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. Three numbers stack up between the name on the brochure and the oil you can actually burn, and they are different numbers on every tank ever made.
Three numbers, three reasons
| 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 is roughly 240 gallons of usable oil. That is normal, not a short delivery.
The whistle decides what full means
As the driver pumps, oil rising in the tank pushes air out through the vent pipe, and a whistle in that vent (the vent alarm) sounds the whole time air is escaping. When the oil reaches the whistle, the sound stops, and silence is the driver's signal to stop pumping. That moment always arrives while an air gap remains at the top of the tank, which is why no normal delivery ever reaches the geometric ceiling, let alone the number painted on the brochure. The whistle has a full guide of its own: the oil tank vent alarm.
How to check a delivery anyway
If a fill ever feels light, you can verify 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 numbers almost always agree, but ten minutes with a stick beats wondering.
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 is a feature, not shorted gallons.
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 actually computes a little over its label at 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 tanks are never moved with fuel in them, why indoor tanks belong on a real concrete floor, and why a leaning tank leg is a service call rather than a shim-it-yourself job.