Gas for a 150-mile trip each way costs $43.54 round trip, or $0.145 a mile
Gas cost is the miles you drive divided by your car's miles per gallon, times the price of a gallon. Drive 150 miles each way and you cover 300 miles. At 30 mpg that burns 300 ÷ 30 = 10 gallons, and at the EIA's U.S. average for regular of $4.354 a gallon (week of October 5, 2026) the bill is 10 × $4.354 = $43.54. A one-way drive costs half of that, $21.77, and two people splitting the round trip pay $21.77 each.
Cost per mile is the gas price divided by mpg: $4.354 ÷ 30 = $0.145 a mile. It depends only on your car and the pump price, not on the trip, so once you know it you can price any drive by multiplying by the miles. The sections below cover when to run the numbers, the steps, a monthly commute, how mpg and price change the total, a cost table by distance, converting mpg, business mileage and common mistakes.
When is this calculation useful?
- Planning a road trip: a map app gives you miles and time but not dollars. Enter the round-trip miles and your mpg to get the gas budget, then add tolls and parking.
- Splitting gas money: for a carpool, a ride to a wedding or a shared road trip, adding gas and tolls and dividing by the number of people is easier to defend than splitting whatever the driver paid at the pump.
- Commute cost per month: multiply the round-trip cost by the days you actually drive. Comparing that with a transit pass or a parking garage tells you what driving costs in fuel alone. Insurance, registration and depreciation are separate; the car ownership cost calculator covers them.
- Reimbursing business miles: employers often pay a flat rate per mile instead of actual gas. See the business mileage section below for how that compares.
How the calculation works
The calculator runs the steps below in order. It keeps full precision in the middle and rounds only the final dollar amounts to the cent. The example is 150 miles each way, round trip, 30 mpg, regular at $4.354, with $20 in tolls (an assumed figure) and three people splitting.
| Step | Formula | Example |
|---|---|---|
| 1. Total miles | one-way miles × (1 one way, 2 round trip) × number of trips | 150 mi × 2 × 1 = 300 mi |
| 2. Gallons used | total miles ÷ mpg | 300 mi ÷ 30 mpg = 10 gal |
| 3. Gas cost | gallons × price per gallon | 10 gal × $4.354 = $43.54 |
| 4. Total | gas cost + tolls and parking × number of trips | $43.54 + $20.00 = $63.54 |
| 5. Per person | total ÷ people, rounded up to the cent | $63.54 ÷ 3 = $21.18 |
| Cost per mile | price per gallon ÷ mpg | $4.354 ÷ 30 = $0.145 |
The per-person share is rounded up so the money collected never falls short of the total. When the split is uneven, the pot can end up a cent or two over.
Commute gas cost per month
For the same drive every day, pick round trip and enter the number of days you really drive, not calendar days. Take out work-from-home days, vacation and days you ride transit. The table assumes 30 mpg, regular at $4.354, 21 driving days a month and 252 a year (21 × 12).
| One-way miles | Per day (round trip) | Per month (21 days) | Gallons per month | Per year (252 days) | Per month, 2 people |
|---|---|---|---|---|---|
| 10 mi | $2.90 | $60.96 | 14 gal | $731.47 | $30.48 |
| 20 mi | $5.81 | $121.91 | 28 gal | $1,462.94 | $60.96 |
| 30 mi | $8.71 | $182.87 | 42 gal | $2,194.42 | $91.44 |
| 40 mi | $11.61 | $243.82 | 56 gal | $2,925.89 | $121.91 |
A 20-mile commute each way over 21 days is 840 miles and 28 gallons, which costs $121.91 in gas. If two people ride together every day and split it, each pays $60.96. Errands and weekend trips are separate drives, so calculate them on their own and add them.
How much does mpg change the cost?
The same 300 miles at $4.354 a gallon costs very different amounts at different mpg. The bar chart under the calculator runs this comparison with your own inputs.
| Fuel economy | Gallons | Gas cost | Difference from 30 mpg |
|---|---|---|---|
| 20 mpg | 15 gal | $65.31 | +$21.77 |
| 25 mpg | 12 gal | $52.25 | +$8.71 |
| 30 mpg | 10 gal | $43.54 | $0.00 |
| 35 mpg | 8.57 gal | $37.32 | −$6.22 |
| 40 mpg | 7.5 gal | $32.66 | −$10.88 |
Equal mpg gains do not save equal dollars. Going from 20 to 25 mpg saves $13.06, while 35 to 40 mpg saves only $4.66, because gallons used is miles divided by mpg and the curve flattens as mpg rises.
