Determine the unit you want to convert to, for example, liters to milliliters, gallons to cups or hogsheads to gallons.
Find the conversion factor on a unit conversion chart. If the chart does not have a conversion factor between the units you choose, you may need to combine conversion factors. For instance, a unit conversion chart might not give you the conversion factor for gallons to cups but may tell you that there are 8 pints in a gallon and 2 cups in a pint. In this case, you must use both the gallons-to-pints and the pints-to-cups conversion factors.
Convert the large unit into a fraction by giving it a denominator of one. For example, 5 gallons becomes 5 gallons/1 gallon.
Express each conversion factor as a ratio in fractional form. For instance, 8 pints per gallon becomes 8 pints/1 gallon.
Set up an equation that multiplies the large unit by as many conversion factors as necessary to reach the small unit. For example, to convert 5 gallons to cups, you would set up the following equation: 5 gallons/1 gallon * 8 pints/1 gallon * 2 cups/1 pint.
Cancel any units that occur in the numerator of one fraction and denominator of another. The unit that remains is the target unit. For instance, gallons and pints occur in the equation's numerator and denominator, leaving the unit "cups."
Complete the multiplication and simplify. For instance, 5 * 8 * 2/1 * 1 * 1 simplifies to 80. Therefore, 5 gallons equals 80 cups.