Heating Oil Tank Sizes, Explained

Residential heating oil tanks come in a small family of standard sizes, and once you know the family the measurements identify your tank instantly.

The most common by far is the 275-gallon tank: 27 inches wide, 44 inches tall, 60 inches long when installed vertically (the upright oval profile you see in most basements). The same tank installed on its side is a "275 horizontal" — same capacity, lower clearance, common in crawl spaces. The 330-gallon is identical in section but a foot longer at 72 inches.

The shape matters more than people realize: these tanks are obround — flat sides, semicircular top and bottom — designed at 27 inches wide so they fit through a 30-inch basement door, which a cylindrical tank of equal capacity could not. That flat-sided profile is also why generic "oval tank" calculators misread them by several percent.

Underground tanks are usually true cylinders: 500-gallon (~48-inch diameter), 550 and 1000-gallon sizes are typical. Outdoor above-ground installs use the same obround family on legs.

Two practical notes. A tank is never filled to its geometric brim: a 275 takes about 250 gallons at fill-up to leave expansion room, a 330 about 300. And if your tank predates you, check the nameplate before trusting any chart — manufacturers stamp capacity and dimensions on a plate near the top.