Tank Volume Calculator – Gallons in Vertical, Horizontal and Rectangular Tanks

Use the free Tank Volume Calculator below: your data never leaves your browser — all work happens on your device, and nothing is uploaded to any server.

Tanks come in awkward shapes, and working out how much they hold is harder than it looks. A vertical cylinder is simple enough, but a horizontal cylinder is only partly full, and a rectangular tank has three dimensions to multiply. The Tank Volume Calculator handles all three shapes, including partial fill for horizontal tanks. You choose the shape, enter the dimensions in feet, and it gives the volume in cubic feet, US gallons, and litres, so you can plan fills, doses, and deliveries without guessing.

Using the tool starts with the shape. For a vertical cylinder, enter the diameter and the height, and the tool uses the full height. For a rectangular tank, enter the width, the length, and the height. For a horizontal cylinder, enter the diameter and the length, plus the fill depth: how deep the liquid sits, measured from the bottom of the tank upward. That fill depth is the clever part, because a horizontal tank that looks half full by eye may hold more or less than half its volume, depending on where the liquid level sits in the circle.

The maths for each shape is different, and the tool handles all three. A vertical cylinder uses pi times the radius squared times the height. A rectangular tank multiplies width by length by height. A horizontal cylinder uses the circular segment formula: it works out the area of the part of the circle below the liquid level, using an inverse cosine and a square root, then multiplies that area by the tank length. A horizontal tank four feet across and six feet long, filled to half its depth, holds about 37.7 cubic feet, or roughly 282 gallons, which is exactly half of its full volume only because half depth is the special case.

Partial fill is where intuition fails most often. In a horizontal cylinder, the volume does not rise in a straight line with depth. The first foot of depth adds less volume than the middle foot, because the tank is narrow at the bottom and widest at the middle. A tank that is a quarter full by depth holds less than a quarter of its volume, and a tank that is three quarters full by depth holds more than three quarters. The tool’s segment formula captures that curve exactly, which is why it beats a ruler and a guess every time.

This tool is an educational estimator only, not professional plumbing advice. It assumes a perfect shape with flat ends and constant dimensions, while real tanks have dished ends, internal fittings, and walls of varying thickness. Results are estimates for planning fills, treatments, and orders. Check your local codes, and consult a licensed plumber or tank professional before any real installation, modification, or work on a tank that holds fuel, chemicals, or pressurised contents. Never enter a tank, and never assume a tank is safe to open.

Good measuring habits matter here more than anywhere. Measure the inside dimensions if you can, because wall thickness can steal a surprising amount of volume on a small tank. For a horizontal tank, measure the fill depth with a dipstick from the bottom, not by eye through a sight glass, and measure at the middle of the tank where the shape is truest. If the tank has dished ends, the tool will slightly overestimate, so treat the result as an upper figure and keep a small margin when ordering a delivery.

The Tank Volume Calculator is free, it runs in your browser, and your dimensions never leave your device. Use it before you order a delivery, dose a treatment, or plan a transfer between tanks. Choose the shape, enter the size honestly, and read the volume in the units your supplier uses. Tanks hide their volume in their shape, and this tool makes that hidden number plain, so you can stop guessing and start planning.

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