Almost every decision in fish keeping starts with a number: how many litres are in the tank? The volume tells you how much water treatment to add, how big a filter to buy, how large a water change should be and how heavy the finished tank will be. The sum is simple for a rectangular tank, but there are several traps: using the wrong units, measuring the outside instead of the inside, forgetting that the water does not reach the top and confusing US and UK gallons. This guide shows the formulas, works through examples in metric and imperial units, covers tanks that are not boxes and explains why your real water volume is smaller than the figure on the box.
The basic formula
For a rectangular tank, volume is length times width times height. The unit depends on what you measure in.
| You measure in | Formula | Result |
|---|---|---|
| Centimetres | L × W × H ÷ 1,000 | Litres |
| Millimetres | L × W × H ÷ 1,000,000 | Litres |
| Inches | L × W × H ÷ 231 | US gallons |
| Inches | L × W × H ÷ 277.42 | UK (imperial) gallons |
Why 1,000? Because one litre is 1,000 cubic centimetres. Why 231? Because a US liquid gallon is defined as exactly 231 cubic inches (3.785411784 litres). The UK gallon is a different, larger unit of 4.54609 litres, which is why a tank has a smaller number in UK gallons than in US gallons. The Aquarium Volume Calculator handles the conversions for you.
Worked examples
A metric tank. Inside length 100 cm, width 40 cm, water height 45 cm. Volume = 100 × 40 × 45 ÷ 1,000 = 180 litres. In US gallons that is 180 ÷ 3.785 = 47.6, and in UK gallons 180 ÷ 4.546 = 39.6.
An imperial tank. Inside 48 in long, 13 in wide and 21 in high. 48 × 13 × 21 = 13,104 cubic inches. ÷ 231 = 56.7 US gallons. In litres that is 56.7 × 3.785 = 214.7.
A small tank. 30 cm × 20 cm × 25 cm: 15,000 cubic centimetres, or 15 litres, about 4.0 US gallons.
Write down each measurement before you multiply, and check the units. A mix of centimetres and inches is the most common error.
Measure the inside, and measure the water
The listing on the box usually gives outside dimensions. Glass thickness varies, often from a few millimetres in small tanks to a centimetre or more in large ones, and it takes space from both ends and both sides. For an accurate result, measure inside: length from glass to glass, width from front to back and water depth from the substrate surface to the water line, or from the bottom of the tank to the water line if you want the gross volume.
| Measure | Outside | Inside |
|---|---|---|
| Length | 100 cm | 98 cm |
| Width | 40 cm | 38 cm |
| Height | 50 cm | 48 cm |
| Volume | 200 L | 178.7 L |
In the example, 12-mm glass reduces length and width by 2 cm each and height by 2 cm, so the inside volume is about 11% below the outside figure. For a large tank, the difference can be significant. The Aquarium Water Change Calculator works from the real volume, so use your best measurement.
The water does not reach the top
Tanks are not filled to the brim. A gap of a couple of centimetres below the rim stops splashing and leaves room for a lid or hood. Use the water height, not the glass height. A tank that is 50 cm high filled to 46 cm has 92% of the brim volume. If you do not want to measure, estimate a gap of 3 to 5 cm.
Things in the tank displace water
Substrate, rocks, wood, pumps, heaters and decorations take up space that water would otherwise fill. The effect is usually an overall reduction of 10 to 20% or more, depending on how much hardscape you use. A tank with a deep sand bed and lots of rock can hold far less water than the nominal figure. Measuring the real volume is easy: fill the tank from a bucket of known size and count the buckets. Otherwise, treat the calculated number as an upper limit and be careful when dosing treatments for the first time. When in doubt, measure out the amount for the smaller volume or follow the label, since many treatments say how to adjust.
Tanks that are not boxes
Cylinders. Volume = π × radius² × water height. A radius of 20 cm and a height of 50 cm give 3.1416 × 400 × 50 = 62,832 cm³, which is 62.8 litres.
Bow-front tanks. The front is curved, so volume is more than the box and less than the widest width would suggest. A reasonable approach is to measure the maximum width, the width at the ends and average them, but a better one is to fill and count.
Hexagons and corner tanks. Calculate the floor area from the shape, then multiply by water height. A hexagon with side s has an area of 2.598 × s².
Pentagon or custom shapes. Use the floor area × height if the walls are straight; otherwise measure by filling.
For irregular tanks the filling method is the most reliable. Use a measuring jug or a bucket of known volume, and note how many go in.
Turning volume into decisions
Once you have the number, use it.
- Water treatments. Measure out treatments by the label for your volume, adjusting for what you know of displacement.
- Filter. Many filters are rated by tank volume, and some owners aim for a flow of several times the tank volume per hour. Our Aquarium Filter Turnover Calculator gives the turnovers.
- Water changes. A 25% change of a 180 L tank is 45 L. The Aquarium Water Change Calculator does it.
- Weight. A litre of water weighs about one kilogram, so 180 litres is about 180 kg before glass, stand and substrate. See how much a fish tank weighs.
- Stocking. Volume is one factor in how many fish a tank can hold, along with filtration, fish size and species; ask the shop or a specialist.
Conversion table
| Litres | US gallons | UK gallons |
|---|---|---|
| 20 | 5.3 | 4.4 |
| 40 | 10.6 | 8.8 |
| 60 | 15.9 | 13.2 |
| 100 | 26.4 | 22.0 |
| 150 | 39.6 | 33.0 |
| 200 | 52.8 | 44.0 |
| 300 | 79.3 | 66.0 |
| 500 | 132.1 | 110.0 |
The conversions use 3.785 litres to a US gallon and 4.546 to a UK gallon. Our guide to fish tank size chart in gallons lists the common tank sizes with their dimensions.
