Tank ullage is the empty space above the liquid in a tank - the distance from the surface of the product up to a fixed reference point at the top. It is the complement of the liquid depth, and the two are measured and used in different ways. This guide explains ullage versus innage, how each is gauged for inventory, and why the reference-height convention is essential for custody accuracy.
Tank Ullage in one line: Ullage, also called outage, is the vertical distance from the liquid surface up to a fixed reference point at the top of a tank - the empty space above the product. Innage is the opposite measurement: the depth of the liquid from the tank bottom, or a datum near it, up to the surface. Both are used in tank gauging to determine how much product a tank holds, and each depends on a defined reference height to be accurate.
Ullage and innage describe the same tank from opposite directions. Innage is measured from the bottom up: it is the depth of the liquid, the height of product standing in the tank. Ullage is measured from the top down: it is the empty space between the liquid surface and a fixed reference point above, so it tells you how much room is left rather than how much liquid is present. Outage is another name for ullage - the terms refer to the same top-down empty-space measurement.
Which one is measured depends on how the tank is gauged and what is practical. Gauging from the top of the tank down to the liquid surface naturally reads ullage, while dipping to the bottom and reading the wetted length reads innage. A radar or servo gauge mounted at the top essentially measures the distance down to the surface, which is an ullage-style measurement that the system then converts into liquid level.
The two are linked by the tank's reference height. If you know the total reference height of the tank and the ullage, you can calculate the innage by subtraction, and vice versa. That relationship is why a gauging system can report level however the operator prefers, as long as the reference height is correct.
Tank inventory - the volume of product a tank holds - is derived from the measured level, whether that level was obtained as ullage or innage. A manual gauge might lower a tape from a reference gauge point at the top: reading where the liquid marks the tape gives ullage directly, and subtracting from the reference height gives innage. Automatic tank gauges do the same conceptually, sensing the surface and reporting a level.
There are practical reasons to prefer one over the other. Ullage measurement is convenient when the tank is nearly full, because the small empty space at the top is easy to read precisely, and it avoids disturbing bottom sediment. Innage is intuitive when reading the actual product depth and is useful when bottom conditions and any water layer matter. Careful operations often account for both the product and any water bottom, since the salable liquid is the oil above the water.
Once the level is established, it is converted to volume using the tank's calibration - a capacity or strapping table that relates height to volume for that specific tank's geometry. Temperature is also accounted for, since liquid volume changes with temperature, so a raw height reading becomes a corrected, standardized volume for inventory purposes.
Everything in tank gauging is anchored to a defined reference height - the fixed, surveyed distance from the gauge reference point at the top to the datum at the bottom. Because ullage is measured down from that reference point, an error or ambiguity in the reference height throws off the derived innage, and therefore the volume, by the same amount. For custody transfer, where quantities are being bought, sold, and reconciled, that error translates directly into a disputed or incorrect measured quantity.
This is why the reference height convention is treated so carefully: the reference gauge point is a specific, marked location, the datum is defined, and the tank's capacity table is built against those references. Using the wrong reference point, or a gauge whose zero does not match the surveyed reference height, introduces a consistent bias into every reading. Custody accuracy depends on the whole chain - reference height, level measurement, capacity table, and temperature correction - being consistent.
A cloud SCADA such as Merobix trends the tank level continuously against that established reference, so operators see inventory in real time rather than only at manual gauging events. Continuous level history helps reconcile receipts, deliveries, and sales, and it flags anomalies - a level that moves when it should not, or does not move when a transfer is running - that could indicate a gauging problem, a leak, or a valve left open, all of which matter when the numbers are the basis for custody.
Ullage, also called outage, is the empty space measured from the liquid surface up to a fixed reference point at the top of the tank. Innage is the depth of the liquid measured from the bottom, or a datum near it, up to the surface. They describe the same tank from opposite directions and are linked through the tank's reference height.
Measuring ullage - the distance down from a top reference point to the surface - is convenient when a tank is nearly full, because the small empty space is easy to read precisely, and it avoids disturbing bottom sediment. Top-mounted radar and servo gauges naturally sense that distance, and the system converts it to liquid level using the known reference height.
Ullage is measured down from a fixed reference point, so any error in the surveyed reference height carries straight into the derived liquid level and the volume calculated from it. In custody transfer, where measured quantities are bought and sold, that bias becomes a real discrepancy, which is why the reference gauge point, datum, and capacity table must all be consistent.
Merobix reads your field devices into a cloud SCADA - the real thing behind these terms, live in days from any browser.