Tank Volume Calculator › Tank Volume Conversion
Tank Volume Conversion
Once you have calculated or measured a tank's volume, convert it between the common capacity units — US gallons, imperial gallons, litres, cubic metres, cubic feet and US oil barrels — using the exact factors, quick tables and worked tank examples below. Every figure on this page is for tank volume and tank capacity specifically, using the US liquid gallon (231 cubic inches), not the dry gallon.
These conversions are exact unit definitions. The accuracy of your final figure still depends on how accurately the underlying tank volume was calculated or measured — see calculated volume vs usable capacity below.
Convert tank volume between units
The table below gives the exact factor for one unit of each common tank capacity unit. To convert, multiply your volume by the factor in the column you want. All values use the US liquid gallon and the petroleum (US oil) barrel of 42 US gallons.
| 1 unit of… | US gallons | Imperial gallons | Litres | Cubic metres | Cubic feet | US oil barrels |
|---|---|---|---|---|---|---|
| US gallon | 1 | 0.832674 | 3.785412 | 0.003785 | 0.133681 | 0.023810 |
| Imperial gallon | 1.200950 | 1 | 4.546090 | 0.004546 | 0.160544 | 0.028594 |
| Litre | 0.264172 | 0.219969 | 1 | 0.001 | 0.035315 | 0.006290 |
| Cubic metre | 264.172 | 219.969 | 1,000 | 1 | 35.3147 | 6.28981 |
| Cubic foot | 7.480519 | 6.228835 | 28.316847 | 0.028317 | 1 | 0.178108 |
| US oil barrel (42 gal) | 42 | 34.9723 | 158.987295 | 0.158987 | 5.614583 | 1 |
US-gallon tanks in other units
Common round tank capacities in US gallons, converted to litres, cubic metres, cubic feet and US oil barrels.
| US gallons | Litres | Cubic metres | Cubic feet | US oil barrels |
|---|---|---|---|---|
| 100 | 378.5 | 0.379 | 13.37 | 2.38 |
| 500 | 1,892.7 | 1.893 | 66.84 | 11.9 |
| 1,000 | 3,785.4 | 3.785 | 133.68 | 23.81 |
| 5,000 | 18,927.1 | 18.927 | 668.4 | 119.05 |
Metric tanks in US and imperial units
Tank volumes given in litres or cubic metres, converted to US gallons, imperial gallons and cubic feet.
| Litres | Cubic metres | US gallons | Imperial gallons | Cubic feet |
|---|---|---|---|---|
| 1,000 | 1 | 264.2 | 220 | 35.31 |
| 5,000 | 5 | 1,320.9 | 1,099.8 | 176.57 |
| 10,000 | 10 | 2,641.7 | 2,199.7 | 353.15 |
| 25,000 | 25 | 6,604.3 | 5,499.2 | 882.87 |
Quick conversion formulas
The eight conversions you are most likely to need for a tank volume, each with its exact factor:
US gallons → litres litres = US gallons × 3.785411784
litres → US gallons US gallons = litres ÷ 3.785411784
cubic feet → US gallons US gallons = cubic feet × 7.480519481
cubic metres → litres litres = cubic metres × 1000
cubic metres → US gallons US gallons = cubic metres × 264.172052
US oil barrels → US gallons US gallons = barrels × 42
imperial gallons → litres litres = imperial gallons × 4.54609
US gallons → imperial gallons imperial gallons = US gallons × 0.832674
Worked tank examples
Rectangular tank: cubic feet → US gallons
A rectangular tank measuring 6 ft long × 4 ft wide × 5 ft high has a volume of 6 × 4 × 5 = 120 cubic feet. Multiplying by 7.480519481 gives 120 × 7.480519481 ≈ 897.7 US gallons. That is the full-shell figure; deduct for fittings and freeboard to get usable capacity.
Vertical cylindrical tank: cubic metres → litres
A vertical cylindrical tank computed at 12.5 cubic metres holds 12.5 × 1000 = 12,500 litres. If you also need US gallons, 12.5 × 264.172052 ≈ 3,302 US gallons.
Dip-chart reading: US gallons → litres
Suppose a dip chart or the tank dip chart generator tells you the current level is 250 US gallons. Converting the measured (partial) volume: 250 × 3.785411784 ≈ 946.4 litres. Because unit conversion is linear, a partial-fill volume converts exactly like a full one.
US gallons → imperial gallons
A tank rated at 500 US gallons is 500 × 0.832674 ≈ 416.3 imperial gallons — clearly not the same as 500 imperial gallons, which is why the gallon type must always be stated.
US gallons → US oil barrels
For petroleum storage, 1,000 US gallons ÷ 42 ≈ 23.81 US oil barrels, using the 42-gallon petroleum barrel defined below.
US gallons vs imperial gallons
The two gallons are different sizes and are the most common source of tank-capacity conversion errors. A US liquid gallon is 3.785411784 litres (231 cubic inches); an imperial gallon is 4.54609 litres. An imperial gallon is therefore about 20% larger than a US gallon, and a US gallon is about 17% smaller than an imperial gallon.
For a tank, this matters directly: 1,000 US gallons is about 833 imperial gallons, and 1,000 imperial gallons is about 1,201 US gallons. Always record which gallon a capacity is quoted in. In the United States, tank capacity is normally in US gallons; in the UK and parts of the Commonwealth you may see imperial gallons or litres.
