The exact amount of oil that should be in a tank is typically around 90% of its rated capacity.
Fuel oil delivery services follow a standard practice to ensure safety and prevent issues related to expansion. They will always leave room in the tank for the oil to expand, especially during warmer temperatures. A good rule of thumb is that they will try to fill the tank to about 90% of the rated capacity. For instance, in a standard 275-gallon tank, they will typically fill it to roughly 250 gallons.
Why Tanks Are Not Filled to 100%
Filling an oil tank to its absolute maximum capacity is avoided for a critical reason: thermal expansion.
- Thermal Expansion: Oil, like most liquids, expands when heated. If a tank is filled to 100% on a cold day, the oil level could rise significantly as temperatures increase, potentially causing spills, damaging the tank, or affecting vent pipes.
- Safety: Leaving a buffer prevents overflow and reduces the risk of environmental hazards and property damage associated with spills.
- Delivery Logistics: The 90% rule provides a reliable target for delivery drivers, simplifying the filling process while adhering to safety standards.
Understanding Tank Capacity and Fill Levels
Different tank sizes will have varying typical fill levels based on the 90% principle.
Here's an illustration using common tank sizes:
| Tank Rated Capacity | Typical Fill Level (Approx. 90%) |
|---|---|
| 165 Gallons | 148.5 Gallons |
| 275 Gallons | 247.5 - 250 Gallons |
| 330 Gallons | 297 Gallons |
Note: The 250-gallon figure for a 275-gallon tank is a commonly cited practical example, slightly below the strict 90% but well within the safe expansion margin.
The goal is always to maintain a safe margin, preventing spills and ensuring the integrity of your heating system and property.
