Rectangular Tank Volume Calculator
Calculate the exact volume and capacity of any rectangular tank — fish tank, water storage tank, rainwater harvester, industrial sump, or swimming pool. Enter length, width, and height in any unit, and instantly see results in litres, gallons, cubic metres, cubic feet, and more.
Built by He Loves Math with mixed-unit support, a 3D tank preview, complete step-by-step working, and all the conversion tables you need alongside the calculator.
Quick Answer: The Volume Formula
The volume of any rectangular tank (also called a rectangular prism or cuboid) is found by multiplying its three interior dimensions together:
Where L = internal length, W = internal width, H = internal height. The result is in cubic units matching your measurements. To get litres from cubic metres: multiply by 1,000. To get US gallons: multiply cubic metres by 264.172.
Rectangular Tank Volume Calculator
Enter the internal length, width, and height of your tank. Each dimension can have a different unit — the calculator converts automatically. Select your preferred output units, then click Calculate Volume.
Volume — All Units
The Rectangular Tank Volume Formula — Full Explanation
A rectangular tank is geometrically a rectangular prism (also called a cuboid). It has six rectangular faces — three pairs of parallel, congruent rectangles. The volume enclosed by the six faces is the usable capacity of the tank.
The Fundamental Formula
This formula works because volume is a measure of three-dimensional space. The length gives the first dimension, the width the second, and the height the third. Multiplying them together — in any order, since multiplication is commutative — gives the volume in cubic units.
Converting to Litres
Where dimensions are in metres. This works because 1 cubic metre = 1,000 litres exactly.
Converting to US Gallons
Converting from Centimetres to Litres Directly
This is the most convenient formula for fish tank calculations because aquarium dimensions are almost always given in centimetres. Multiply the three centimetre dimensions together and divide by 1,000 to get litres directly.
Unit Conversion Reference
The calculator handles all conversions internally, but understanding the relationships between volume units helps you verify results and perform quick mental estimates.
| Unit | Equals | Equals | Equals |
|---|---|---|---|
| 1 m³ | 1,000 L | 264.172 US gal | 35.3147 ft³ |
| 1 Litre (L) | 0.001 m³ | 0.264172 US gal | 61.0237 in³ |
| 1 US Gallon | 3.78541 L | 231 in³ | 0.133681 ft³ |
| 1 UK Gallon | 4.54609 L | 1.20095 US gal | 0.160544 ft³ |
| 1 ft³ | 28.3168 L | 7.48052 US gal | 1,728 in³ |
| 1 in³ | 0.016387 L | 0.004329 US gal | 16,387.06 mm³ |
| 1 cm³ | 1 mL | 0.001 L | 0.0610237 in³ |
Worked Examples
Example 1 — Standard Fish Aquarium
An aquarium measures 120 cm × 50 cm × 50 cm (internal dimensions).
A practical tip: the usable aquarium volume is typically 10–20% less than the geometric volume due to substrate (gravel/sand), decorations, and the fact tanks are not filled to the brim. So this 300 L tank typically holds 240–270 L of water in practice.
Example 2 — Domestic Water Storage Tank
A concrete rainwater tank measures 2.4 m × 1.8 m × 1.5 m.
Example 3 — Industrial Oil Sump (Mixed Units)
A workshop oil sump is measured as: length = 72 inches, width = 48 inches, height = 24 inches.
Partial Fill Volume
A tank is often not completely full. To find the volume of liquid in a partially filled rectangular tank, replace the full height (H) with the actual liquid level (h):
The percentage capacity used is:
Example: A 300-litre tank (120 cm × 50 cm × 50 cm) has water at a depth of 35 cm. The partial volume is:
Surface Area of a Rectangular Tank
Surface area calculations are needed whenever you are buying liner material, calculating heat loss, sizing insulation, or ordering paint for a tank. There are two versions depending on whether the tank has a lid:
Example: For a fish tank 120 cm × 50 cm × 50 cm (open top):
Weight of Water in a Rectangular Tank
One of the most practically important calculations for tank installation is the total weight of a full tank — both to ensure structural support is adequate and to plan logistics for moving tanks.
For fresh water at room temperature, the density is \( \rho \approx 1.00 \text{ kg/L} \) (exactly 1 kg/L at 4°C). For salt water (aquariums), \( \rho \approx 1.025 \text{ kg/L} \). For the industrial applications involving other liquids, use the appropriate density.
Add the tank's own weight (varies enormously: a glass aquarium might weigh 20–50 kg, a concrete cistern 500–2,000 kg) to get the total load on the supporting surface.
