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Horizontal Cylindrical Tank Volume Calculator

Find how much liquid is in a horizontal cylindrical tank (propane, fuel, water, septic) from its diameter, length, and fill height — accounting for the tank's curved cross-section.

Filled volume
15,708 L
Filled volume (US gallons)
4,149.6 gal
Percent full
50%
Total tank capacity
31,415.9 L
Total tank capacity (US gallons)
8,299.2 gal

How it works

A horizontal cylindrical tank's liquid level is deceptive: because the circular cross-section is narrower near the top and bottom than through the middle, the same rise in dipstick height fills much less volume near empty or full than it does near the halfway mark. The filled cross-section is a circular segment — for a tank of radius r filled to height h from the bottom, the segment's central angle is θ = 2 × arccos((r − h) ÷ r), and its area is A = (r² ÷ 2) × (θ − sin θ), a standard closed-form geometry result. Multiplying that area by the tank's length gives the filled volume; at h = r (exactly half the diameter) this correctly works out to precisely half the tank's total volume, since arccos(0) = π/2 makes θ = π and sin(π) = 0.

Diameter, length, and fill height all get this page's metric/imperial toggle, converted to meters before the formula runs, so the reported liters and US gallons stay consistent regardless of which unit system was used to enter the tank's dimensions. A quick sanity check the formula passes: a tank filled to just a quarter of its diameter holds well under a quarter of its total volume, precisely because of that narrowing cross-section.

FAQ

Why isn't a tank filled to half its height exactly half full by volume, like a box would be?

It is, at exactly half height (h equal to the radius) — that's the one point where the geometry works out to exactly 50%. But away from the midpoint, the circular cross-section curves inward, so a reading a quarter of the way up the gauge holds noticeably less than a quarter of the total volume, and a reading three-quarters up holds noticeably more than three-quarters. A flat-sided rectangular tank wouldn't have this effect; a cylindrical one always does.

Does this work for a vertical (standing) cylindrical tank too?

No — a vertical tank's cross-section at any height is the same full circle, so its filled volume is simply πr² × fill height, directly proportional to how full it reads. The curved-cross-section correction this calculator makes only matters for a tank lying on its side, which is how most propane, heating-oil, and septic tanks are actually installed.

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