US oil barrels and other "barrel" units
When this page refers to barrels, it means the US oil (petroleum) barrel of exactly 42 US gallons (about 158.99 litres), the standard unit for crude oil and many petroleum products. It is easy to confuse with other "barrel" units, so for tank conversions use only the 42-gallon definition unless a specific industry tells you otherwise:
- US oil barrel — 42 US gallons (petroleum). Used on this page.
- US liquid barrel — 31.5 US gallons; a general liquid measure, not the oil barrel.
- US beer barrel — 31 US gallons; brewing-specific.
- Imperial barrel — 36 imperial gallons; a different system entirely.
Mixing these up changes a tank figure by 10–35%, so confirm the barrel definition before converting.
Calculated volume vs usable tank capacity
Converting units never makes an estimate more certain. A calculated tank volume is the ideal geometric volume of the tank shell; the usable capacity is what the tank actually holds and can dispense, and it is generally smaller. Real tanks differ from ideal geometry because of:
- wall and insulation thickness (inside vs outside dimensions),
- internal fittings, baffles, heating coils and draw-off pipes,
- rounded corners and weld seams not captured by a simple shape,
- manufacturing tolerances and in-service deformation or dents,
- usable fill limits — freeboard left for thermal expansion, venting, or an unusable heel below the outlet.
So treat a converted figure as a planning estimate. For billing or custody transfer, use a certified gauge or a measured strapping table rather than a converted geometric volume. The conversion factors here follow standard unit definitions, set out in full on the methodology page.
How this connects to the tank calculators
This page converts a volume you already have. To produce that volume in the first place, use the tank volume calculator on the homepage, which reports results in US gallons, imperial gallons, litres, cubic metres and cubic feet directly. For specific shapes, see the horizontal cylindrical, vertical cylindrical and rectangular tank calculators. To convert a level reading to a volume first, build a depth-to-volume table with the tank dip chart generator, and for the formulas and exact unit definitions behind every result, see the methodology page.
Common mistakes
- Confusing US and imperial gallons: they differ by about 20%. Always note which gallon a capacity is in.
- Confusing US oil barrels with other barrels: use 42 US gallons for the petroleum barrel, not 31.5, 31 or 36.
- Converting dimensions instead of volume: convert the computed volume, not the length, width or height. Convert dimensions to one unit system first, then compute the volume.
- Rounding too early: keep full precision through the calculation and round only the final answer, or small errors compound.
- Mixing inside and outside dimensions: capacity comes from internal dimensions; outside dimensions overstate it.
- Treating an estimate as certified capacity: a converted geometric volume is a planning figure, not a custody-transfer measurement.
Frequently asked questions
How do I convert tank volume from cubic feet to gallons?
Multiply the volume in cubic feet by 7.480519481 to get US liquid gallons (the exact factor is 1,728 ÷ 231, because a cubic foot is 1,728 cubic inches and a US gallon is 231 cubic inches). For example, a tank holding 120 cubic feet contains 120 × 7.480519481 ≈ 897.7 US gallons. For imperial gallons, multiply cubic feet by 6.228835 instead.
How many litres are in a US gallon?
One US liquid gallon is exactly 3.785411784 litres. This is the US gallon used for tank capacity in the United States — not the US dry gallon, which is a different and rarely used unit. To go the other way, one litre is about 0.264172 US gallons.
How many litres are in an imperial gallon?
One imperial (UK) gallon is exactly 4.54609 litres. It is noticeably larger than the US gallon (3.785411784 litres), so the two must never be mixed when reading or converting a tank capacity.
How many gallons are in a cubic metre?
One cubic metre holds 1,000 litres, which is about 264.172 US gallons or about 219.969 imperial gallons. So a 12 m³ tank holds roughly 3,170 US gallons or about 2,640 imperial gallons.
Should I use US gallons or imperial gallons for tank capacity?
Use whichever your tank, supplier and region are specified in, and label it clearly. A US gallon is about 17% smaller than an imperial gallon, so a tank rated at 500 US gallons is not the same as 500 imperial gallons. In the United States, tank capacity is normally in US gallons; in the UK and some Commonwealth countries it may be in imperial gallons or litres.
Can I convert partial-fill tank volume?
Yes. Unit conversion is linear, so a partial-fill volume converts exactly the same way as a full-tank volume. If a dip chart or shape calculator tells you the tank currently holds 250 US gallons, that is 250 × 3.785411784 ≈ 946.4 litres, regardless of how full the tank is. Convert the actual volume you measured, not the tank's rated capacity.
What is the difference between calculated tank volume and usable tank capacity?
Calculated volume is the ideal geometric volume of the tank shell. Usable capacity is what the tank can actually hold and dispense, which is generally smaller because of wall thickness, internal fittings, baffles, rounded corners, manufacturing tolerances, deformation, and fill limits left for thermal expansion or venting. Converting between units does not change this: an estimate stays an estimate in any unit. For billing or custody transfer, rely on a certified gauge or strapping table rather than a converted geometric figure.
Disclaimer: The conversion factors here are exact unit definitions, but any tank volume you convert is only as accurate as the calculation or measurement behind it. Use converted figures for planning and estimating, not for billing or custody-transfer measurement.