You need to know the vehicle's average economy and have an estimate of how many miles you plan on driving.
To calculate the car's average fuel economy, you need to fill the tank and write down the vehicle's mileage and then drive it until empty (using your normal driving habits). when you fill the tank again, get the mileage and subtract the first mileage reading from that. Take that number and divide it by the number of gallons you purchased and that will tell you the car's average mileage.
*(Mileage 1- Mileage 2)/ gallons= average fuel economy*
To calculate future fuel usage, take your average economy and multiply that by the number of miles you plan on driving.
*average fuel economy X estimated miles= estimated future fuel usage*