In real life, I am a staunch defender of the metric system. But even I have to admit that it sounds out of place in a heroic fantasy setting. When I'm the GM in a Dungeons & Dragons game, I'm a bit iffy about using metric units, at least when speaking in-character as an NPC.
Since the original English-language D&D books use American customary units, I will continue to use them with my English-language group, even though I never do in real life. I use slightly rounded values: a US foot is actually 30.48 cm, but I will assume it is exactly 30 cm instead. Most translations of D&D books use metric units with this simplified conversion rate.
Here are a few other units that I will occasionally use. Most of my players aren't used to US units, so I've given them simple metric values. These units are supposed to be plausible, but do not make a coherent system. Don't ask me how many inches there are in a furlong, that's not the point.
Length
Line: 2 mm (about 1/12 inch)
Inch: 2.5 cm
Palm: 10 mm (4 inches)
Foot: 30 cm
Cubit: 50 cm
Step: 1.5 m (%)
Furlong: 200 m
Mile: 1.6 km
League: 4 km (2.5 miles)
(%) D&D rules often reference the "5-foot step", and the combat grid has 5-foot squares.
Areas
Square foot: 0,09 m2
Rood: 56 m2 = 5x5 steps
Acre: 0,4 ha = 1 furlong x 1 chain (=20 m)
Volumes
Ounce: 30 mL
Pint: 500 mL
Gallon: 4 L
Bushel: 40 L
Muid: 2 m3
Weights
Grain: 50 mg = 1/20 g
Penny: 1 g
Ounce: 30 g
Pound: 500 g