Gallons per inch of tank
For straight-walled tanks, one number converts every gauge reading to gallons. Round horizontal tanks vary by depth — use the charts for those.
vertical cylinder: gal/inch = π × (D/2)² ÷ 231
rectangular: gal/inch = L × W ÷ 231Multiply gallons-per-inch by your stick reading and you have your volume. This is exactly how delivery drivers do quick mental math on straight-walled tanks — and why they carry printed charts for the round ones.
FAQ
What is gallons per inch?
How many gallons one inch of liquid depth represents in your tank. For straight-walled tanks it's constant: π(D/2)²/231 for a vertical cylinder, L×W/231 for a rectangle.
Why doesn't this work for horizontal cylinders?
Because the surface width changes with depth — the middle inches hold the most. Use the inch-by-inch tank charts for round horizontal tanks instead.
What's the gallons-per-inch of a 275 oil tank?
It varies by depth (the tank is obround). Around mid-tank it's about 7 gal/inch; near the top and bottom it's much less — see the 275V chart.