🛢️ Tank Capacity Calculator

A tank capacity calculator answers one question quickly: how much does this container hold? For a cylindrical tank the volume is V = πr²h, where r is the internal radius and h is the internal height. For a rectangular tank (aquarium, sump, cistern) it is simply V = L × W × H. The tool returns the answer in liters, US gallons, imperial gallons and cubic meters so you can order the right size water tank, estimate fuel volume, or size an aquarium before you fill it.

Aquarium keepers use this to check whether a new tank is the advertised volume; farmers use it for rainwater harvesting barrels; engineers use it for process vessels. Because the tool converts between liters, gallons and cubic meters on the fly, it also serves as a quick unit sanity check for anyone mixing metric and imperial plumbing.

The cylinder mode applies V = πr²h using the internal dimensions—wall thickness is deliberately ignored because capacity is what matters, not overall size. The rectangular mode applies V = L × W × H. All inputs are in meters.

A useful derived number is the fill weight: one liter of water weighs almost exactly 1 kg, so a 1,000-liter tank holds one tonne of water. That figure drives floor-loading decisions for rooftop cisterns and stands. If you need to know how much your tank weighs when empty, combine the shell volume with the material density in our density calculator.

1. Choose tank shape: cylindrical or rectangular.
2. Enter the internal dimensions in meters.
3. Click Calculate to get the volume in liters and gallons.
4. Check the cubic meter and cubic foot equivalents if needed.
5. Use the fill-weight line to check floor-loading limits.

Why does my 100-gallon tank only hold 95 gallons?

The nominal rating usually includes a small air gap for expansion and overflow protection. For liquid tanks, manufacturers often rate capacity to the overflow line, not the absolute brim, so the usable volume is slightly less.

How do I convert liters to gallons?

Divide liters by 3.785 for US gallons, or by 4.546 for imperial gallons. The calculator displays both because the two units differ by about 20%, and confusing them can double your fuel order.

Does the calculator include the wall thickness?

No—it uses the internal dimensions you enter. If you only know external dimensions, subtract the wall thickness twice from the diameter or width, and once from the height for an open tank.

Can I use this for a horizontal cylinder?

The full-volume formula V = πr²h works for any orientation. For partial fill of a horizontal cylinder, the liquid surface is a chord, so the filled volume is more complex and this tool does not calculate it yet.

What is the weight of water in my tank?

One liter of water weighs about 1 kg, so a 1,000-liter tank holds roughly 1,000 kg (2,205 lb) of water. Multiply the capacity in liters by 1.0 to get kilograms, then by 2.2046 for pounds.

How much capacity do I need for rainwater harvesting?

A rough rule: 1 mm of rain on 1 m² of roof yields 1 liter. A 100 m² roof receiving 30 mm of rain produces about 3,000 liters. Size your tank to capture the biggest storms you expect, balanced against the available space.

Always use internal dimensions for capacity. A 2 cm wall on a 1 m tank removes about 8% of the volume. For horizontal cylinders that are partially filled, the volume is not simply proportional to fill height because the cross-section is a circle—the low part of the tank holds less per centimeter of height than the middle. When comparing gallons, note that one US gallon is 3.785 liters but one imperial gallon is 4.546 liters; the calculator shows both. For pressurized vessels, capacity is irrelevant—you design for pressure rating, not volume.