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Knowing your true aquarium volume is one of the most helpful skills you can learn as a fishkeeper. It affects stocking, filtration, heater size, medication and fertilizer dosing, water change amounts, and even how heavy the tank will be on your stand. In this guide, you will learn simple, beginner-friendly ways to calculate volume in both litres and gallons for common tank shapes, how to allow for glass thickness, freeboard, and decorations, and how to avoid the most common mistakes.
Quick Reference: Litres and Gallons
Two kinds of gallons you should know
There are two different gallon units used in the aquarium world. US gallons are most common in North America. Imperial (UK) gallons are used in the UK and in some older books and lists. They are not the same size. 1 US gallon is 3.785 litres. 1 Imperial gallon is 4.546 litres. If a label or guide just says “gallons” and you are not sure which one it means, assume US gallons unless you know the source is UK based.
Essential conversions
1 litre = 0.2642 US gallons = 0.21997 Imperial gallons. 1 US gallon = 3.785 litres. 1 Imperial gallon = 4.546 litres. For quick mental math, litres to US gallons is roughly litres divided by 3.8. US gallons to litres is roughly gallons times 3.8.
A note on water weight
Water is heavy, and safe stands depend on accurate volume. 1 litre of water weighs about 1 kilogram. 1 US gallon weighs about 8.34 pounds, or 3.785 kilograms. A 200 litre aquarium holds around 200 kilograms of water before you even add glass, substrate, and rocks.
Measuring Your Tank Correctly
Tools and tips
Use a tape measure or a rigid ruler. Measure interior dimensions if possible because glass thickness reduces the inside space. If you only have outside measurements, you can subtract twice the glass thickness from the length and width, and subtract the bottom glass thickness from height. Typical glass thickness on hobby tanks ranges from 4 to 12 mm depending on size, but it is best to check your exact tank.
Measure to the water line, not the rim
Most aquariums are not filled to the very top. The gap between the water surface and the top rim is often called “freeboard” or “air gap.” If you keep a 2 to 5 cm gap, or you have a rimmed tank that hides the water line, make sure to measure height only up to the planned water level. This small detail can change your final volume by many litres.
Rectangular and Cube Tanks
Simple formulas
Rectangular tanks are the easiest. If you measure in centimetres: Litres = Length (cm) × Width (cm) × Water Height (cm) ÷ 1000. If you measure in inches: US gallons = Length (in) × Width (in) × Water Height (in) ÷ 231. The number 1000 converts cubic centimetres to litres. The number 231 converts cubic inches to US gallons. To get Imperial gallons from inches, first find litres using the cm method or convert US gallons to Imperial by multiplying US gallons by 0.8327.
Worked example in centimetres
Tank size listed as 120 × 45 × 45 cm. You keep a 3 cm air gap, so water height is 42 cm. Inside glass thickness reduces length and width by about 1 cm each if the glass is 5 mm. New internal length 119 cm, width 44 cm. Volume in litres = 119 × 44 × 42 ÷ 1000 = 219.7 litres. In US gallons, 219.7 ÷ 3.785 = 58.0 US gallons.
Worked example in inches
Tank size listed as 36 × 18 × 18 in. You plan a 1 inch air gap, so water height is 17 in. US gallons = 36 × 18 × 17 ÷ 231 = 47.6 US gallons. In litres, multiply by 3.785: 47.6 × 3.785 = 180.2 litres.
Cylinders, Columns, and Round Tanks
Cylinder formula
Many column tanks, breeder barrels, and some nano tanks are cylindrical. Volume formula is V = π × r² × h. r is the inside radius, h is the water height, and π is about 3.1416. If you measure in centimetres, divide by 1000 at the end to get litres.
Worked example
Round tank with inside diameter 30 cm and water height 60 cm. Radius r = 15 cm. Volume in litres = 3.1416 × 15² × 60 ÷ 1000 = 3.1416 × 225 × 60 ÷ 1000 = 42.4 litres. In US gallons, 42.4 ÷ 3.785 = 11.2 US gallons.
