Litres / Gallons conversion

Hope this doesn't sound like too daft a question... but I'm English and a noob so please humour me :)

When I am reading recipes, brewing instructions, calculations, etc. from American documentation and they are referring to "gallons" are these imperial gallons or US gallons?

To me, 1 gallon = 4.5 litres. Google tells me that 1 US gallon = 3.8 litres. Quite a difference. Clearly, since we both split gallons into the same number of quarts and pints, this would affect the measurement of those too.

Although this affects pretty much everything I need to learn, I am asking specifically because of point 14 at the bottom of Palmer's page here http://www.howtobrew.com/section3/chapter18-6.html - using US gallons in this calculation gives my first every AG brew an extraction efficiency factor of 30 (yay!) whereas using imperial gallons gives me 25 (boo!).

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Your extraction efficiency should not change whether you use metric or Imperial measurements. Be sure to consistently use punds of grain/gallons of water, or kilos of grain/liters of water. Also, Appendix E in "How to Brew" has handy tables for Imperial-metric (and vice versa) conversions. – Chino Brews Dec 9 '13 at 23:55

Palmer is referring to U.S. customary measurements.

One U.S. gallon is 3.79 liters.

One liter is 0.26 U.S. gallons, or 1.05 U.S. quarts.

For the U.S. customary measurement system-impaired (i.e., the whole world except the U.S and maybe parts of the U.K.), a U.S. quart is slightly smaller than a liter, and there are two 8-ounce cups in a pint, two 16-ounce pints in a quart, and four 32-ounce quarts in a U.S. gallon. Thus, a gallon is 128 ounces.

Because many U.S. recipes call for five-gallon batches, you should know that 19 liters = five U.S. gallons.

When using extraction efficiency formulas, remember that one point per pound per gallon (ppg) = 8.3454 points per kg per liter (pkl).

Edit: clarified that we use U.S. measures in th U.S., even though we sometimes erroneously refer to them as "Imperial" or "english" measures. Imperial measures are often very different than U.S. measures.

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Ha :) Typical! That's thrown the cat amongst the pigeons! Incidentally, in the UK, although "technically" we went metric in 1971, in reality we're a mixed bunch... people of a certain age (myself included) measure humans in feet/inches, and stone/pounds... long distances in miles, shorter distances in metres/millimetres, temperatures in Celsius, petrol (gasoline) in gallons, beer in pints, and recipes in grammes. – Doug Dec 10 '13 at 0:00
...oh yes, that means my extraction efficiency came out at 30! That gets a "yay" from me :) – Doug Dec 10 '13 at 0:01
Congratulations. On further reading of your quandary, and for future readers, I should note that our measurement system in the U.S. is called, sometimes, the "Imperial" or "English" system of measurement, but the technical name is the "U.S. Customary System" or "U.S. Standard System", I believe. Thus, a U.S. gallon is not an Imperial gallon -- it is quite a bit smaller. All beer recipes using non-metric (U.S.) liquid measurements should be in U.S. standard ounces, pints, and gallons. I will fix that above. – Chino Brews Dec 10 '13 at 0:11
Thanks - I was just returning to this thread to ask exactly that - I've been watching a couple of BeerSmith videos and they're talking about "English" units... so these English units aren't actually English then! :) – Doug Dec 10 '13 at 0:29