How to Measure an Underground Tank You Can't See

Underground tanks hide their geometry, but you can recover it with a stick, the fill pipe, and one delivery receipt.

Get the depth first. Stick the tank through the fill pipe to find total interior depth — bottom to the top of the tank, not the top of the pipe. Subtract the riser: stick to the bottom, mark; pull up until the stick tip just catches the tank top lip if you can feel it, mark again. The difference is tank diameter (for a cylinder lying horizontally, interior depth = diameter).

Identify the family. Residential underground oil tanks are almost always horizontal cylinders in standard sizes: a 48-inch stick depth suggests a 500 or 1000 (length is the difference), 42 inches suggests the long 550 variant. Match your measured diameter against the standard sizes on our chart hub.

Confirm with a delivery. The decisive test: stick the tank before and after a metered delivery. If 200.0 gallons raised your level from 14 to 27 inches on a 48-inch-diameter chart, and the chart agrees, your identification is right. If it disagrees consistently, your tank is the other length — recheck against that chart.

Two cautions. Never assume a buried tank is sound based on measurements alone — age, water at the stick bottom (use water-finding paste), or sudden unexplained level drops are reasons to call a professional. And if a property purchase is involved, geometry is not an inspection.