The price of gasoline can change on any given day. Although a number of factors determine the price per gallon, the price of crude oil makes the most impact. The per-barrel price of crude oil is most directly affected by world supply and demand. By closely monitoring the price of crude -- as well as keeping tabs on a few other factors -- you can estimate the cost to fill up.
Break down the per-gallon price of gasoline. According to the U.S. Energy Information Administration, the price of crude oil accounts for about 67 percent of the per-gallon gas price. Another 7 percent is based on the price to refine crude. Distribution and marketing account for 11 percent, and the remaining 15 percent comes from taxes. Keep in mind these percentages can change, particularly because each state has a different gas tax.
Divide the day's crude oil price by 42. One barrel of crude contains 42 gallons. This will tell you the dollar amount per gallon of refined gasoline attributed to crude. For example, if crude oil is $100 per barrel, then about $2.38 of the price of a gallon of gas comes from the crude price.
Divide the dollar amount per gallon of refined gas attributed to crude in half. This will give you a dollar amount that accounts for about one-third of the total price per gallon. By multiplying this amount by three, you can get an estimated price per gallon. For example: using $2.38, divide by two to get $1.19. Multiply $1.19 by three to get $3.57, an average cost per gallon of gasoline.
Monitor websites such as bloomberg.com to get daily updates on the price of crude. Look up your state and county gas taxes and add these figures to the federal rate of 18 cents per gallon.
Refining costs depend greatly on the operability of refineries, which often depends on the weather. One of the reasons gas prices rise in the summer is because crude sources in the Gulf of Mexico are threatened by hurricanes.
Always consider demand. The more people driving, the higher the demand for gas, thus the higher the demand for crude. Crude oil is a dwindling natural resource, so the more people who need it, the higher its price.