What’s That: Ullage

When a fuel tank is rated to hold a certain amount, that does not necessarily mean operators should plan to fill it completely. Tanks need space above the liquid and that empty space is known as ullage.

As fuel levels change, so does the amount of ullage available. Understanding that space helps operators determine how much room remains in a tank while accounting for operating limits and changes in fuel volume.

Why Does Ullage Matter?

As the amount of fuel in a tank increases, ullage decreases. As fuel is consumed or removed, ullage increases. In practical terms, the measurement helps operators understand how much space remains above the product.

That remaining space serves an important purpose. Petroleum products expand and contract as temperatures change, so tanks need appropriate room to accommodate changes in fuel volume. Ullage also helps operators plan deliveries without exceeding appropriate tank fill levels.

However, ullage should not automatically be treated as the number of gallons a tank can safely accept. Tank geometry, rated capacity, operating limits, delivery procedures, and applicable regulations all affect safe delivery quantities.

Ullage vs. Sounding

Ullage and sounding are two different methods used to describe the liquid level in a tank.

Ullage measures downward from a designated reference point near the top of the tank to the surface of the liquid. Sounding, also called an innage measurement, determines the depth of the liquid from a reference point at or near the bottom of the tank to the liquid surface.

Put simply, sounding indicates the vertical depth of the fuel, while ullage indicates the vertical space above it. The American Petroleum Institute (API) maintains petroleum measurement standards covering areas such as tank calibration, tank gauging, and the calculation of petroleum quantities.

How Is Ullage Measured?

Tank levels can be determined manually or through electronic gauging equipment. Traditional manual measurement uses specialized equipment, such as a calibrated tape and weighted end, lowered through a designated gauge point. For an ullage measurement, the operator determines the distance between the tank's reference point and the liquid surface.

Many modern storage systems instead use automatic tank gauges or other electronic level-measurement technologies. According to the EPA's underground storage tank guidance, automatic tank gauging systems can monitor product levels and help identify inventory discrepancies.

Why Does Ullage Matter?

Ullage becomes particularly important when planning fuel deliveries. Knowing how much space is available helps determine whether a planned delivery can be safely accommodated and reduces the risk of overfilling a tank.

Accurate tank measurements can also support better inventory management. Businesses can use tank-level data to coordinate deliveries, monitor fuel consumption, and identify discrepancies between fuel delivered and fuel dispensed.

For fuel buyers with onsite storage, understanding the distinction between tank capacity, current inventory, ullage, and safe delivery capacity is an important part of managing fuel supply. A tank may have empty space remaining, but not all of that space is available for another delivery. Having accurate tank information and a well-planned delivery schedule can help businesses maintain the fuel they need without creating unnecessary storage or delivery challenges.

Mansfield helps businesses manage on-site fuel supply with delivery solutions built around their operational needs. From planning fuel deliveries to keeping tanks supplied, Mansfield provides the fuel and support businesses need to keep operations moving. Contact Mansfield today to learn more.