Using excel to do conversions in volume

I want to use Excel to convert from one volume unit to another (e.g. gallons to liters). I have a total of four units that I want to convert from / to. I want to use a drop down list for the "from" and "to" units. Each pair of units would have a unique conversion factor (e.g. gallons to liters would be 3.7854). In this example, if I have four (4) gallons, the number of liters = 4 x 3.7854.

Once the user selects the "from" and "to" units, I want Excel to select the appropriate conversion factor. Is there a better way to do this than by nesting twelve IF statements? A lookup table maybe?

Thanks for your help!

They are gallons, fluid ounces, liters, and milliliters.