Bow-Front, Corner, and Other Shapes
Bow-front approximations
Bow-front tanks curve outward, so the depth is not constant. A simple, accurate method is to use the average depth. Measure the depth at the ends (near the sides) and at the center of the bow. Average depth = (left depth + right depth + center depth) ÷ 3. Then use the rectangular formula with length as the straight front length and depth as this average. This gives a good estimate for most bow-front designs.
Quarter-cylinder corner tanks
Corner tanks often look like a pizza slice. Many are quarter-cylinders. Use V = (π × r² ÷ 4) × h. r is the radius from the corner to the front edge, and h is the water height. If measuring in cm, divide by 1000 for litres. For example, radius 40 cm, water height 50 cm. Volume in litres = (3.1416 × 40² ÷ 4) × 50 ÷ 1000 = (3.1416 × 1600 ÷ 4) × 50 ÷ 1000 = (1256.6 ÷ 4) × 50 ÷ 1000 = 314.2 × 50 ÷ 1000 = 15.7 litres. That seems too small because we made a math slip. Let’s correct it. 3.1416 × 1600 = 5026.6. Divide by 4 gives 1256.6. Multiply by 50 gives 62,829. Divide by 1000 gives 62.8 litres. That is the correct result.
Hexagon tanks
Regular hexagon tanks are built from six equal panels. The easiest way to estimate area is to measure flat-to-flat width across the hexagon, call this F. The top area of a regular hexagon with flat-to-flat width F is Area = (3 × √3 ÷ 8) × F². √3 is about 1.732. So the constant (3 × 1.732 ÷ 8) is about 0.6495. Area in square centimetres = 0.6495 × F² if F is in cm. Then Volume in litres = Area × water height (cm) ÷ 1000. Example: a hex tank with flat-to-flat width 40 cm and water height 50 cm has area = 0.6495 × 40² = 0.6495 × 1600 = 1039.2 cm². Volume = 1039.2 × 50 ÷ 1000 = 51.96 litres.
Odd shapes: water-fill and grid methods
If your tank has an unusual shape or heavy sculpted back panels, you can measure volume by filling. Place the empty aquarium where it will live, level it, and fill it to your planned water line using a container of known volume such as a 10 litre jug or a 5 US gallon bucket. Count the fills. Subtract at the end to account for displacement from substrate and décor if you have not added them yet. Another method is a grid method: measure several depths at equal intervals across the width, average them, and then use length × average depth × height. The more measurements you take, the closer the estimate.
Subtracting Substrate and Décor Displacement
Why displacement matters
Your fish and filters care about the volume of water, not the glass box capacity. Substrate, rocks, driftwood, and internal equipment take up space. A typical lightly planted tank with a modest hardscape displaces about 5 to 10 percent of the raw volume. Heavy aquascapes with big stones can displace 15 to 25 percent or more. You can estimate or measure after setup by topping off with a known volume pitcher.
Simple method to estimate
First calculate the raw water volume using your interior measurements and water height. Then multiply by a displacement factor. For a moderate setup, subtract 10 percent. Example: a 220 litre rectangular tank with a planted substrate and a few rocks might have actual water volume about 220 × 0.9 = 198 litres. For a rocky aquascape, you might use 0.8, giving 176 litres.
Including Sumps and Total System Volume
Sump basics
Marine tanks and some freshwater systems use a sump. The sump adds to your total water volume, which improves stability. To calculate total system volume, add the display’s actual water volume and the sump’s operating water volume. Do not use the sump’s full capacity because you leave headroom for drain-down when the return pump is off.
How to calculate sump operating volume
Measure the sump water length, width, and operating water height in each chamber and add them together, or simply measure the overall sump interior dimensions and the average operating water height. If your return section fluctuates, use the normal running water level. Remember to allow space for back-siphon water from the display. A common safety margin is to leave 20 to 30 percent free space in the sump, so operating volume may be 70 to 80 percent of the total internal sump capacity.
Example
Display tank actual water volume 180 litres after displacement. Sump interior 80 × 35 × 35 cm, normal water height 25 cm. Sump operating volume = 80 × 35 × 25 ÷ 1000 = 70 litres. Total system volume = 180 + 70 = 250 litres.
