100 US gallons is 378.5411784 liters

The conversion is a fixed ratio, not an estimate. Under the definitions NIST prints in Handbook 44 Appendix C, one US liquid gallon is 231 cubic inches exactly, and one inch is 2.54 centimeters exactly. Multiply 231 by 16.387064 (the milliliters in a cubic inch) and you get 3,785.411784 milliliters, so:

100 gal = 100 x 3.785411784 = 378.5411784 liters

The same chain gives you the smaller units without any new factors. A gallon holds 4 quarts, 8 pints, 16 cups and 128 fluid ounces, so 100 gallons is 400 quarts, 200 pints, 1,600 cups and 12,800 US fluid ounces. A NIST Special Publication 811 table confirms the liter factor at 3.785 411 784 for the US gallon.

For completeness against the other gallon: 100 US gallons is 83.267418463 imperial gallons and 85.936696267 US dry gallons. If someone in Canada or the UK hands you a number quoted in their gallon, do not reuse the 3.785 factor. See the imperial section below.

NIST Special Publication 811, Appendix B.8 lists the gallon to liter factors, and Handbook 44 Appendix C (PDF) gives the 231 cubic inch definition the factor comes from.

Sizing a 100 gallon aquarium from its outside dimensions

Aquarium volume comes from inches, not liters, so the box dimensions are the number you measure. The rule is:

gallons = length x width x height (in inches) / 231

A tank measuring 55 inches long, 20 inches front to back and 21 inches tall is 55 x 20 x 21 = 23,100 cubic inches, which divides by 231 into 100 gallons exactly, or 378.5411784 liters. Any tank whose inside volume reaches 23,100 cubic inches holds 100 gallons to the brim, and 378.541 liters of water.

Weight matters more than volume once the tank is full. Water weighs about 8.34 pounds per gallon, so 100 gallons weighs about 834 pounds (378.3 kilograms) on its own. Add 150 to 250 pounds of glass, stand, gravel and rock and you are parking over 1,000 pounds on one floor section. That is why a 100 gallon setup usually lives on a ground floor slab or a load rated stand, and why the fish store quotes you both gallons and liters before delivery.

Water changes and dosing at 100 gallons

Big tank owners do water changes by percentage, and the percentage turns straight into liters:

• 10 percent of 100 gallons = 10 gal = 37.85411784 L
• 20 percent = 20 gal = 75.70823568 L
• 25 percent = 25 gal = 94.6352946 L
• 50 percent = 50 gal = 189.2705892 L

Each 5 gallon bucket you carry holds 18.92705892 L, so a 25 percent change on a 100 gallon tank is 94.635 liters, or 20 full buckets back and forth. Many people buy a 100 liter dosing or holding bin for exactly this job: 100 L is 26.4172 gallons, a bit more than the 25 gallon target, so a 25 percent change fits with 1.4172 gallons (5.365 L) to spare.

Conditioners and medications dose per gallon or per 10 gallons. A product that asks for 1 mL per 10 gallons takes 10 mL for the whole 100 gallons; if the label says 5 mL per 10 gallons, the full tank is 50 mL. Dose by the gallons the water volume holds, not the tank label, because gravel and rock displace water and the true figure runs below the nameplate.

Stock tanks, rain barrels and 100 gallon storage

A 100 gallon stock tank or rain barrel holds 378.5411784 liters. Filling it is a pump and hose question: a garden hose delivering 5 gallons per minute needs 20 minutes for the full 100 gallons, and an 8 gallon per minute line does it in 12.5 minutes. Rain barrels fill on roof runoff, where even a light shower on a 1,000 square foot roof sheds hundreds of gallons, so a 100 gallon barrel overflows quickly and the useful question is how much you can drain and reuse between storms.

Compare containers: the common 55 gallon steel drum holds 208.1976481 L, so a 100 gallon tank is just under two drums (1.82 drums). A standard 275 gallon IBC tote is 1,041 liters and holds 2.75 of these tanks. If you are matching a European product listed in liters, a "400 liter" cistern is 105.67 US gallons, slightly more than the 100 gallon tank you priced in the US catalog.

Pools: what 100 gallons does to a water level

100 gallons sounds big until it meets a pool. In a 10 foot round inflatable pool, the water surface is about 11,310 square inches, so 23,100 cubic inches of top up water raises the level only about 2.04 inches. A large inground pool at 15,000 gallons would need 150 refills of this size. So 100 gallons is the scale of a top up after a hot week, a splash pad refill, or a small hot tub, not a pool fill.

Chemistry follows the same number. Pool chemicals dose per 10,000 gallons, and one dose spread over a small pool is dangerous, so the CDC's healthy swimming guidance tells owners to read the label per volume and to keep pool water treated and tested. If your splash pool holds 100 gallons, that is 378.541 liters, and a product dosed at 1 ounce per 1,000 gallons wants 0.1 ounce for the whole pool, which is why small pools get spoon sized doses instead of cup sized ones.

Quarts, cups, cubic feet and cubic yards in one number

Here is 100 gallons in every unit a US homeowner meets, each derived from the same 231 cubic inch gallon:

The cubic foot figure is worth remembering for plumbing and soil orders: 100 gallons is 13.368 cubic feet, so a wheelbarrow rated at 4 cubic feet carries about 30 gallons, and a 2 cubic yard delivery of soil is about 404 gallons by volume.

Where the 100 gallon figure goes wrong

The classic mistake is the imperial gallon. One imperial gallon is 4.54609 L, so 100 imperial gallons is 454.609 liters, about 76.07 liters more than 100 US gallons. Canadian and UK product pages use the imperial gallon, and a "100 gallon" tank quoted there is a different object from the one on a US page. The US dry gallon is 4.404884 L, so 100 dry gallons would be 440.4884 liters, though the dry gallon almost never appears in tank or pool work.

Second, tank labels are nominal. A tank sold as 100 gallons is dimensioned at 100 gallons to the brim; fill lines, trim, filter boxes and rock displace water, so the working volume might be 92 to 96 gallons, or 348 to 363 liters. Dosing by the label slightly overdoses; dosing by measured water volume is correct.

Third, check which gallon a calculator used. A calculator that rounds to 3.785 is off by 0.041 liters at 100 gallons, and one that uses 3.78 is off by 0.541 liters. This page and its widget use 3.785411784 throughout.

Barrels, kegs, drums and jerry cans at 100 gallons

Trade containers make 100 gallons easy to picture in bulk terms. The US oil barrel is 42 gallons, so 100 gallons is 2.3809523810 barrels. The 31 gallon beer barrel gives 3.2258064516 barrels, and the 15.5 gallon half barrel keg used in most US taprooms is 6.4516129032 kegs. A 55 gallon drum takes 1.8181818182 drums, and a standard 5 gallon jerry can means exactly 20 of them.

Fuel sellers mix the two scales too. A pump selling gasoline at 3.50 dollars per gallon is charging 0.92456 dollars per liter, since one liter of that gasoline is 0.2641720524 gallons. Run the ratio the other way and a 378.5411784 liter tank drop reads as a 100 gallon delivery.