Off the top of my head:
3 teaspoons is 1 tablespoon. 2 tablespoons is 1 ounce ( or when measuring coffee, 1 "scoop"). 8 Ounces is 1 cup. 2 cups is 1 pint. 2 pints is one quart. 4 quarts is one gallon.
7.5 gallons to a square foot (I just changed this, it was wrong actually). Also, 128 square feet is a cord. Also also, an acre foot is 325 thousand gallons.

It's hard being American, because that's just the beginning! You also need to know the metric system if you buy imported items or talk to people from overseas.

So for conversions you should know 5 millilitres is a teaspoon. And in American hospitals the use CC in place of ml, because using millilitres for liquid volume and cubic centimeters for solid volumes is too confusing for american doctors. 1CC = 1ml.

And for the metric system itself you need to know that 1000 ml is a litre. 1000 litres is a Kilolitre... I guess that's everything you need.