It's about as big as a Concrete Mixer Truck
(a.k.a. cement mixer, a.k.a. cement mixer truck, a.k.a. cement transport truck, a.k.a. concrete mixer, a.k.a. concrete transport truck, a.k.a. in-transit mixer)(standard size; drum capacity)
A concrete mixer truck has a drum volume of up to 2,900 gallons. Concrete mixer trucks can be in transit for about 90 minutes before the concrete dries — if the concrete dries while in the drum, jackhammers or even dynamite must be used to remove it.
It's about fifteen-and-a-half times as big as a Hot Tub
In other words, 2,900 gallons is 15.3 times the size of a Hot Tub, and the size of a Hot Tub is 0.065 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, four-person hot tub holds 190 gallons of water. Modern hot tubs became popular outside of Japan starting in about 1940.
It's about 70 times as big as a Bathtub
In other words, 2,900 gallons is 69 times the size of a Bathtub, and the size of a Bathtub is 0.01 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 150 times as big as a Car Gas Tank
In other words, 2,900 gallons is 145 times the size of a Car Gas Tank, and the size of a Car Gas Tank is 0.007 times that amount.(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 mass-produced car — had a fuel capacity of just 5.280 gallons.
It's about 200 times as big as a Beer Keg
In other words, 2,900 gallons is 187 times the size of a Beer Keg, and the size of a Beer Keg is 0.0054 times that amount.(Half barrel; maximum capacity)
The most common keg size sold by US beer sellers is a half-barrel 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 one-two-hundredth as big as a Hot Air Balloon
In other words, 2,900 gallons is 0.01 times the size of a Hot Air Balloon, and the size of a Hot Air Balloon is 200 times that amount.(four-person capacity; envelope volume; average)
The average volume of a hot air balloon sufficient to lift four adults is 580,000 gallons. Typically made of nylon or dacron, a balloon envelope of this size typically weighs just 88 kg (190 lbs).
It's about one-two-hundred-fiftieth as big as an Olympic Swimming Pool
In other words, 2,900 gallons is 0.004 times the size of an Olympic Swimming Pool, and the size of an Olympic Swimming Pool is 230 times that amount.(FINA regulation; (minimum capacity)
According to the Fédération Internationale de Natation (FINA), a swimming pool for Olympic and world championship events must measure 50 m by 25 m with a depth of at least 2 m for a total volume of 660,000 gallons. A pool of this size was to be used in Olympic time trials in New Jersey in early 2008, but its component pieces were stolen from a warehouse, likely to be sold for scrap metal.