Most people today did not learn imperial units in school. Since the 1970s, we only teach the metric system (that is, until recently in the UK (see my previous post for details)). So with that in mind, it’s time for a lesson on the most common units used in the imperial system. I will go through the history of these units and some of the other units in a future post, but for now I’m keeping it simple!
So sit up straight, take out that chewing gum, and pay attention. We shall begin with units of mass and work our way through volume, length and area (power, pressure etc are not included here). The tables below describe each unit, they’re value in relation to previous unit, and how they can be subdivided.
Mass
Unit  Value  Subdivisions  Metric (approx)  Notes 

Ounce (oz)  Many moons ago, there existed a unit called the dram, which was ^{1}⁄_{16}th of an ounce, but that’s not used now and today the ounce is subdivided by decimal points or fractions.  28.35 g  The oz abbreviation comes from the Italian word onza.  
Pound (lb)  16 oz  Pounds can subdivided using ounces or fractions, for example:
Sometimes decimal places are also used. 
453.59 g  The pound is derived from the Roman libra pondo, which is where the lb abbreviation comes from. 
Stone (st)  14 lb  Stones are subdivided in the same way as with pounds. Examples:
Disclaimer: these values are in no way representative of my current weight… 
6.35 kg  When quoting a weight by stone, it’s common to use the singular, for example “14 stone” as apposed to “14 stones“.
Aparantly, in Co Clare there was a time when potatoes were sold using 16 lbs to the stone in the summer and 18 lbs to the stone in the winter. How odd… 
Ton (t)  160 st (2240 lb)  Tons are subdivided using fractions or decimal places.  1.016 t  There are three common tons; the long ton of 2240 lb, the short ton of 2000 lb (used in the US and Canada) and the metric tonne of 1000 kg. It’s also used as a slang word for lots of something, like “I have tons of money” (I wish). 
Volume
Unit  Value  Subdivisions  Metric (approx)  Notes 

Fluid ounce (fl oz)  There is a fluid dram of 1/8th of a fluid ounce, but that’s not used any more, fluid ounces are usually subdivided using decimal places.  28.41 ml  The fluid ounce (not to be confused with ounce, which is a uint of mass) was originally defined as the volume taken up by an ounce of some liquid such as wine or water.  
Pint (pt)  20 fl oz  Pints can be subdivided using fl oz or fractions, for example:
Decimal places might also be used. 
568.26 ml  Ahh the humble pint. Still in use today in pubs and in the sale of milk (in the UK). The word has become synonymous with certain alcoholic beverages. 
Gallon (gal)  8 pt  In theory you should be able to subdivide gallons using pints as with pints and fluid ounces, but it’s more common to use fractions or decimal places.  4.546 L  There used to be three common gallons, the corn gallon, the ale gallon and the wine gallon. The most current gallon in these parts is close to the ale gallon, whereas the gallon used in the US is more akin to the wine gallon. 
Cubic foot (ft^{3}) 
Length
Unit  Value  Subdivisions  Metric  Notes 
Inch ( ” or in)  Inches are subdivided into fractions, usually from halves up to 32nds of an inch (^{1}⁄_{2}, ^{1}⁄_{4}, ^{1}⁄_{8}, ^{1}⁄_{16}, ^{1}⁄_{32}) for example:

25.4 mm  If you’ve lost your measuring tape, use your thumb, the width of which is about an inch.  
Foot ( ‘ or ft)  12″  Feet are subdivided using inches, for example:

304.8 mm  Unless you have a large, ahem, foot, it probably isn’t quite 12 inches. The foot is based on a person’s boot rather than foot. 
Yard (yd)  3′  Yards are subdivided using fractions.  0.9144 m  The Welsh once called it a pace and the Romans called it a step. 
Furlong (fur)  220 yd  Furlongs are subdivided by fractions.  201.168 m  You’ll most commonly find the furlong in horse racing these days, but they used to be used in farming and road signs (and still are used in road signs in some countries). 
Mile (mi)  8 fur (1760 yd)  Miles are subdivided by furlongs or fractions, for example:

1.609344 km  The mile is a Roman invention; which was originally 1000 paces of an army, or 5000 feet. Traditionally, miles are abbreviated m; but to avoid confusion with metres, mi is now more commonly used. 
Area
Unit  Value  Metric (approx)  Notes 
Square Inch (sq in)  1 x 1 inch  645 mm²  It’s about the surface area of a male thumb from the knuckle to the top (depending on the size of your thumb!) 
Square Foot (sq ft)  1 x 1 foot (144 sq in)  0.093 m²  Used in specifying indoor area sizes (rooms, houses, office space etc) 
Square Yard (sq yd)  1 x 1 yard (9 sq ft)  0.836 m²  Used in the sale of carpet or other flooring 
Acre  1 chain (22 yd) x 1 furlong (220 yd) (4840 sq yd) 
0.4047 ha  Originally based on the amount of land a team of oxen can plough in a day. It of course does’t have to be specifically 1 chain x 1 furlong, any size area of 4840 sq yds is an acre. 
Square Mile (sq mi)  640 acres  2.59 km²  Used to describe large areas such as an island or country, it’s also used when describing population density. 