You should know at least one of these for each type of unit. These are the ones I remember:

1 gallon = 3.785 liters
(1 gal = 3.785 L)

1 inch = 2.54 centimeters [exact] (1 in = 2.54 cm)

1 pound = 453.6 grams
(1 lb = 453.6 g)

1 terameter (Tm) = 10¹² meters (m)

1 gigameters (Gm) = 10⁹ meters

1 megameter (Mm) = 10⁶ meters

1 kilometer (km) = 10³ meters

1 meter = 10² centimeters (cm)

1 meter = 10³ millimeters (mm)

1 meter = 10⁹ nanometers (nm)

1 meter = 10¹² picometers (pm)

1 foot (ft) = 12 inches (in)

1 mile (mi) = 5280 feet

1 gallon (gal) = 4 quarts (qt)

1 quart = 2 pints (pt)

1 pint = 16 fluid ounces (fl.oz.)

1 pound (lb) = 16 ounces (oz)

1 ton = 2000 pounds

1 mL = 1 cm³

(1 milliliter = 1 cubic centimeter)