Why Accurate Volume Matters
Stocking decisions
Simple rules like “one inch of fish per gallon” are crude and often misleading, but they do rely on a clear gallon figure. More important is the bio-load your filter can process and the swimming space your fish need. Accurate litres or gallons keeps your planning honest and prevents overcrowding, especially with larger species.
Heater sizing
Heater size depends on volume and the difference between room temperature and target water temperature. A practical rule for most homes is 1 watt per litre, or 3 to 5 watts per US gallon, for a 8 to 10 °C (15 to 18 °F) temperature rise. If your home is warm, you can use less; if it is cold or the tank has a large uncovered surface, you may need more. A 100 litre tank often needs about a 100 W heater. Many aquarists prefer two smaller heaters for redundancy.
Filter flow rate
Filter turnover is usually measured as litres or gallons per hour relative to tank volume. A common target for freshwater is 4 to 8 times per hour. For planted tanks and high-flow species, 8 to 10 times can work well if the current is not too strong. For example, a 200 litre tank with a 6× target needs about 1200 litres per hour of real-world flow.
Dosing and treatments
Medications, dechlorinators, liquid fertilizers, and salt dosages are all based on water volume. Overdosing can harm fish and plants; underdosing may not work. If your real water volume after displacement is 160 litres but you dose for 200 litres, you are overdosing by 25 percent. Always calculate based on actual water, not the box label.
CO2 and nutrients
Pressurized CO2 advice and EI or lean dosing schedules use litres to estimate ppm and weekly totals. Knowing your true water volume lets you dial in stable CO2 and avoid surface film or stressed fish.
Worked Examples You Can Copy
Example 1: A common 55 gallon tank
A standard 55 US gallon display is often listed as 48 × 13 × 21 inches. You plan a 1 inch air gap and a moderate substrate that reduces volume by 10 percent. Water height is 20 inches. Raw US gallons = 48 × 13 × 20 ÷ 231 = 54.0 US gallons. Allow for 10 percent displacement: 54.0 × 0.9 = 48.6 US gallons of water. In litres, 48.6 × 3.785 = 184 litres. This is the number you should dose and size equipment for.
Example 2: A 60 × 30 × 36 cm “tall” tank
Interior measures 59 × 29 × 35 cm because of glass thickness and a small air gap. Raw litres = 59 × 29 × 35 ÷ 1000 = 59.8 litres. You have a big root and deeper substrate, so estimate 15 percent displacement. Actual water volume ≈ 59.8 × 0.85 = 50.8 litres. US gallons = 50.8 ÷ 3.785 = 13.4 US gallons.
Example 3: Corner quarter-cylinder
Radius to the front panel is 45 cm, and water height is 50 cm. Volume in litres = (π × 45² ÷ 4) × 50 ÷ 1000. First, 45² = 2025. π × 2025 ≈ 3.1416 × 2025 = 6361.7. Divide by 4 = 1590.4. Multiply by 50 = 79,519. Divide by 1000 = 79.5 litres. If you add live rock that displaces about 20 percent, your actual water volume is roughly 63.6 litres. In US gallons, 63.6 ÷ 3.785 = 16.8 US gallons.
Common Mistakes and How to Avoid Them
Using outside measurements only
Measuring the outside of the tank without subtracting glass thickness can overestimate volume. Interior dimensions can be 1 to 3 cm smaller than the outside, which adds up across all three dimensions.
Forgetting the air gap
If you calculate to the rim, you will overestimate. Most aquariums run at least a couple of centimetres lower than the rim. Measure water height, not glass height.
Using the wrong gallon
Always confirm US gallons versus Imperial gallons. Mixing them up can be a difference of about 20 percent. If your calculation seems off, check which gallon your source used.
Ignoring displacement
Hardscape can remove a large volume of water, especially in reef tanks with live rock or freshwater iwagumi layouts. Start with a 10 percent subtraction for moderate setups and adjust based on how packed your aquascape is.
