Rectangular Tank Formulas Explained
I built this calculator for anyone sizing a fish tank, rain water collection tank, storage container, or concrete pour. Rectangular tanks (also called cuboids or rectangular prisms) are the most common shape for tanks, bins, and boxes — and their formulas are elegantly straightforward.
The Key Formulas
- Volume:
V = length × width × height - Total surface area:
SA = 2(lw + lh + wh) - Capacity in litres (metric):
litres = V in cm³ / 1000 - Capacity in gallons (US):
gallons = V in inches³ / 231 - Space diagonal:
d = √(l² + w² + h²)
The unit conversion for volume is worth remembering: 1 litre = 1000 cm³, and 1 cubic metre = 1000 litres. This makes litres the most practical unit for tank capacity in metric contexts.
Real-World Uses for Tank Calculations
Knowing the exact capacity of a rectangular tank matters in a surprising number of situations — from calculating the weight of water in a fish tank (to ensure the floor can support it) to ordering the right amount of concrete for a rectangular form.
- Aquariums: a standard 120 cm × 45 cm × 45 cm aquarium holds approximately 243 litres — and weighs around 243 kg when full
- Rainwater tanks: calculate whether a tank provides enough storage for a given roof catchment area
- Concrete formwork: volume gives the exact quantity of concrete to order, avoiding waste or shortfall
- Shipping containers: internal volume determines maximum cargo load by size
Frequently Asked Questions
How do I convert cubic centimetres to litres?
Divide the volume in cm³ by 1000. For example, a tank measuring 60 cm × 30 cm × 40 cm has a volume of 60 × 30 × 40 = 72,000 cm³, which equals 72,000 / 1000 = 72 litres. This works because one litre is defined as exactly 1000 cm³ (one cubic decimetre).
How much does a full tank weigh?
One litre of water weighs almost exactly 1 kg. So a tank with a capacity of 72 litres, when full of water, weighs approximately 72 kg plus the weight of the tank itself. For saltwater (like marine aquariums), use approximately 1.025 kg per litre. This calculation is critical for checking that shelves, floors, and stands can support the load.
What is the difference between volume and capacity?
Volume is the total amount of space a three-dimensional object occupies. Capacity specifically refers to the amount a container can hold. For a tank with walls, the internal capacity is slightly less than the external volume. In most practical calculations with thin-walled tanks, the difference is negligible and the two terms are used interchangeably.