Horizontal Tank Capacity
Diameter and length in, a full fill-height-to-volume table out — the real geometry of a tank lying on its side, where fill height and fill volume are never the same percentage.
Fill-Height-to-Volume Table
Every 5% of diameter, from empty to full, using the dimensions entered above.
| Fill % | Height (in) | Volume |
|---|
The Formula Used
Why the Table Isn't a Straight Line
A vertical tank has the same cross-section at every height, so its fill-height-to-volume relationship is linear — double the height, double the volume. A horizontal tank does not, because the wetted cross-section is a circular segment that starts at zero width at the bottom, widens through the middle, and narrows back to zero at the top. That is why the first few inches off the bottom of a horizontal tank read as much less volume than the same few inches near the middle, and it is exactly what the table on the left is built to show rather than approximate.
Where This Fits
This is the same family of geometry-to-volume math as the drill pipe tally calculator's capacity and displacement figures, just applied to a different shape — a circular segment here instead of a full circular cross-section. If the number you actually need is pump output or strokes rather than a static tank volume, the lag & volume calculator covers that side of the circulating system.
For where a portable fuel, water, or chemical tank like this one fits relative to the rig's own fluid processing equipment, see the mud pit / solids control system page — a different, rectangular class of tank doing a different job.
Frequently Asked Questions
Why is 25% fill height not 25% of the tank's volume?
Because a horizontal cylinder is not a uniform cross-section top to bottom — the circular shape is narrowest at the bottom and widest through the middle, so the same few inches of fill height cover a lot less volume near the bottom than it does near the middle. At 25% of diameter this tank holds about 19.5% of its full volume, and at 75% of diameter it holds about 80.5% — that gap is the geometry, not an error, and it mirrors exactly around the 50% mark where fill height and fill volume always agree.
Does this work for vertical tanks too?
No — a vertical (upright) cylindrical tank has a constant circular cross-section at every height, so its fill-height-to-volume relationship is a straight line: volume = cross-sectional area x height, full stop. This calculator is specifically for a tank lying on its side, where the circular segment shape is what makes the relationship curve instead of a straight line.
What if my tank has rounded (dished) ends instead of flat ends?
This calculator treats the tank as a plain cylinder and does not add volume for dished, elliptical, or hemispherical end caps. Most small-to-mid oilfield fuel and water tanks run flat ends where that's not a meaningful source of error, but a tank with true dished heads will hold measurably more than this number, and that extra volume needs adding separately from the head manufacturer's own capacity figure for that end style.
Is the tank assumed to be perfectly level?
Yes. Any pitch or list changes the true wetted cross-section at a given dipstick reading, which this formula does not account for. For a tank on a fixed skid or foundation that's usually a safe assumption; for one on questionable ground, check level before trusting a fill-height reading against this table.
Can I use this for a mud tank or other oilfield vessel, not just fuel?
The geometry is generic — any horizontal cylindrical vessel with flat ends follows the same formula, whether it holds fuel, water, or drilling fluid. The mud pits and tanks in an actual solids control system are usually rectangular, not cylindrical, so this specific shape is more common for portable fuel, water, and chemical tanks on a location than for the pit system itself — see the mud pit system page for how those rectangular tanks fit into fluid processing.