Fuel Needed Calculator – Gallons, Tank Range & Fuel to Add
Calculate total trip fuel, how much gas to add from a partial tank, estimated driving range with a reserve, and whether one tank can cover the trip.
QUICK ANSWER // TRIP FUEL
How many gallons of gas do I need?
Divide the miles you plan to drive by your vehicle’s MPG. A 540-mile trip at 36 MPG needs 15 gallons of fuel for the driving itself. If you know a previous trip instead, use the same proportion: 7 gallons for 252 miles equals 36 MPG, so 540 miles at that rate also needs 15 gallons.
Formula: gallons needed = miles ÷ MPG. Then account for fuel already in the tank and any reserve you want at arrival.
Try an example
Enter your numbers and calculate.
FUEL PROPORTION MODE // SEARCH SIGNAL
If a car used X gallons for Y miles, how many gallons for Z miles?
Use the known trip directly. This assumes the same fuel economy for both distances.
Enter the known distance, fuel used and new distance.
Worked examples
Trip
Fuel needed
What it means
500 miles at 25 MPG
20 gallons
A 14-gallon tank cannot cover the entire trip without refueling.
300 miles at 30 MPG
10 gallons
With 5 gallons already in the tank and a 2-gallon arrival reserve, add about 7 gallons.
Frequently asked questions
How many gallons do I need for a road trip?
Divide your total trip miles by the MPG you realistically expect.
Should I use highway MPG?
Use highway MPG for mostly highway driving and a lower combined estimate for mixed driving.
Does this include a fuel reserve?
Yes. Trip fuel is consumption only. Total fuel to buy includes your chosen arrival reserve and subtracts fuel already in the tank.
How much gas do I need for a trip?
Gallons needed = trip miles ÷ MPG. For example, 450 miles at 30 MPG requires about 15 gallons for the driving itself.
How far can I go on the gas already in my tank?
Estimated range above reserve = (fuel in tank − reserve) × MPG. If you have 5 gallons, want to keep 2 gallons in reserve, and expect 30 MPG, the usable 3 gallons represent about 90 miles of range.
How far can I go on a full tank?
Full-tank range above reserve = (tank capacity − reserve) × MPG. This makes it easy to see whether the trip can be completed on one tank before planning a fuel stop.
These estimates are fuel used while driving, before adding your chosen arrival reserve. All gallons and MPG on this page use US units.
Miles
20 MPG
25 MPG
30 MPG
40 MPG
100
5.00 gal
4.00 gal
3.33 gal
2.50 gal
300
15.00 gal
12.00 gal
10.00 gal
7.50 gal
500
25.00 gal
20.00 gal
16.67 gal
12.50 gal
1000
50.00 gal
40.00 gal
33.33 gal
25.00 gal
Why total fuel to buy can exceed tank capacity
A 500-mile drive at 25 MPG uses 20 gallons. Starting with 5 gallons and keeping a 2-gallon reserve means buying 17 gallons in total. With a 14-gallon tank, add at most 9 gallons before leaving and buy the remaining 8 along the route. You cannot add all 17 gallons at the first stop.
Measuring your actual MPG
After filling up, record the odometer or reset the trip meter. At the next fill-up, divide the miles driven by the gallons needed to refill to the same level. Use several comparable trips if possible. A dashboard average or official rating may differ from the MPG on your route.
The proportion mode above answers textbook-style questions from a known trip. The main calculator uses MPG, current fuel, tank capacity and a reserve to plan a real trip. For more worked solutions, see the fuel proportion guide.