A vertical tank half full holds half its capacity. A horizontal one does too — but at a third of the way up it holds barely a fifth, and that is where people get caught. This gives the volume at any depth, plus a dipstick table.
Notes
Why horizontal tanks are counter-intuitive
The cross-section is a circle, so the area of the wetted part changes with the square of the depth near the bottom and top. Filling the first quarter of the height adds far less than filling the middle quarter.
The formula for a partly full cylinder
The wetted area is the circular segment: r squared times (theta minus sin theta) over two, where theta is twice the arccosine of (r minus depth) over r. Multiply by the length and you have the volume.
Making a dipstick
Print the table, mark the depths on a straight stick, and you have a gauge calibrated to your actual tank. Far more reliable than a float gauge and it never fails.
What this does not do
It assumes flat ends and a level tank. Dished or hemispherical ends add capacity, and a tank sitting even slightly out of level reads wrong at both extremes.