.5 to 8 ft	512 ft²	5.75 ft	2,944 ft³ = 22,023 gal
16 x 32 ft rectangle, flat 4 ft	512 ft²	4 ft	2,048 ft³ = 15,320 gal
24 ft round, 4 ft of water	452 ft²	4 ft	1,810 ft³ = 13,536 gal
15 x 30 ft ellipse, 4 ft	353 ft²	4 ft	1,414 ft³ = 10,575 gal
10 x 5 m rectangle, 1.2 to 2 m	50 m²	1.6 m	80 m³ = 80,000 L

What 1 ppm means

Pool chemistry is measured in parts per million, which for water means milligrams per litre. Raising free chlorine by 1 ppm in 10,000 US gallons (37,854 litres) needs 37,854 milligrams, or 37.9 grams, of available chlorine. Every chlorine dose follows from that number and the strength of the product:

available chlorine (g) = ppm x litres / 1,000
product (g)            = available chlorine / strength      (solids, strength as a fraction)
product (mL)           = available chlorine / (strength% x 10) x 1,000   (liquids, trade percent)

Liquid pool chlorine is sold by trade percent: grams of available chlorine per 100 millilitres. A 12.5% product holds 125 grams per litre. Household bleach is labelled by weight of sodium hypochlorite, which works out close to its trade percentage; treat a 6% bleach as roughly 6% available chlorine and expect a little less from old stock.

Chlorine doses per 10,000 gallons

Product	Strength	To raise 1 ppm in 10,000 US gal	Side effect per 1 ppm
Liquid chlorine	12.5%	about 10.2 fl oz	none (raises pH slightly)
Liquid chlorine	10%	about 12.8 fl oz	none
Household bleach	about 6%	about 21 fl oz	none
Cal-hypo	65%	about 2.05 oz	+0.7 ppm calcium hardness
Cal-hypo	73%	about 1.8 oz	+0.7 ppm calcium hardness
Dichlor	56%	about 2.4 oz	+0.9 ppm cyanuric acid
Trichlor	90%	about 1.5 oz	+0.6 ppm cyanuric acid

The side effects come straight from the chemistry. A trichlor molecule carries three chlorine atoms on one cyanuric acid ring; three equivalents of available chlorine (3 x 70.9 g) arrive with one ring (129 g of cyanuric acid), so every 1 ppm of chlorine adds 129 / 212.7 = 0.61 ppm of CYA. Dichlor has two chlorines per ring, so 0.91 ppm. Cal-hypo carries one calcium for two hypochlorites, about 0.7 ppm of calcium hardness (as calcium carbonate) per ppm of chlorine.

Those side effects accumulate. A season of trichlor tablets can push cyanuric acid well past 100 ppm, at which point chlorine is so tightly bound that much more is needed to keep the water sanitised. Test CYA every few weeks and switch to liquid chlorine or cal-hypo when it climbs.

For the 22,023 gallon pool above, raising free chlorine from 1 to 3 ppm (2 ppm) takes 2.2 x 2 x 10.2 = 45 fluid ounces of 12.5% liquid, a little over a third of a gallon.

Salt for a salt-water chlorinator

A salt-water pool makes its own chlorine from dissolved salt, and the cell needs the salt within its range, typically about 2,700 to 3,400 ppm with an ideal around 3,200 for many residential cells. Check your cell's manual; some run lower.

Salt in ppm is milligrams per litre, so the dose is:

salt (kg) = (target ppm - current ppm) x litres / 1,000,000
salt (lb) = (target ppm - current ppm) x US gallons x 8.34 / 1,000,000

The 22,023 gallon pool going from 0 to 3,200 ppm needs 3,200 x 22,023 x 8.34 / 1,000,000 = 588 lb, fifteen 40 lb bags. A pool already at 2,800 ppm needs only 74 lb to reach 3,200. Add salt slowly with the pump running and wait 24 hours before re-testing; adding too much is only fixed by draining and refilling.

Filling time

Hose flow varies with pressure and hose size. Time how long your hose takes to fill a 5 gallon bucket: 30 seconds means 10 gallons per minute. Then:

hours = gallons / gallons per minute / 60

At 10 gpm the 22,023 gallon pool takes about 37 hours. Many water utilities offer a pool-fill allowance on the sewer charge if you call ahead, which can save more than the water itself.

Safe dosing habits

Add chlorine with the pump running, in front of a return, and let it circulate before testing.
Never mix chlorine products with each other or with acids, and never add water to chemicals; add chemicals to water.
Pre-dissolve cal-hypo in a bucket of pool water for vinyl pools, so granules do not bleach the liner.
Liquid chlorine loses strength with heat and time. A jug stored through a summer may be half its labelled strength.
Dose to a target measured with a good test kit, not to a schedule.

Sources and further reading

Unit definitions: 1 ft³ = 7.48052 US gallons; 1 US gallon = 3.785411784 L (NIST).
Molar masses for the CYA and calcium ratios: trichloroisocyanuric acid, sodium dichloroisocyanurate, calcium hypochlorite and cyanuric acid (129.07 g/mol); available chlorine expressed as Cl2 (70.906 g/mol).
Salt-chlorinator manuals from major manufacturers for operating ranges.

Work out your own pool in the Pool Volume Calculator. For tanks and other containers the Tank Volume Calculator covers cylinders and capsules, and the Water Heating Time Calculator estimates how long a heater takes.