How Do You Measure Fuel at the Gas Pump?

If you drive a gasoline-powered vehicle, you may wonder if the gas pump is accurate. How do you know you are getting what you pay for when you fill up?

A largely invisible infrastructure ensures that these transactions are fair and equitable. That work starts long before you arrive at the gas station.

Let’s take a look “under the hood” of this process.

How Gasoline Is Dispensed

When you squeeze the pump nozzle, the gasoline dispenser pulls the product from underground storage tanks. The dispenser then registers the amount of gasoline and delivers it to your vehicle.

The dispenser has a tool called a positive displacement meter that converts known gasoline amounts into dollar amounts. If you’ve ever watched the gallon (and price) numbers tick upward as you pump gas, that’s the positive displacement meter at work.

Typically, at least once per year, state and local weights and measures officials test and inspect these dispensers (including the meter) for accuracy and compliance with state and local laws.

One reason fuel is stored underground is that temperature affects its volume. Warmer temperatures expand the volume of fuel, so there’s less energy in a gallon. Underground storage tanks help stabilize temperature, often keeping it near the reference temperature of 60 F, which is the petroleum industry standard. This reduces extreme expansion and contraction with ambient air temperature. So, you’ll get what you pay for at the pump, regardless of the weather outside.

How Gasoline Pumps Are Tested

Weights and measures officials use a certified, precisely measured five-gallon test standard, known as a liquid volume prover.

Officials pump five gallons of gasoline from the dispenser into these provers and compare the display readout to verify that the volume is accurate. They also inspect the dispenser to ensure that it follows other requirements, such as clear displays of quantity and price.

Once the dispenser has passed the test, the official gives the gas station permission to use those machines in its sales.

Economic fairness is important on both sides of a sale. Customers need to know they’re getting what they’re paying for, and businesses need to know they’re not unknowingly giving away their products.

What to Do If You Have a Concern

If you have a concern about a gas pump or gas station, contact your state’s weights and measures office.

NIST’s Role

The National Institute of Standards and Technology (NIST) does not regulate gas stations or gas pumps. NIST provides training for state officials and standards for those officials who test gasoline dispensers. For example, we publish handbooks for inspectors that include detailed specifications for a device and the method of sale (e.g., must be sold by the gallon).

We do this because of our extensive expertise in measuring devices and knowledge of standards used in commercial transactions. Accurate measurements are essential when buying commodities by weight or measure, such as gasoline, food and mulch.