Aquarium Size Calculator
Type the length, width and height of your tank and how deep the gravel or sand will be. You get the real water volume, the filled weight, the filter flow and a sensible number of small fish.
- Real water volume
- Filled weight for your stand
- Filter and heater sizing
Weight includes water, glass and substrate. Add the cabinet and any rockwork.
A tank never holds its full rated volume. Gravel, rocks, wood and the gap below the rim usually take 15% to 20% of the space. A tank sold as 120 litres often holds about 100 litres of water, and every dose of medicine or water conditioner should be based on that lower figure.
Common aquarium sizes
Standard tanks and what they really hold. Search by gallons, litres or dimensions.
| Tank | Size (cm) | Gross (L) | Water (L) | Full weight (kg) |
|---|---|---|---|---|
| Nano 5 gal | 41 × 21 × 25 | 21 | 16 | 26 |
| 10 gal | 51 × 26 × 32 | 42 | 34 | 52 |
| 20 gal long | 76 × 31 × 32 | 75 | 60 | 95 |
| 29 gal | 76 × 31 × 47 | 111 | 94 | 140 |
| 40 gal breeder | 91 × 46 × 41 | 172 | 140 | 210 |
| 55 gal | 122 × 33 × 53 | 213 | 180 | 280 |
| 75 gal | 122 × 46 × 53 | 297 | 250 | 385 |
| 125 gal | 183 × 46 × 56 | 471 | 400 | 640 |
| 60 L UK | 60 × 30 × 35 | 63 | 52 | 80 |
| 180 L UK | 100 × 40 × 45 | 180 | 150 | 230 |
US tanks are sold by gallon and UK and European tanks by litre. The same footprint can be sold under both labels, so always check the actual dimensions.
Longer tank, more water
Width 30 cm and water depth 35 cm here. Drag to change the length and see water, heater and filter needs.
Length adds water without making the tank deeper, which matters because surface area controls how much oxygen gets in. For the same volume, a long low tank keeps fish healthier than a tall narrow one.
How to measure an aquarium
Inside measurements give the true volume. Use a tape measure and follow these steps.
Measure the inside length
Run the tape along the bottom from glass to glass.
Glass on most tanks is 4 to 12 mm thick. On a 120 cm tank with 10 mm glass, measuring outside instead of inside overstates the volume by about 5%.
Freshwater, planted or marine
Same tank, different needs. Pick a setup to see how it changes equipment.
Tetras, rasboras and catfish. Filter turnover of four times the volume per hour suits most fish.
Dense plants use waste and need gentle flow so they do not get pushed around.
Live rock fills a large part of the tank and corals need strong, varied flow.
Marine tanks lose more volume to live rock and a sump, and they need much stronger water movement. Planted tanks need less filtration but more light.
Before you fill the tank
Water is heavy and mistakes are hard to undo. Check these first.
Tick each item as you go.
A 200 litre tank weighs over 250 kg when set up. Place it along a load-bearing wall or across floor joists, not in the middle of a span.
Numbers you can use
Each result answers a buying or dosing question. Each tile explains one result.
The real litres after substrate, decor and the top gap.
Water, glass and substrate together, for the stand and floor.
Four tank volumes per hour as a starting point.
About 1 watt per litre in a heated home.
A cautious guide for small fish, not a target to fill.
Use the water volume for all medication and conditioner doses. Dosing by the gross tank size is a common cause of overdosing.
Frequently Asked Questions
No question matches that search.
How do I calculate aquarium volume in litres?
Multiply length × width × height in centimetres and divide by 1,000. An 80 × 35 × 45 cm tank is 126 litres gross.
How do I convert litres to gallons?
Multiply litres by 0.264 for US gallons or by 0.22 for imperial gallons.
How much does a full aquarium weigh?
Roughly 1.2 to 1.3 kg per litre of tank once glass, water and gravel are included. A 55 gallon tank weighs about 280 kg.
How many fish can I keep?
The old one inch of fish per gallon rule only works for small slim fish. Treat the result as an upper limit and stock slowly.
What size filter do I need?
Most freshwater tanks do well with a filter rated at four to five times the tank volume per hour. Marine and cichlid tanks need more.
What size heater do I need?
About 1 watt per litre in a home kept at 20°C. Use more in a cold room, or two smaller heaters for safety.
Does substrate reduce water volume?
Yes. Five centimetres of gravel in an 80 × 35 cm tank takes 14 litres of space.
Plan for the water, not the label
Measure inside the glass, allow for substrate and decor, and base every dose, filter and heater on the real water volume. Check the filled weight against your stand and floor before you start filling.
More size calculators
Each one has its own calculator, worked examples and answers to common questions.