Common Tank Sizes Reference
| Tank Type | Typical L × W × H | Volume (Litres) | Volume (US Gal) | Full Water Weight |
|---|---|---|---|---|
| Small aquarium | 60 × 30 × 30 cm | 54 L | 14.3 gal | 54 kg |
| Standard 120L aquarium | 100 × 40 × 30 cm | 120 L | 31.7 gal | 120 kg |
| Large aquarium | 150 × 60 × 60 cm | 540 L | 142.6 gal | 540 kg |
| Domestic IBC tote | 120 × 100 × 116 cm | 1,000 L (approx) | 264 gal | 1,000 kg |
| Small water tank | 2 × 1 × 1 m | 2,000 L | 528 gal | 2,000 kg |
| Above-ground pool (small) | 5 × 3 × 0.9 m | 13,500 L | 3,566 gal | 13,500 kg |
| Bathtub | 170 × 75 × 40 cm | 510 L | 134.7 gal | 510 kg |
| Standard skip bin | 350 × 180 × 120 cm | 7,560 L | 1,996 gal | N/A (dry) |
Real-World Applications of the Rectangular Tank Formula
Fish & Aquariums
The most common use of this calculator is sizing aquariums. The rule of thumb "1 cm of fish per litre of water" requires knowing exact tank volume. Hobbyists also use it to calculate how much dechlorinator, fertiliser, or medication to add — most liquid treatments are dosed per 100 litres.
Water Storage & Harvesting
Homeowners and farmers use rectangular tanks and cisterns for rainwater collection, irrigation, and emergency water reserves. Knowing exact capacity helps plan water usage schedules, estimate how long supply will last, and size pumps appropriately for the volume being moved.
Swimming Pools
Above-ground rectangular pools and commercial lap pools use this formula directly. Calculating pool volume is essential for adding the correct dose of chlorine, pH adjusters, and algaecides — all of which are volume-dependent. Under-dosing allows biological growth; over-dosing is unsafe and wasteful.
Industrial & Engineering
Engineers calculate tank volumes for: chemical storage sizing, fuel tank design, effluent holding capacity, hydraulic reservoir sizing, and food processing vats. Each industry has safety regulations tied to stored volumes (e.g., secondary containment requirements for tanks over certain capacities).
Construction & HVAC
Buffer tanks in HVAC systems store hot or chilled water to smooth demand peaks. The tank volume determines how long the system can supply heating or cooling during a demand spike without the chiller or boiler cycling on and off — improving efficiency and reducing equipment wear.
Shipping & Logistics
Liquid cargo in rectangular ISO tank containers is measured by volume, then converted to mass using liquid density. Overloading ship compartments or trucks is controlled by calculating maximum allowable volume at a given liquid density, directly from the container's rectangular geometry.
Frequently Asked Questions
What is the formula for the volume of a rectangular tank?
The formula is \( V = L \times W \times H \), where L is the internal length, W is the internal width, and H is the internal height of the tank. The result is in cubic units matching your input unit. Always measure internal dimensions — exclude the wall thickness — to get the usable capacity of the tank.
How do I convert cm³ to litres?
Divide cubic centimetres (cm³) by 1,000 to get litres: \( V_L = V_{cm^3} / 1{,}000 \). This is exact — 1 litre is defined as 1,000 cm³. So a fish tank measuring 100 cm × 50 cm × 40 cm has a volume of 100 × 50 × 40 = 200,000 cm³ = 200 litres.
How do I convert tank volume to US gallons?
Multiply litres by 0.264172 to get US liquid gallons. Or multiply cubic metres by 264.172. Or multiply cubic feet by 7.48052. Example: a 500-litre tank holds 500 × 0.264172 = 132.1 US gallons.
What is the partial fill formula for a rectangular tank?
For a partially filled rectangular tank, use \( V_{\text{fill}} = L \times W \times h \), where h is the current depth of liquid (not the tank's full height H). The percentage full is \( (h / H) \times 100\% \). This is accurate for any liquid level in a rectangular tank because the cross-sectional area (L × W) is constant throughout the height.
How do I find the volume of a fish tank in litres?
Measure the internal length, width, and height of the aquarium in centimetres. Apply \( V = \frac{L \times W \times H}{1{,}000} \) to get litres directly. Note that the actual water volume is typically 10–20% less than the geometric volume due to substrate, decorations, and the fact that tanks are not filled to the top.
How much does a full rectangular tank weigh?
Water weighs approximately 1 kg per litre at room temperature. So the water weight = volume in litres × 1.00 kg/L. Add the tank's structural weight to get the total load on the supporting surface. A 300-litre aquarium filled with fresh water adds 300 kg of water weight alone — equivalent to about 3–4 adults standing on the stand.
What is the surface area formula for a rectangular tank?
For a closed rectangular tank: \( SA = 2(LW + LH + WH) \). For an open-top tank (no lid): \( SA_{\text{open}} = LW + 2H(L + W) \). Surface area is needed when buying liner material, calculating heat loss through tank walls, sizing insulation, or computing paint coverage for external surfaces.
Should I measure inside or outside the tank?
Always measure inside the tank for volume calculations. The internal dimensions give you the usable capacity — the space actually available for liquid. Measuring external dimensions and not accounting for wall thickness will overestimate the tank's capacity. For glass aquariums, typical glass thickness is 8–12 mm per side, so subtract this from each external dimension to get internal dimensions.
