A measurement of volume
"Item, sack, two gallons ..........5s. 8d."
 Henry IV, Part 1, Act II Scene IV
"The report of the committee of 1757  8, shows that the gallon is of very various content"
 Plan for Establishing Uniformity in the ... Measures of the United States
It was a puzzle to me for a long time. Two nations, separated by more than just language  even our pints and gallons of beer were different sizes. Of course, ever the insular and arrogant Englishman, I put it down to the same American spirit of damnable independence that brought us Noah Webster's misspelling of our words. "Color", indeed.
It took me a long time to discover the real story behind the tiny "girly" pints of our Merkin cousins, but at last, the tale can be told...
A Brief History of the Gallon
In good old British style, the gallon measure was ever somewhat confusing. Whether for reasons of professional pride, indolence, drunkenness or general obstinacy, the vinters and brewers and presumably, the millers, could not agree on what consituted a gallon. For not only were wine and beer gallons different, but the dry measure (for wheat and so on) was different again.
For a long time there were almost as many gallon measures as there were people using them, but by the end of the 18th century, there were three generally fixed and agreed measures.

The ale gallon was 282 in³ (4.62 litres)

The wine gallon, ("Queen Anne's gallon") was 231 in³ (3.79 litres)

The corn gallon, ("Winchester gallon" was c. 268.8 in³ (4.405 litres)
If this appears confusing, consider that in the years since the 12th century, there were gallons measuring between 224 in³ and 282 in³, some of which are worthy of mention for various reasons (all measures in cubic inches, quotations from the Commitee of 1757)

"224 cubic inches, according to the standard wine gallon preserved at [London] Guildhall"

"231, according to the statute of 5th of Anne"

The Rumford gallon of 1228 (266.25 in³ or 264.8 in³, depending on whether you believed the ancient quart measure, or ancient gallon measure)

"271, less 2 spoonfuls, according to a standard gallon of Henry VII., and another dated 1601, marked E. E., both in the Exchequer"

"277.18 as established for the measure of coal by the statute 12 Anne"

"282 according to the standard gallon for beer and ale in the Treasury"
There are, moreover, varieties on these varieties, from the barrel to the ton, inclusive; for, if the barrel be of herrings, it must contain 28 gallons by the statute 13 Eliz. c. 11. If of wine, it must contain 31 1/2 gallons by the statute 2 Henry VI. c. 11, and 1 Rich. III. c. 15. If of beer or ale, it must contain 34 gallons by the statute 1 William and Mary, c. 24, and the higher measures in proportion.
The English had set their measure by some strange standards, intertwined with their strange, predecimal currency. For instance, in 1266, Henry III decreed that a penny should weigh as much as 12 grains of wheat, twenty pence to an ounce, twelve ounces to the pound. Eight pounds was the weight of a gallon of wine. This is how we got to have 240 pennies in the Pound Sterling. No wonder the poor colonials were in such a state.
Standardised Gallons
The need for standardisation was obvious, and the fledging USA made a decision on the size of the gallon (and the bushel too). The standard US gallon would be the "Queen Anne" wine gallon, of 231 cubic inches (3.785 litres). The "corn gallon" or dry gallon was used in the US until fairly recently recently for grain and other dry goods, being 268.8025 cubic inches, oneeighth of a Winchester bushel. Oddly though, the initial recommendation was to
Let the measures of capacity, then, for the United States be 
A gallon of 270 cubic inches;
The gallon to contain 2 pottles;
The pottle 2 quarts;
The quart 2 pints;
The pint 4 gills;
Meantime, the UK struggled on with the many and various "standards" until 1824, when the Imperial Gallon was defined, and all others abolished. This new measure (close in size to the old ale gallon) was defined as being "the volume of 10 lb. of distilled water weighed in air with brass weights with the barometer standing at 30 inches of mercury and at a temperature of 62 °F". This was subsequently refined to be the volume occupied by 10 lb of distilled water of density 0.998 859 g/mL weighed in air of density 0.001 217 g/mL using weights of density 8.136 g/mL. This means that the new gallon is 4.546 litres (277.4416 in³).
That the British declined to standardise on the US gallon is mystifying, for now there are two systems bearing the same name, with wildly differing volumes. One hopes that one day, we'll all be metric, and drink beer by the litre.
Summary and Conversions
The gallon is still of "very various content". Whichever variant of the gallon Falstaff had his sack in, it was unlikely to be either of those available today.
Imperial gallon = 4.54609 litres (277.419 in³)
US (liquid) gallon = 3.785411784 litres (231 in³)
US (dry) gallon = 4.4048841 litres (268.8 in³)
1 Imperial gallon = 8 pints
1 Imperial pint = 20 fluid ounces
1 US gallon = 8 pints
1 US pint = 16 fluid ounces
1 US pint = 0.832673844 Imperial pints
1 Imperial pint = 1.20095042 US pints
1 US fluid ounce = 1.0408423 Imperial fluid ounces
1 Imperial fluid ounce = 0.960760333 US fluid ounces
Google calculator
Encyclopædia Britannica
http://en.wikipedia.org/wiki/Gallon
http://en.wikipedia.org/wiki/English_unit#Comparison_of_the_Imperial_and_US_customary_systems
"Plan for Establishing Uniformity in the Coinage, Weights, and Measures of the United States"  http://yale.edu/lawweb/avalon/jeffplan.htm
Heh. Now I discover
US Survey feet and wonder why they are larger than a reg'lar ol' foot.