Formulas
Miles driven = one-way miles × 2 for a round trip, or one-way miles for a one-way trip.
Gallons = miles driven ÷ MPG. Gas cost = miles driven ÷ MPG × price per gallon (the same fuel formula the commute and car calculators use).
Trip total = gas cost + tolls and parking entered for the whole trip.
Each share = trip total ÷ people paying, where people paying = everyone in the car, or everyone except the driver.
Cost per mile = price per gallon ÷ MPG. Miles per $20 = $20 ÷ price per gallon × MPG.
Scope, assumptions, and limitations
You type the miles; the calculator doesn’t plan routes, check distances or add detours.
One MPG and one price apply to the whole trip. Real fuel economy changes with speed, load, terrain and weather, and prices change along the way.
EIA’s weekly prices are averages of Monday retail prices for an area, with all taxes included. They are not the price at a particular station, and they are updated once a week.
Only gas and the tolls or parking you enter are counted. Wear, maintenance, insurance, depreciation, food and lodging are not. No IRS mileage rate is used.
Amounts keep full precision until display; shares are rounded to the cent.
Confidence and precision
Calculations retain precision until display formatting. The input-quality indicator is based on your classification of the source groups, not whether a field happens to contain a value. It does not measure the probability that a future outcome will match the estimate.
Example inputs are for learning the tool. Replace them with your own values, and compare a less favorable scenario to understand sensitivity.
Primary sources
U.S. Energy Information Administration: Gasoline and Diesel Fuel Update: regular gasoline, weekly ↗
Week of September 28, 2026 · released September 29, 2026 · next release October 6, 2026 · Checked October 4, 2026
FuelEconomy.gov (U.S. Department of Energy and EPA): Find and compare cars ↗
Live lookup of EPA fuel economy ratings · Checked September 27, 2026
FuelEconomy.gov (U.S. Department of Energy and EPA): Driving more efficiently ↗
Current page (undated) · Checked September 27, 2026
Internal Revenue Service: Standard mileage rates ↗
Live reference table; for July 1 to December 31, 2026 (IR-2026-29): 76 cents a mile for business, 23.5 cents for medical or military moving, 14 cents for charity. Page updated July 28, 2026 · Checked September 27, 2026
Methodology version 1.0 · Reviewed September 27, 2026.