For the mpg you enter, your dashboard's trip average or your last fill-ups (miles driven divided by gallons added) are closer to real life than the sticker rating. FuelEconomy.gov, run by the U.S. Department of Energy and EPA, says aggressive driving such as speeding, rapid acceleration and braking can lower gas mileage by roughly 15% to 30% at highway speeds and 10% to 40% in stop-and-go traffic. If you expect heavy traffic, run the calculation with a lower mpg and compare.
Gas prices and fuel type
Every 10 cents per gallon adds $1.00 to a 300-mile trip at 30 mpg (10 gal × $0.10). The table uses the EIA's U.S. average prices for the week of October 5, 2026, with typical mpg values assumed for each fuel. Your car's mpg will differ.
| Fuel | U.S. average price per gallon | Assumed mpg | Cost per mile | Gas cost for 300 mi |
|---|---|---|---|---|
| Regular | $4.354 | 30 mpg | $0.145 | $43.54 |
| Diesel | $6.199 | 35 mpg | $0.177 | $53.13 |
| Premium | $5.400 | 28 mpg | $0.193 | $57.86 |
Diesel costs far more per gallon, and a diesel car that gets better mpg narrows the gap per mile only partly in this example. When you switch fuels in the calculator, change the mpg too. The EIA figures are national averages including all taxes, so your station, and especially a high-tax state, can be well above or below them.
Gas cost by distance
One-way and round-trip gas cost at 30 mpg and $4.354 a gallon. The table under the calculator recalculates with your own mpg and price.
| One-way distance | One way | Round trip | Round-trip gallons |
|---|---|---|---|
| 5 mi | $0.73 | $1.45 | 0.33 gal |
| 10 mi | $1.45 | $2.90 | 0.67 gal |
| 25 mi | $3.63 | $7.26 | 1.67 gal |
| 50 mi | $7.26 | $14.51 | 3.33 gal |
| 100 mi | $14.51 | $29.03 | 6.67 gal |
| 150 mi | $21.77 | $43.54 | 10 gal |
| 200 mi | $29.03 | $58.05 | 13.33 gal |
| 300 mi | $43.54 | $87.08 | 20 gal |
| 500 mi | $72.57 | $145.13 | 33.33 gal |
Each row is calculated from the total miles, not from a rounded one-way figure doubled, so a round-trip number can differ by a cent from twice the one-way number.
Business mileage and the IRS standard rate
If you drive your own car for work, a per-mile rate is the usual way to be paid back or to deduct the trip. The IRS lists 2026 standard mileage rates in two periods: for business use, 72.5 cents a mile from January 1 to June 30 and 76 cents from July 1 to December 31, 2026. Medical is 20.5 cents and then 23.5 cents, and charity stays at 14 cents. The IRS describes these as the basis for a deduction if you use your car for business, charity or medical purposes.
Take the 300-mile round trip. At 76 cents a mile the rate comes to $228.00, against $43.54 in gas alone. They are different numbers for different purposes: the IRS rate is a deduction rate, not a fuel price, so do not expect the calculator's gas-only result to match it. An employer's own reimbursement policy may differ from the IRS rate, and which one applies is something to confirm with your employer or a tax professional.
Converting mpg to other units
This calculator takes U.S. mpg. If a spec sheet lists L/100 km or km/L, convert to mpg first: mpg = 235.215 ÷ (L/100 km). In the other direction, L/100 km = 235.215 ÷ mpg and km/L = mpg × 0.4251437.
| U.S. mpg | L/100 km formula | L/100 km | km/L |
|---|---|---|---|
| 20 mpg | 235.215 ÷ 20 | 11.76 L/100 km | 8.5 km/L |
| 25 mpg | 235.215 ÷ 25 | 9.41 L/100 km | 10.63 km/L |
| 30 mpg | 235.215 ÷ 30 | 7.84 L/100 km | 12.75 km/L |
| 35 mpg | 235.215 ÷ 35 | 6.72 L/100 km | 14.88 km/L |
| 40 mpg | 235.215 ÷ 40 | 5.88 L/100 km | 17.01 km/L |
| 50 mpg | 235.215 ÷ 50 | 4.7 L/100 km | 21.26 km/L |
Those factors are from NIST's conversion table and use the U.S. gallon of 3.785 liters. A UK figure is usually in imperial gallons (about 4.546 liters), so the same mpg number there means more distance per gallon than a U.S. mpg number, and the conversion above does not apply.
Common mistakes
- Doubling a round-trip distance: if you typed the round-trip miles from your map app and also selected Round trip, the miles are counted twice. Enter the round-trip miles with One way selected, or the one-way miles with Round trip.
- Entering what you paid at the pump as the price: the price field is dollars per gallon. Typing a $50 fill-up there gives an absurd result.
