Horizontal Cylinder Tank calculator
Volume of a horizontal cylindrical tank — total and at any liquid depth. Exact geometry, instant results, nothing leaves your browser.
Exact segment geometry — see methodology. Dished-head propane math is documented there too.
Horizontal cylinders are the workhorse shape for fuel storage, underground oil tanks, water haulers and air receivers. Because the tank is round and lying on its side, each inch of depth holds a different number of gallons: the inches near the centerline hold the most, the first and last inches hold the least. That is why you cannot estimate a half-reading by halving the capacity of the top half — and why this calculator solves the circular-segment area exactly at every depth.
Enter the inside diameter and length, then drag the fill slider to your dipstick reading. The output panel shows liquid gallons, liters, percent full, liquid weight for your chosen fluid, and how many gallons it takes to top off.
How this is calculated
V = πr²LV(h) = [r²·acos((r−h)/r) − (r−h)·√(2rh−h²)] · LLiquid forms a circular segment in the round cross-section; its area × the tank length gives the volume. See the full derivation, a worked example and a fill-ratio table on the horizontal cylinder formula page.
FAQ
TVC-101/FAQHow accurate is this for my underground oil tank?
Exact for a true cylinder. Real tanks have slightly dished ends and fittings, which typically change capacity by 1–3%. For standard sizes use our pre-computed tank charts.
Why isn't 25% depth the same as 25% volume?
Because the chord of a circle is narrow near the bottom. At 25% depth a horizontal cylinder holds only about 19.6% of its volume.
Can I measure in centimeters?
Yes — switch the dimension unit; the math is unit-exact.