Common mistakes
- Mixing centimetres and inches.
- Using the outside dimensions.
- Using the glass height, not the water height.
- Forgetting the displacement of substrate and decor.
- Confusing US and UK gallons.
- Trusting the “nominal” gallons on the box.
- Rounding too early.
Questions to ask before you buy a tank or treatment
- Are the dimensions inside or outside?
- What is the actual water volume?
- How much does the glass weigh and how thick is it?
- How is the filter rated?
- Which gallon does the label mean?
A worked step-by-step example with real measurements
Suppose you buy a second-hand tank, and want to add a treatment in the correct amount. The tank’s outside dimensions are 90 cm long, 35 cm wide and 45 cm high, with glass of about 6 mm.
- Inside length: 90 − 2 × 0.6 = 88.8 cm.
- Inside width: 35 − 2 × 0.6 = 33.8 cm.
- Inside height: 45 − 0.6 (base) = 44.4 cm.
- Gross inside volume: 88.8 × 33.8 × 44.4 ÷ 1,000 = 133.3 litres.
- Water level 4 cm below the top rim: height of water 40.4 cm. Volume: 88.8 × 33.8 × 40.4 ÷ 1,000 = 121.3 litres.
- Subtract an estimate for substrate and decor, say 10%: about 109 litres.
So the tank sold as 142 litres on outside dimensions (90 × 35 × 45 ÷ 1,000 = 141.75) holds about 109 litres of real water, a difference of nearly a quarter. If you measured out the treatment for 142 litres, you would be giving about 30% too much. This is why measuring inside and allowing for displacement is worth the extra minutes.
Checking by filling
If you want certainty, fill the tank with a bucket of known size. Count the buckets and add the part-bucket at the end. Note that the first fill is usually done without substrate; if you add substrate later, it displaces water and the level rises, so you may take water out. Record the final figure for your tank, and keep it with the equipment manuals.
Filters, heaters and lights scale with volume
Equipment ratings are given for a volume. A heater is usually rated in watts for tanks of a certain size, and a filter in litres per hour for a range of tank volumes. If your real volume is 109 litres, not the 142 on the box, choose equipment for the real number, and allow some margin. Our Aquarium Filter Turnover Calculator shows how many times per hour the filter turns over the tank. Ask a specialist shop for advice on the right equipment for your fish and plants.
Water changes with real volume
A weekly change of 20% of 109 litres is about 22 litres; of 142, about 28. The difference matters when you add water conditioner per litre. Use the Aquarium Water Change Calculator with your real volume. Keep a bucket for aquarium use only, mark the 10-litre level, and count buckets. Match the temperature of new water to the tank, and add conditioner as the label says.
Quick reference formulas
| Shape | Formula | Notes |
|---|---|---|
| Rectangle | L × W × H | Use inside measurements |
| Cube | side³ | A special case |
| Cylinder | π × r² × H | r is the inside radius |
| Hexagon | 2.598 × s² × H | s is the side length |
| Quarter-circle corner tank | (π × r² ÷ 4) × H | Plus any rectangular part |
Divide cubic centimetres by 1,000 for litres, and cubic inches by 231 for US gallons. When in doubt, fill and count.
Quick summary
- Litres = L × W × H (cm) ÷ 1,000; US gallons = L × W × H (in) ÷ 231.
- A US gallon is 3.785 litres; a UK gallon is 4.546 litres.
- Measure inside the glass and use the water height.
- Substrate and decor displace water, so real volume is lower.
- Cylinders use π × r² × h; irregular tanks are best measured by filling.
- Use the Aquarium Volume Calculator for any tank.
Frequently asked questions
How do I calculate aquarium volume in litres?
Length × width × height in centimetres, divided by 1,000. A tank 100 × 40 × 50 cm holds 200 litres to the brim.
How do I calculate aquarium volume in US gallons?
Length × width × height in inches, divided by 231. A tank 48 × 13 × 21 in is 13,104 cubic inches, about 56.7 US gallons.
What is the difference between US and UK gallons?
A US gallon is 3.785 litres and a UK (imperial) gallon is 4.546 litres, so the same tank has a bigger US gallon number than UK number.
Should I measure inside or outside the tank?
Inside, because glass takes up space. Outside measurements overstate the volume, and thick glass makes a bigger difference.
Why is my real water volume lower?
Substrate, rocks, wood, equipment and the gap below the rim all take space. Real water volume is often noticeably less than the nominal figure.
How do I calculate a cylinder tank?
π × radius squared × water height. A radius of 20 cm and height of 50 cm gives about 62.8 litres.
Why do I need the volume?
For dosing water treatments by label, choosing a filter, planning water changes and working out the weight of the tank.
Sources
- NIST Guide to the SI, Appendix B.8 (exact US liquid gallon = 231 cubic inches = 3.785411784 litres)
- Pure geometry: volume = length × width × height; 1 litre = 1,000 cubic centimetres
Related calculators
- Aquarium Volume CalculatorWork out your fish tank's volume in litres and gallons from its dimensions.
- Aquarium Water Weight CalculatorWork out how much the water in a fish tank weighs so you can check your stand and floor.
- Aquarium Water Change CalculatorWork out how much water to replace for the percentage change you choose.
Keep reading
- Fish Tank Size Chart: Gallons, Litres and WeightA fish tank size chart for common US gallon sizes with typical dimensions, volume in gallons and litres and water weight, plus how to read nominal sizes.
- How Much Does a Fish Tank Weigh? Full Tank WeightHow to estimate the weight of a filled fish tank: water at 1 kg per litre or 8.35 lb per US gallon, plus glass, substrate, rock and stand, with examples.