How this calculator works
Base area times internal height gives capacity. Dividing by each gallon definition produces two different counts for the same liquid. The fill table assumes an upright cylinder: capacity then grows linearly with liquid depth. A horizontal tank needs circular-segment geometry instead. Wall thickness and headspace are excluded.
Formula
V = πr²h = πd²h/4; US gallons = V/0.003785411784; imperial gallons = V/0.00454609, with V in m³. r = sqrt(V/(πh)); h = V/(πr²).
Worked example
A cylinder with internal radius 1 m and height 1 m holds π m³, about 829.92 US liquid gallons or 691.05 imperial gallons. At half its height it holds half those amounts.
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Frequently asked questions
Which gallon should I use in Canada?
Check the specification. Canadian products may use litres, US gallons or imperial gallons; the label must identify the definition.
Can I enter a diameter?
Yes. Choose diameter and height; the calculator halves the diameter before finding the base area.
Is half depth half capacity?
For an upright straight cylinder, yes. The quarter-depth rows do not apply to a cylinder lying on its side.