Tank Size Calculator
Calculate the volume of rectangular, cylindrical or oval tanks instantly. Results in litres, UK gallons, US gallons and cubic metres. Perfect for fish tanks, aquariums, water storage tanks, hot water cylinders and oil tanks.
Tank Volume Calculator
An oval (elliptical) tank has two radii — the semi-major axis (half the long width) and semi-minor axis (half the short width).
Common Tank Sizes — Quick Reference
Use this reference table to find the volume of standard fish tank, aquarium and storage tank sizes used in the UK.
Fish Tank & Aquarium Sizes
| Tank Description | Dimensions (L × W × H cm) | Volume (litres) | UK Gallons | Approx. Fish Capacity |
|---|---|---|---|---|
| Nano / Starter tank | 35 × 20 × 25 | 17.5 litres | 3.8 gal | 3–5 small fish (e.g. neon tetras) |
| Small fish tank | 60 × 30 × 30 | 54 litres | 11.9 gal | ~5–8 small fish |
| Medium fish tank | 90 × 40 × 35 | 126 litres | 27.7 gal | ~10–15 medium fish |
| Standard 4ft tank | 120 × 45 × 45 | 243 litres | 53.5 gal | ~20–25 fish |
| Large fish tank | 120 × 40 × 40 | 192 litres | 42.2 gal | ~18–22 fish |
| 6ft display tank | 180 × 60 × 60 | 648 litres | 142.5 gal | ~50+ fish (depends on species) |
| Jewel Rekord 70 | 61 × 31 × 37 | 70 litres | 15.4 gal | ~8–10 small fish |
| Jewel Rio 125 | 81 × 36 × 43 | 125 litres | 27.5 gal | ~12–15 fish |
| Jewel Rio 240 | 121 × 41 × 51 | 253 litres | 55.7 gal | ~25–30 fish |
Hot Water Tank (Cylinder) Sizes — UK Homes
| Household Size | Cylinder Volume | Typical Dimensions | Suitable For |
|---|---|---|---|
| 1–2 people | 120 litres | ø45cm × 90cm | Flat, studio or small house with 1 bathroom |
| 2–3 people | 150 litres | ø45cm × 105cm | 2-bed house with 1–2 bathrooms |
| 3–4 people | 180 litres | ø45cm × 125cm | 3-bed house with 1–2 bathrooms |
| 4–5 people | 210 litres | ø45cm × 145cm | 4-bed house with 2 bathrooms |
| 5+ people / Heat pump | 250–300 litres | ø50cm × 150cm+ | Large homes, heat pump systems (need larger buffer) |
Water Storage Tank Sizes
| Tank Type | Volume | UK Gallons | Common Use |
|---|---|---|---|
| Water butt (small) | 100–200 litres | 22–44 gal | Garden rainwater collection |
| Water butt (large) | 500 litres | 110 gal | Large garden, allotment |
| Loft tank (cold water storage) | 50–100 litres | 11–22 gal | Older UK homes with gravity-fed systems |
| IBC tote | 1,000 litres | 220 gal | Commercial water storage, agriculture |
| Domestic oil tank | 1,000–2,500 litres | 220–550 gal | Home heating oil storage |
| Large storage tank | 5,000–10,000 litres | 1,100–2,200 gal | Farm, commercial, fire suppression |
How to Calculate Tank Volume (Step-by-Step)
Rectangular Tank Volume
Most fish tanks, water storage tanks and oil tanks are rectangular (also called cuboid or box-shaped). The formula is:
Volume (litres) = Length (cm) × Width (cm) × Height (cm) ÷ 1,000
Example: A fish tank measuring 90cm long, 40cm wide and 35cm tall:
- Volume in cm³ = 90 × 40 × 35 = 126,000 cm³
- Convert to litres: 126,000 ÷ 1,000 = 126 litres
- Convert to UK gallons: 126 ÷ 4.546 = 27.7 UK gallons
Cylindrical Tank Volume
Round or cylindrical tanks (vertical or horizontal) use the circle area formula:
Volume (litres) = π × radius² (cm) × height (cm) ÷ 1,000
Example: A cylindrical water butt with diameter 50cm (radius 25cm) and height 120cm:
- Volume = 3.14159 × 25² × 120
- = 3.14159 × 625 × 120 = 235,619 cm³
- Convert to litres: 235,619 ÷ 1,000 = 235.6 litres
Oval (Elliptical) Tank Volume
Oval tanks, such as some oil tanks and specialist aquariums, use the ellipse area formula:
Where a = semi-major axis (half the long width), b = semi-minor axis (half the short width), h = height.
Unit Conversions for Tank Volume
When measuring in different units, convert to centimetres first for simplicity, or use the multipliers below:
| From | To | Multiply by |
|---|---|---|
| Cubic centimetres (cm³) | Litres | ÷ 1,000 |
| Cubic metres (m³) | Litres | × 1,000 |
| Litres | UK (Imperial) gallons | ÷ 4.546 |
| Litres | US gallons | ÷ 3.785 |
| UK gallons | Litres | × 4.546 |
| US gallons | Litres | × 3.785 |
| Litres | Cubic metres | ÷ 1,000 |
| Cubic feet | Litres | × 28.317 |
| Inches | Centimetres | × 2.54 |
| Feet | Centimetres | × 30.48 |
Litres vs Gallons: UK and US Differences
One of the most common points of confusion when calculating tank volume is the difference between UK (imperial) gallons and US gallons. These are not the same and using the wrong one will give an incorrect result.
