21 ft round pool volume

A 21 ft round pool holds

9,930gal 37,600L

Measured at the water, not at the wall: a 52 in wall less 6 in of freeboard leaves 46 in of water.

Gallons at every fill

Wall height is what the box says; the water stops 6 in below it, and the gallons are worked from the water.
Wall	Water	Holds
48 in	42 in	9,070 gal 34,300 L
52 in	46 in this page	9,930 gal 37,600 L
54 in	48 in	10,400 gal 39,400 L

At 9,930 gal

What to add

One product for each adjustment, worked at this page's own gallons. Follow a quantity to set your own reading.
Fixes	Product	Add
Lower pH −0.5 pH	HASA Dry Acid	1.24 lb
Raise total alkalinity +40 ppm	HASA Alkalinity Up	5.57 lb
Raise pH +0.3 pH	HASA Soda Ash	2.98 lb
Raise free chlorine +2.5 ppm	HASA CHLOR	0.79 qt
Shock the pool +9.5 ppm	HASA CHLOR	3.02 qt

Not quite your pool?

Diameter

feet, across the middle

Wall height

inches, the figure on the box

Water below the top of the wall

The gap between the water and the top rail, usually 4 to 6 inches

Change any measurement and every figure on this page is worked again: the gallons, the drawing, the fill table and each dose.

Pools near this one

10,900 gal How many gallons a 22 ft round pool holds
7,300 gal 18 ft round
13,000 gal How many gallons a 24 ft round pool holds
5,070 gal How many gallons a 15 ft round pool holds
16,400 gal 27 ft round
3,240 gal 12 ft round

Updated October 2026

Sources and dates

Ranges are quoted from the code named beside them, product strengths from the manufacturer's own product page, each retrieved on the date shown.

COMAR 10.17.01.45 A(1) · https://www.law.cornell.edu/regulations/maryland/COMAR-10-17-01-45 · 2026-09-17
HASA · https://hasa.com/pool-chemical-calculator · 2026-09-17
COMAR 10.17.01.45 A(2) · https://www.law.cornell.edu/regulations/maryland/COMAR-10-17-01-45 · 2026-09-17
Florida 64E-9.004 (1)(d)2 · http://flrules.elaws.us/fac/64e-9.004 · 2026-09-17
HASA Dry Acid · https://hasa.com/product/dry-acid · 2026-09-17
HASA Alkalinity Up · https://hasa.com/product/alkalinity-up · 2026-09-17
HASA Soda Ash · https://hasa.com/product/soda-ash · 2026-09-17
HASA CHLOR · https://hasa.com/product/hasachlor · 2026-09-17

This site publishes calculated water volumes and chemical quantities for home pool and spa owners. It is not a health, safety or professional service, and it does not replace the instructions printed on the product you are holding. Not affiliated with, endorsed by or sponsored by HASA, Pyrock Chemical, Pentair, Hayward, Sonoma Hot Tubs, the Centers for Disease Control and Prevention, or any state health department. Product names are the trademarks of their owners and appear here only to identify the strength of what is in the container.