How to Read a Tank Chart (Without Squinting)

A tank chart converts a depth measurement into gallons. Three things trip people up, and all three are fixable in thirty seconds.

First, measure from the right place. The chart's inch column is depth of liquid from the inside bottom of the tank — not from the fill pipe, not from the gauge tube top. Lower a clean stick until it touches bottom, keep it vertical, pull it out and read the wet line.

Second, know your tank's orientation. A 275-gallon oil tank has completely different per-inch values standing up versus lying flat, because the rounded zones sit at different depths. The 275 vertical and 275 horizontal charts are not interchangeable — at 12 inches the vertical tank holds about 60 gallons while the horizontal holds about 119.

Third, don't extrapolate linearly on round tanks. In a cylinder the middle inches hold the most. If you're between chart rows, use the interactive stick reader on our chart pages — it computes the exact value rather than averaging neighbors.

One more habit worth keeping: write your tank's chart values for quarter, half and three-quarter on a card near the fill gauge. When the float gauge gets sticky — and they all do — the stick and the card are your ground truth.