Start with the level you can measure

Horizontal tank weight calculator

Estimate the liquid volume in a partly filled cylinder lying on its side. Add the liquid’s density to turn that volume into pounds.

For a level, circular cylinder with flat vertical ends. Rounded or dished ends, tilted tanks and internal obstructions are outside this model.

Use internal measurements. Depth runs vertically from the inside bottom to the liquid surface, from 0 to the diameter. Switching units converts entered dimensions.

Use the actual liquid’s density at a matching temperature. Find and interpret a product density. Leaving density blank still gives volume.

Liquid currently in the tank

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Enter your inside dimensions and liquid depth.

Liquid weight

Liquid only. The tank, fittings and any other load are excluded. Full geometric capacity is not a recommended operating fill limit.

Why a quarter of the depth is not a quarter of the gallons

The bottom of a horizontal round tank is narrow. As the surface rises toward the middle, each additional inch of depth covers a wider slice. The same depth increase therefore adds different amounts of volume at different levels.

At half the inside diameter, a flat-ended horizontal cylinder is half full by volume. At one quarter of the diameter, it is only about 19.55% full. Multiplying full capacity by the depth percentage would overestimate that amount. This is the reason for using a circular-segment calculation.

From liquid level to volume

Let r be the inside radius, h the liquid depth from the bottom and L the inside length, all in the same unit. The cross-sectional area below the liquid surface is a circular segment. Multiplying that area by length gives the liquid volume.

A = r² × arccos[(r − h) ÷ r] − (r − h) × √(2rh − h²)
Liquid volume = A × L
Full cylinder volume = π × r² × L

The inverse cosine uses radians. The formula follows Wolfram MathWorld’s circular-segment derivation. Empty, half-full and completely full states are handled explicitly. Near the extremes, the implementation uses a numerically stable equivalent to avoid subtracting almost equal values.

Then use density to calculate the liquid weight

Kilograms = liquid liters × density in kg/L
Pounds = kilograms ÷ 0.45359237

Dimensions determine the volume; density determines the weight. A tank’s nominal gallon rating does not tell you how much liquid it currently holds, and a volume does not identify the liquid. The density field starts blank so neither water nor fuel is silently assumed.

The example button uses a 4-foot inside diameter, an 8-foot inside length and a 1-foot liquid depth, with an illustrative density of 1 kg/L. That depth is one quarter of the diameter, so the liquid occupies about 19.55% of the cylinder’s geometric capacity. The example is arithmetic, not a density or safe-fill recommendation for any named product.

Check the tank shape before using the result

  • Horizontal and level: the long cylinder axis must be level. A tilted tank has a different depth along its length.
  • Circular cross-section: an oval or irregular tank does not follow this geometry.
  • Flat ends: length runs between the two flat internal end faces. Domed, dished or hemispherical end sections add geometry this tool does not calculate.
  • Internal dimensions: outside diameter and outside length include wall thickness and can overstate the contained volume.
  • Unobstructed interior: the calculation does not subtract internal equipment, sediment or other displaced volume.

A tank-specific calibration chart describes that tank more directly than an idealized shape calculation. Use the documented chart when actual construction or calibration makes the simple cylinder model unsuitable. This calculator does not determine operating headspace, permissible loading or equipment capacity.

Read the level on the correct basis

Enter vertical liquid depth measured upward from the internal bottom. A reading measured down from the top is empty space, sometimes called an ullage reading; for this geometry, subtract that reading from the inside diameter to obtain liquid depth. Do not enter it directly as the filled depth.

For example, an internal diameter of 48 inches and empty space of 36 inches correspond to a filled depth of 12 inches. Changing the unit selector to inches converts an existing 4-foot diameter to 48 inches without changing the physical tank. Keep all three dimensions in the selected unit.

Gallons and temperature still matter

The result shows US liquid gallons prominently and also gives liters and Imperial gallons. A US liquid gallon is 3.785411784 liters; an Imperial gallon is 4.54609 liters. Those unit relationships and the pound conversion follow the NIST conversion factors.

The calculator does not correct dimensions, volume or density for temperature. Pair an observed volume with a density appropriate to the same conditions. Our liquid density guide explains how to read density units and reference temperatures.

If you already know the gallons, use the main gallons to pounds converter. For a cited diesel density range, use the diesel conversion tool. For water and a separate container total, use the water weight calculator.

Sources and corrections

Geometry and unit references were reviewed September 17, 2026. Results are estimates from your dimensions and density, not a certified tank calibration. All calculations run in your browser; nothing is uploaded or saved. Browse the liquid tools directory or send a correction to contact@gallonstopounds.org.