Basics:

1 mile = 1.609 km

1 km = 0.6214 miles

Acre- One acre comprises 4,840 square yards or 43,560 square feet (which can be easily remembered as 44,000 square feet, less 1%). Because of alternative definitions of a yard or a foot, the exact size of an acre also varies slightly. Originally, an acre was a selion of land one furlong long and one chain wide. However, an acre is a measure of area, and has no particular width, length or shape.

Wikipedia

Chain- A chain is a unit of length; it measures 66 feet or or 22 yards (20.1168m). There are 10 chains in a furlong, and 80 chains in one statute mile. An acre is the area of 10 square chains (that is, an area of one chain by one furlong). The chain has been used for several centuries in England and in some other countries influenced by British practice.

Wikipedia

Furlong- A furlong is a measure of distance in imperial units and U.S. customary units. It is equal to one-eighth of a mile or 660 feet or 201.168 metres.

Five furlongs are approximately 1 kilometre (1.00584 km to be exact). Since the original definition of the metre was one-quarter of one ten-millionth of the circumference of the Earth (along the great circle coincident with the meridian of Longitude passing through Paris), the circumference of the Earth is about 40,000 km or about 200,000 furlongs.

Wikipedia - Roman Origins

Hectare- A hectare (symbol ha, pronounced /ˈhɛktɛər/) is a unit of area equal to 10,000 square metres, or one square hectometre, and commonly used for measuring land area. A 100 m square is one ha. Its base unit, the are, was defined by older forms of the metric system, but is no longer part of the modern metric system. The Comité International des Poids et Mesures classifies the hectare as a unit that is accepted for use with SI.

Wikipedia - Metric & Imperial

League- A league is a unit of length or area long common in Europe and Latin America, although no longer an official unit in any nation. The league most frequently expresses the distance a person, or a horse, can walk in 1 hour of time (usually about 3.5 miles or 5.5 kilometres).

Wikipedia (also info on Roman Measurements)