Navigate your browser to a mapping site such as Google Maps, Yahoo! Maps or MapQuest (see the links in the Resources section).
Use your mapping site of choice to determine the mileage for your trip.
In Google Maps, type your destination address at the top of the screen, then click "Search Maps." Click "Directions" on the left side of the screen. Type the address from which you plan to drive in the "A" field, then click "Get Directions."
In Yahoo! Maps, type the address from which you plan to drive in the "A" field. Type your destination address in the "B" field. Click "Go."
In MapQuest, click "Get Directions." Type the address from which you plan to drive in the "A" field. Type your destination address in the "B" field.
Calculate the necessary amount of gas for your trip by dividing the number of miles in your trip, determined in Step 2, by your vehicle's gas mileage (see your vehicle's owner's manual to find out its gas mileage). For example, for a trip of 200 miles in a vehicle that gets 30 miles to the gallon, divide 200 (miles) by 30 (miles per gallon); in this case, you will use about 6.67 gallons for your trip.
