Why a 275 gallon tank holds about 240
The ticket says 240-some gallons into a tank sold as a 275, and the gauge reads full. Nobody shorted you. The size on the label, the volume the shell really holds, and the oil your burner can draw out of it are three different numbers, and the same is true of every tank size.
Three numbers for the same tank
| Number | Gallons | What it is |
|---|---|---|
| Nominal size | 275 | The catalog label. It rounds the shell up. |
| Geometric capacity | about 274 | The computed volume of the shell, dished heads included. See the 275 vertical chart. |
| Typical delivery fill | 230 to 250 | What fits into a near-empty tank before the vent whistle stops the driver, with an air gap still at the top. |
| Usable oil when "full" | roughly 240 | A few gallons of the fill sit below the outlet pickup, out of the burner's reach. |
So a "full" 275 holds roughly 240 gallons of usable oil. That is normal for the size. Dimensions, lifespan and what a swap involves are on the 275 gallon oil tank page.
Why the driver stops at silence
As the driver pumps, oil rising in the tank pushes air out through the vent pipe. A whistle in that vent, the vent alarm, sounds the whole time air is escaping. When the oil reaches the whistle, the sound stops. That silence is the driver's signal to stop pumping, and it always comes while an air gap is still open at the top of the tank. No normal delivery reaches the geometric capacity, let alone the number on the label. For more on the whistle itself, see the oil tank vent alarm.
How to check a delivery anyway
If a fill ever feels light, you can check it without an argument. Stick the tank before the delivery and again after, convert both readings with the chart, and compare the difference to the gallons on the ticket. Delivery meters are sealed and inspected, so the two numbers almost always agree. Ten minutes with a stick confirms it.
Common questions
Was my 240 gallon delivery short?
Almost certainly not. A 230 to 250 gallon fill into a near-empty 275 is exactly what the geometry and the vent whistle produce. To verify any delivery, stick the tank before and after, convert both readings with the chart, and compare against the ticket.
Can the driver keep pumping past the whistle?
The whistle is the overfill protection. Pumping after it goes silent risks pushing oil out the vent and onto the house, so a careful driver stops at silence every time. The air gap it leaves behind is supposed to be there.
Do 330s and buried tanks work the same way?
Yes, every size has the same three-number stack: a nominal label, a computed capacity, and a smaller realistic fill. A 330 computes to about 329 gallons, and a 550 underground computes a little over its label, about 564. The whistle and the pickup behave the same on all of them.
How heavy is a full 275?
Over 2,000 pounds, per NORA's tank manual. That is why a tank is never moved with fuel still in it, and why an indoor tank belongs on a real concrete floor. A leaning tank leg is a service call. Do not shim it yourself.
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