Gas pump price display representing how to calculate cost to fill up a tank

Cost to Fill Up: Formula, Examples, and a Quick Calculator

February 23, 2026 9 min read

Quick Summary "Cost to fill up" is a simple multiplication problem once you estimate how many gallons you will buy. The only tricky part is translating your fuel gauge (fraction of a tank) into gallons.

Gallons to buy ≈ tank capacity × (1 − fuel level fraction).
Fill-up cost ≈ gallons to buy × price per gallon.
For road trip planning, pair fill-up cost with cost per mile and tank range.

What We Know (Sourced)

Fuel cost is a function of fuel consumption and price. EPA label documentation is designed to help consumers compare fuel economy and fuel costs using standardized assumptions, while DOE fuel economy guidance explains that real-world results vary with driving conditions and behavior.

For practical budgeting, what matters is using consistent assumptions and replacing them with your own measured numbers over time.

The Fill-Up Cost Formula

Step 1: Estimate gallons to buy.

Gallons to buy ≈ tank capacity × (1 − fuel level fraction)

Example fuel level fractions: 1/2 = 0.5, 1/4 = 0.25

Step 2: Multiply by fuel price.

Fill-up cost ≈ gallons to buy × price per gallon

Result: approximate dollars for the next fill

Accuracy tip: Fuel gauges are approximate. If you want a precise fill-up amount, record the gallons dispensed at your last few fill-ups and use that as your typical "fill from 1/4 to full" benchmark.

Worked Examples

Example A: 14-gallon tank, you're at 1/4 tank, fuel is $3.50/gal.

Gallons to buy ≈ 14 × (1 − 0.25) = 10.5 gallons
Cost ≈ 10.5 × $3.50 = $36.75

Example B: 18-gallon tank, you're at 1/2 tank, fuel is $4.00/gal.

Gallons to buy ≈ 18 × (1 − 0.5) = 9 gallons
Cost ≈ 9 × $4.00 = $36.00

How to Use It for Budgeting

Fill-up cost becomes more useful when you connect it to:

Cost per mile (fuel budgeting). Related: cost per mile.
Commute totals (weekly/monthly). Related: commute gas cost.
Tank range (when you'll need the next fill). Related: range on a tank.

Want to calculate it instantly?

Use our fuel cost tools to estimate total trip cost, cost per mile, and fill-up cost.

Try the Fuel Cost Calculator

What's Next

If you're planning a trip: estimate fuel needed and cost, then plan stop intervals. Related: planning fuel stops.
If you're trying to spend less: focus on the big levers (speed, idling, tire pressure). Related: maintenance checklist.
If prices fluctuate: update your calculator inputs and compare scenarios.

Frequently Asked Questions

Why doesn't my fill-up match the gauge fraction exactly?

Fuel gauges are not precise instruments and can vary with vehicle slope and driving. Using the last few receipts is often the fastest way to estimate your typical gallons-per-fill pattern.

Should I always fill up from 1/4 tank?

That's a personal risk-management choice. For remote routes or winter driving, a larger buffer can reduce the chance of running low unexpectedly.

Does better MPG lower my fill-up cost?

Not directly for a single fill from the same fuel level, but better MPG means you fill less often for the same miles. That's why cost per mile is the better long-run budgeting metric.