Trusting the box label blindly
Manufacturers often list nominal sizes. Rim designs, bracing, curved glass, and thick silicone can reduce real volume. Always measure if accuracy matters for dosing or equipment sizing.
Handy Formulas to Memorize
Rectangular or cube tanks
Litres = L(cm) × W(cm) × water height(cm) ÷ 1000. US gallons = L(in) × W(in) × water height(in) ÷ 231. To convert cubic centimetres directly to US gallons, divide by 3785.
Cylinders
Litres = π × r(cm)² × water height(cm) ÷ 1000. US gallons = π × r(in)² × water height(in) ÷ 231.
Hexagons
Area(cm²) ≈ 0.6495 × F(cm)² where F is flat-to-flat width. Litres = Area × water height ÷ 1000.
Conversions
US gallons = litres × 0.2642. Imperial gallons = litres × 0.21997. Litres = US gallons × 3.785. Litres = Imperial gallons × 4.546.
Calculator-Free, Step-by-Step Recipes
Quickly from centimetres to litres
Multiply the three interior measurements in centimetres. Count digits to keep track. For example, 100 × 40 × 40 = 160,000. Move the decimal three places left to divide by 1000. 160,000 ÷ 1000 = 160 litres. If there is an air gap, subtract that from height first.
Quickly from inches to US gallons
Multiply length × width × water height. Divide by 231. Many aquarists remember that 24 × 12 × 12 is 3456. 3456 ÷ 231 ≈ 15 US gallons for a 20 high filled a bit low. Estimating works if you round carefully.
Average depth method for bows and irregulars
Measure depth at left, right, and middle. Add them and divide by three to get average depth. Use the rectangular formula with that depth. If the curve is very gentle, averaging left and middle might be close enough. The more samples you measure, the more accurate it gets.
Special Notes for Marine and Planted Tanks
Marine aquariums with live rock
Live rock is very porous and bulky. It can displace 10 to 30 percent of the display volume depending on how you aquascape. Always re-check your actual system volume after the display and sump are running by tracking how many litres or gallons you add to reach your final water line. Use that measured number for dosing alkalinity, calcium, and medications.
High-tech planted tanks
Pressurized CO2 and liquid fertilizer dosing schedules are given per litre. If your effective water volume is much lower than the glass capacity, CO2 may become too strong, and fertilizer concentrations can rise. Start dosing for 80 to 90 percent of your calculated volume and observe plant and fish response, then adjust. Good surface agitation helps match CO2 and oxygen.
Troubleshooting: When Your Numbers Don’t Match
If your calculation and reality differ
If your calculated volume says 200 litres but you only add 180 litres to reach your water line, check these items. Confirm you used interior dimensions. Re-measure water height to the actual water line. Account for substrate and hardscape displacement. Verify you used the correct gallon type if converting. Consider unusual glass bracing or curved panels that reduce internal volume. It is normal for the final number to be 5 to 15 percent lower than the simple rectangular math once you include all real-world details.
Putting It All Together
Step-by-step checklist
First, measure interior length, width, and planned water height. Second, calculate base volume using the correct shape formula. Third, convert units if needed to litres, US gallons, or Imperial gallons. Fourth, subtract a displacement percentage for substrate and décor. Start with 10 percent for a typical setup. Fifth, add sump operating volume if you have one. Finally, record this real-world volume and keep it handy for dosing, water changes, and equipment planning.
Conclusion
Simple math for better fishkeeping
Calculating aquarium volume is not only about numbers. It is about making your aquarium easier to manage and safer for your fish. With a tape measure and a few simple formulas, you can estimate litres and gallons for rectangles, cylinders, bow-fronts, corners, and hexagons. Remember to measure interior dimensions, use the real water height, pick the correct gallon type, and subtract displacement from substrate and décor. Include your sump’s operating water if you run one. These small habits give you the accurate volume you need for stocking, filtration, heater sizing, water changes, and precise dosing. Once you set your tank up, verify your estimate by tracking how much water it takes to fill to the line. From then on, you will have a reliable, beginner-friendly number that makes every part of fishkeeping simpler and more consistent.