We have many, many commonly used subdivisions of gallons:
- 1/4th of a gallon is a quart
- 1/2 of a quart is a pint
- 1/2 of a pint is a cup
- 1/8th of a cup is an ounce
- 1/2 an ounce is a tablespoon
- 1/3rd of a tablespoon is a teaspoon
- One rundlet is 18 gallons of wine
- One firkin is 84 gallons of wine or 8 gallons of ale
- One tierce is 42 gallons of wine
- One barrel is 31.5 gallons of wine or 4 firkins of ale or 42 gallons of oil
- One hogshead is 63 gallons of wine or 1.5 barrels of ale
- One pipe is 1.5 firkins of wine
- One tun is 2 pipes of wine
Maybe millions of gallons would be used too infrequently for people to remember how many gallons are in the units. But, I think we should start using metric prefixen:
- 300 megagallons of water
- 16 teradollars of national debt
- 50.9 to 50.4 megavotes