What size oil tank do I need?
Bigger is not better, because heating oil ages in storage: heat, oxygen, moisture, and microbes slowly turn dormant fuel into sludge. NORA's sizing rule balances delivery frequency against fuel freshness in one line: estimate your annual gallons and divide by three.
Not sure? Two delivery slips and the usage calculator will tell you, or estimate from home size there.
NORA's worked examples
| Annual usage | Divided by 3 | NORA's recommendation |
|---|---|---|
| 1,800 gallons | 600 | A 500 gallon tank or two 275s |
| 1,000 gallons | 333 | A 275 or 330 gallon tank |
| 750 gallons or less | 250 | No larger than a 275 |
From the NORA Advanced Storage Tanks manual. Local conditions and customer preference can justify deviating; near salt water NORA suggests leaning smaller still, with more frequent water checks.
The logic: a tank around a third of annual usage turns its fuel over a few times a season, so oil never sits long enough to age badly, while deliveries stay at a reasonable rhythm. Even sized this way, some fuel in the tank is always over a year old, which is why water management matters regardless of size.
Common questions
Why not just install the biggest tank that fits?
Because the fuel at the bottom of an oversized tank can sit for years, aging into sludge and acids that shorten the tank's own life and clog filters. NORA's rule exists to keep fuel moving. Fewer deliveries is a real convenience, but it trades against fuel freshness.
I want fewer deliveries. What are my options?
Move up one step inside the rule, not three: at 1,500 gallons a year the math supports a 500 or twin 275s, which already cuts deliveries roughly in half versus a single 275. Automatic delivery on a K-factor schedule removes the chore either way.
Does the rule apply to buried tanks?
The freshness logic is the same, but the industry trend NORA documents runs the other way: aging underground tanks are commonly replaced with smaller above ground tanks, singly or paired, rather than like for like. If you are replacing a buried 1000, that conversation belongs in the replacement quote.
What do twin tanks mean for readings?
Two manifolded 275s fill and drain together, so each tank's stick reading converts with the normal 275 chart and the totals add. Read both; a stuck level difference between twins is worth showing your service tech.