How much oil is in the tank, and how long it will last
Updated October 2026
Find the gauge
On top of the tank there is a clear plastic vial about the size of a pill bottle, with a disc inside it. The disc is on the end of a hinged arm with a float that rides on the oil. As the oil drops, the disc drops. The marks read F, 3/4, 1/2 and 1/4.
The most common basement tank is the 275: 275 gallons, standing on its narrow side, 44 inches tall and five feet long. For that tank the marks mean roughly this.
| Gauge reads | Gallons in the tank |
|---|---|
| F | about 250 |
| 3/4 | about 210 |
| 1/2 | about 138 |
| 1/4 | about 60 |
Full is 250 and not 275 because the driver leaves about six inches of air at the top.
A quarter on the gauge is a little less than a quarter of the oil. The float measures height, and the tank is rounded, so it is narrower at the bottom than in the middle.
The 330-gallon size is the same tank a foot longer, and it holds about a fifth more at every mark. A 275 lying flat on its wide side reads about the same at the gauge, but the stick chart further down does not apply to it.
When the gauge is wrong
The gauge is an estimate. It can also stick. Sludge builds up on the float over the years until it stops riding on the oil, and then the disc sits at the same mark for weeks while the tank goes down.
If the reading has not changed since the last cold week, test it. Unscrew the plastic vial by hand and press the disc down gently with a finger. It should go down and then float slowly back up. If it stays down or does not move, the gauge is stuck and the reading means nothing.
Measuring without the gauge
A technician can measure the oil directly with a stick through one of the openings on top of the tank. For a standard upright 275, inches of oil convert to gallons like this.
| Inches of oil | Gallons |
|---|---|
| 4 | 14 |
| 8 | 38 |
| 12 | 65 |
| 16 | 94 |
| 20 | 123 |
| 24 | 151 |
| 28 | 180 |
| 36 | 236 |
| 38 | 249 |
The numbers are not even because the tank is rounded at the top and bottom. The bottom four inches hold 14 gallons. Four inches in the middle hold almost 30.
Do not count on the last few inches. That is where the sludge and water settle, and the burner should not be drinking from it.
How long it will last
Dealers use a rule of thumb for an average house with the thermostat at 68. It is an approximation, not a measurement of your house.
| Outside temperature | Gallons a day |
|---|---|
| 40 | about 3.7 |
| 30 | about 5.3 |
| 20 | about 7 |
| 15 | about 7.8 |
Divide the gallons in the tank by the gallons a day. A quarter tank is about 60 gallons. At 30 degrees that is about 11 days on paper, and fewer in practice once you leave the bottom alone.
Your own number is better than the rule of thumb. Take the gallons on your last delivery ticket and divide by the days since the delivery before it. That is what your house burned per day in that weather.
When to order
At a quarter. That leaves enough days for the truck to get to you in a cold week and keeps the burner off the bottom of the tank.
If you would rather not go down the stairs to look, a tank monitor sits on top of the tank, measures the level, and texts you when it reaches a quarter.