- Averaging mpg across two kinds of driving: 100 miles of city driving at 25 mpg uses 4 gallons and 100 highway miles at 40 mpg uses 2.5 gallons, so 200 miles takes 6.5 gallons, which is 30.77 mpg overall. Averaging the two numbers to get 32.5 mpg would give 6.15 gallons, about 0.35 gallons too few. Calculate each leg and add them.
- Multiplying the displayed gallons by the price: a 100-mile drive at 35 mpg uses 2.857… gallons, shown as 2.86. The calculator multiplies the unrounded amount, so a 50-mile round trip is $12.44, while 2.86 × $4.354 would give $12.45.
- Using an instant mpg reading: the number flashing on the dash on a flat highway is for that moment. Use the average for the whole trip, including starts, traffic and hills.
Limits of this calculator
- It does not look up routes. Enter the driving distance from a map app; a straight-line distance will understate the cost.
- The default prices are the EIA's national weekly averages for October 5, 2026, not live prices. Replace them with the price at the station where you fill up.
- One mpg figure is applied to the whole trip. Traffic, speed, weather, A/C and cargo change real mileage, so treat the result as an estimate.
- Electric charging and the electric part of a plug-in hybrid are not included, and neither are insurance, registration, maintenance or depreciation.
- Per-person shares are rounded up to the cent, so the total collected can be a cent or two above the bill.
As of The default fuel prices are the EIA's U.S. weekly averages for the week of October 5, 2026 (regular $4.354, premium $5.400, diesel $6.199 per gallon), checked on October 14, 2026 when the EIA page listed October 14 as the next release date. The IRS mileage rates were checked on irs.gov on October 14, 2026.
Sources
- 01U.S. Energy Information Administration: Gasoline and Diesel Fuel UpdateWeek of October 5, 2026, U.S. average, all taxes included: regular $4.354, diesel $6.199 per gallon; premium $5.400 from the weekly retail table
- 02IRS: Standard mileage rates2026 business 72.5 cents (Jan. 1 to June 30) and 76 cents (July 1 to Dec. 31); medical 20.5 then 23.5 cents; charity 14 cents
- 03NIST Special Publication 811, Appendix B.9: Factors for units listed by kind of quantitymile per gallon (U.S.) to km/L 0.4251437; L/100 km = 235.215 ÷ mpg
- 04FuelEconomy.gov (U.S. DOE and EPA): Driving More EfficientlyAggressive driving can lower gas mileage roughly 15% to 30% at highway speeds and 10% to 40% in stop-and-go traffic
Frequently asked questions
How do I calculate gas cost for a trip?
Divide the total miles by your mpg to get gallons, then multiply by the price per gallon. A 150-mile trip each way is 300 miles; at 30 mpg that is 10 gallons, and at $4.354 a gallon (EIA U.S. average for regular, week of October 5, 2026) the gas costs $43.54.
What is the gas cost per mile?
Divide the price per gallon by your mpg. At 30 mpg and $4.354 a gallon that is $0.145 a mile. It depends only on your car and the fuel price, so multiplying it by any distance gives the gas cost for that drive.
How much does gas for my commute cost per month?
A 20-mile commute each way driven 21 days a month is 840 miles. At 30 mpg and $4.354 a gallon that uses 28 gallons and costs $121.91, or $5.81 per round trip. Choose Round trip and enter the days you actually drive.
I already know the round-trip miles. What do I enter?
Enter the round-trip miles in the distance field and choose One way. If you choose Round trip with a round-trip distance, the miles are doubled and the cost comes out twice as high. If the route back is different, add the two distances together and choose One way.
How do I split gas money with passengers?
Enter the number of people. The calculator adds gas, tolls and parking, divides by the people and rounds up to the cent so the money collected is never short. A $43.54 round trip plus $20.00 in tolls is $63.54; split three ways it is $21.18 each.
Do I enter my $50 fill-up as the gas price?
No. The price field is dollars per gallon, as shown on the pump sign. What you paid at the pump covers gas that may not all be used on this trip, and it will not match the cost of one drive.
My car lists L/100 km or km/L. How do I convert it to mpg?
For L/100 km, divide 235.215 by the number: 8 L/100 km is about 29.4 mpg. For km/L, divide by 0.4251437 (about the same as multiplying by 2.352): 12 km/L is about 28.2 mpg. These use the U.S. gallon from NIST's conversion table.
Can I use this for an electric car?
No. The calculator works for gasoline and diesel priced per gallon. An EV needs miles per kWh and a price per kWh, which are different units. To compare the running costs of gas and electric cars, use the car cost comparison tool.