
It's about as big as a Car Gas Tank.
(for 2009 Ford Taurus)
A car gas tank (for example, that of a 2009 Ford Taurus) holds 20 gallons of gas. In contrast, the Curved Dash Oldsmobile — the first massproduced car — had a fuel capacity of just 5.280 gallons.
It's about oneandthreetenths times as big as a Beer Keg.
In other words, 20 gallons is 1.290 times the size of a Beer Keg, and the size of a Beer Keg is 0.7750 times that amount.
(Half barrel; maximum capacity)The most common keg size sold by US beer sellers is a halfbarrel type with a volume of 15.50 gallons. This type of keg contains the equivalent of 165 0.0937 gallons cans of beer.
It's about half as big as a Bathtub.
In other words, 20 gallons is 0.470 times the size of a Bathtub, and the size of a Bathtub is 2.10 times that amount.
(a.k.a. bath, a.k.a. tub) (for Bootz Industries Aloha 5 ft. Left Hand Drain Soaking Tub)The capacity of a typical bathtub is 42 gallons. Historically, bathtubs have been made of various materials including marble, terracotta, silver, gold, and iron but the size and shape of the tubs has changed little in their 7,000 year history.
It's about seven times as big as a Basketball (packed).
In other words, 20 gallons is 6.82035369750 times the size of a Basketball (packed), and the size of a Basketball (packed) is 0.146619962010 times that amount.
(64% packing density) (NBA official ball standards, Size 7)A NBA official ball, manufactured by Spalding, is a Size 7 ball measures about 1.87673551360 gallons. These balls have 4,118 pebbles each with a diameter of 2.5 mm
It's about nine times as big as a Bowling Ball (packed).
In other words, 20 gallons is 9.19531370450 times the size of a Bowling Ball (packed), and the size of a Bowling Ball (packed) is 0.108751047780 times that amount.
(64% packing density) (USBC specifications; tenpin)According to United States Bowling Congress specifications, a tenpin bowling ball should measure at least 10.795 cm (4.25 in) in radius for a total volume of about 1.39201341150 gallons. Because regulation balls measure the same diameter irrespective of their weight, ball density varies by size and balls weighing less than about 5 kg (11 lbs) will float in water.
It's about onetenth as big as a Hot Tub.
In other words, 20 gallons is 0.1050 times the size of a Hot Tub, and the size of a Hot Tub is 9.520 times that amount.
(for Lifesmart Rock Solid Simplicity Plug and Play 4 Person Spa With 12 Jets) (a.k.a. Spa, a.k.a. Jacuzzi, a.k.a. Whirlpool) (Advertised Capacity)An average, fourperson hot tub holds 190 gallons of water. Modern hot tubs became popular outside of Japan starting in about 1940.
It's about tenandahalf times as big as a Basketball.
In other words, 20 gallons is 10.6568026530 times the size of a Basketball, and the size of a Basketball is 0.093836775679 times that amount.
(NBA official ball standards, Size 7)A NBA official ball, manufactured by Spalding, is a Size 7 ball and measures about 1.87673551360 gallons. The surface of these balls have 4,118 pebbles with a diameter of 2.5 mm each. 