- 1 UK (Imperial) gallon = 4.546 litres
- 1 US gallon = 3.785 litres
- 1 UK gallon is approximately 20% larger than 1 US gallon
This matters when buying aquarium equipment. A US fish tank labelled "20 gallons" holds 75.7 litres, while a UK "20 gallon" tank holds 90.9 litres. Most fish tank sizes sold in the UK are described in litres to avoid this confusion, but older guides and some imported products still use US gallons.
| Litres | UK Gallons | US Gallons | Cubic Metres |
|---|---|---|---|
| 10 L | 2.2 gal | 2.6 gal | 0.010 m³ |
| 25 L | 5.5 gal | 6.6 gal | 0.025 m³ |
| 50 L | 11.0 gal | 13.2 gal | 0.050 m³ |
| 100 L | 22.0 gal | 26.4 gal | 0.100 m³ |
| 200 L | 44.0 gal | 52.8 gal | 0.200 m³ |
| 500 L | 110.0 gal | 132.1 gal | 0.500 m³ |
| 1,000 L | 219.9 gal | 264.2 gal | 1.000 m³ |
| 2,500 L | 549.9 gal | 660.4 gal | 2.500 m³ |
Fish Tank Capacity Guide
Knowing your tank's volume in litres is essential for fishkeeping. It determines how many fish you can keep, how much water conditioner to add, medication doses, and the size of filter you need.
The "1 Inch Per Gallon" Rule
The traditional fishkeeping guideline is "1 inch of fish per gallon of water" (using US gallons). In metric terms, this roughly equals 1 cm of adult fish body length per litre. However, this rule is outdated and overly simplistic. It does not account for:
- Fish body shape — a 3-inch goldfish produces far more waste than a 3-inch neon tetra
- Territorial behaviour — some fish need more space regardless of size
- Filter efficiency — a well-filtered tank can support more fish
- Surface area — oxygen exchange depends on surface area, not just volume
A more conservative and widely accepted modern guideline is 1 cm of adult fish per 2 litres for most tropical community fish, with research into the specific species always recommended.
| Tank Volume | Conservative Stocking (1cm per 2L) | Examples of Suitable Fish | Min. Recommended Size |
|---|---|---|---|
| 20–30 litres | 10–15 cm total | Nano shrimp, small tetras, betta (solo) | Nano / shrimp tank |
| 54 litres | ~25 cm total | 5–6 neon tetras, 3–4 platies | Starter community tank |
| 100–125 litres | ~50 cm total | Tetras, corydoras, guppies, small cichlids | Community tank |
| 200–250 litres | ~100 cm total | Medium cichlids, angelfish, larger community | Medium community / cichlid |
| 500+ litres | ~250 cm total | Oscars, large cichlids, shoaling species | Large specimen / predator tank |
Water Weight
Fresh water weighs approximately 1 kg per litre. This means a fully filled 100-litre aquarium, including the tank glass, gravel and equipment, can weigh 130–150 kg. Always ensure your floor and furniture can support the total weight before filling a large tank. The calculator above shows the water weight at the fill level you select.
Frequently Asked Questions
For a rectangular tank: multiply Length × Width × Height (all in centimetres), then divide by 1,000 to get litres. Example: 60cm × 30cm × 30cm = 54,000 cm³ ÷ 1,000 = 54 litres. For a cylindrical tank: π × radius² × height (cm), then divide by 1,000. Use the calculator above for instant results in litres, UK gallons, US gallons and cubic metres.
Common fish tank volumes in the UK: a small 60×30×30cm tank holds 54 litres; a medium 90×40×35cm tank holds 126 litres; a standard 4ft (120×45×45cm) tank holds 243 litres; a large 6ft (180×60×60cm) tank holds 648 litres. Most popular beginner tanks are between 60 and 125 litres.
Divide the number of litres by 4.546 to get UK (imperial) gallons. Example: 100 litres ÷ 4.546 = 21.99 UK gallons. To convert UK gallons back to litres, multiply by 4.546. Important: US gallons are smaller (1 US gallon = 3.785 litres). If a fish tank guide uses US gallons, divide litres by 3.785 instead.
As a guide: 120 litres for 1–2 people; 150 litres for 2–3 people; 180 litres for 3–4 people; 210 litres for 4–5 people; 250+ litres for 5+ people or a heat pump system. Heat pumps require larger cylinders than gas boilers because they heat water more slowly. Your heating engineer will calculate the exact size based on your hot water demand and the type of system installed.
A conservative modern guideline is 1 cm of adult fish body length per 2 litres of water for tropical community fish. So a 100-litre tank can house approximately 50 cm of fish — for example, 10 fish that grow to 5 cm each. Always research the specific species: some fish are very territorial, produce a lot of waste, or need greater swimming space regardless of their size. The filtration capacity of your filter also determines your tank's carrying capacity.
Use the calculator above and select "Inches" from the measurement unit dropdown — it will convert automatically to litres and gallons. Alternatively, convert inches to centimetres first by multiplying by 2.54 (e.g. 24 inches = 60.96 cm), then apply the standard formula. A 24×12×12 inch tank = 60.96×30.48×30.48 cm = 56,634 cm³ = 56.6 litres ≈ 15 US gallons.
Fresh water weighs 1 kg per litre. A fully filled 100-litre tank holds 100 kg of water, plus the weight of the glass or acrylic tank (typically 10–25 kg), substrate and decorations (5–20 kg), and equipment. A 100-litre aquarium when fully set up commonly weighs 120–150 kg. Always check your floor load capacity and ensure your aquarium stand is rated for the total weight before filling large tanks.
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Mustafa Bilgic
Maths & Science Specialist — UK Calculator
Mustafa specialises in practical maths tools for home, DIY and science applications. He develops and maintains volume, area and unit conversion calculators at UK Calculator. Learn More about Mustafa Bilgic.