How many gallons per inch in an oil tank?
There is no single number. Through the middle, a standing 275 gallon tank holds about 7.0 gallons per inch and a lying one about 11.4. Near the bottom and the top an inch is worth less, because the walls curve in. The table shows what one inch of stick is worth at five depths in seven common tanks.
Worked from the outside size (27 x 44 in, 60 in long), the same shape as the 275 gallon chart. A lying 275 holds 11.4 gal in the same inch.
Gallons per inch at five depths
Each cell is the oil between half an inch below and half an inch above the depth in the column heading, so "50% of height" is the inch around the middle. The first column is the first inch from the bottom, and the last column is the top inch of the tank.
| Tank | Height | First inch | 25% | 50% | 75% | Top inch |
|---|---|---|---|---|---|---|
| 275, standing up | 44 in | 1.8 | 6.9 | 7.0 | 6.8 | 1.7 |
| 275, lying down | 27 in | 6.2 | 10.5 | 11.4 | 10.5 | 6.2 |
| 330, standing up | 44 in | 2.1 | 8.3 | 8.4 | 8.2 | 2.1 |
| 330, lying down | 27 in | 7.4 | 12.6 | 13.7 | 12.6 | 7.4 |
| 550 (48 x 72 in round) | 48 in | 2.9 | 13.0 | 15.0 | 13.0 | 2.8 |
| 1000 (48 x 130 in round) | 48 in | 5.2 | 23.4 | 27.0 | 23.4 | 5.2 |
| 1000 (64 x 72 in round) | 64 in | 3.3 | 17.3 | 19.9 | 17.3 | 3.3 |
Wider tanks swing the most. In the 1000 gallon tank that is 48 in across and 130 in long, the first inch holds 5.2 gal and the middle inch 27.0 gal. A standing 275 is flatter through the middle (6.9 to 6.8 gal), but its first and top inches still hold only about 1.8 gal because of the rounded ends. These numbers come from the outside size of each tank, so a tank with thicker steel can hold a little less. Read the pattern from the table, and the gallons from the chart for your size.
What a half inch of misreading costs
A stick is read by eye, so half an inch either way is normal. On a standing 275 dipped at 20 in, the table gives 119.9 gal. Read it at 20.5 in and it says 123.4 gal, a difference of 3.5 gal. On a lying 275 near the middle, 14 in is 139.7 gal and 14.5 in is 145.4 gal, a difference of 5.7 gal. In a lying tank the same slip is worth more oil, so dip twice and wipe the stick between dips.
Water in the bottom looks like oil
Water settles under the oil. The stick reads the oil and the water together, so an inch of water in the bottom inch of a lying 275 means about 6.2 gal less fuel than the chart says. A UK tank supplier suggests coating the end of a stick with water-finding paste to see it, and says to use a wooden stick, because metal can spark (Oil Tank Warehouse). In a heatinghelp.com thread, a poster says that if it is more than a few inches, have the oil provider treat the tank and look into it (heatinghelp.com forum). I can't tell you how much water your tank holds. The stick can only point at it.
Use the table for a quick guess, the chart for a reading
Gallons per inch is a good way to feel how a tank behaves. For an actual reading, add up the inches with the chart for your size, because the per-inch value changes as you go up.
- 275 gallon oil tank chart: every inch for standing and lying tanks, and how to dip.
- 330 gallon oil tank chart: inches to gallons plus the gauge marks.
- 550 gallon oil tank chart and 1000 gallon oil tank chart for the round tanks.
- How long heating oil lasts: two dips a week or two apart give your daily use.