A fish tank volume calculator helps you determine the exact water capacity of your aquarium based on its shape and dimensions. Knowing your tank volume is essential for proper fish stocking, heater sizing, filter selection, and accurate medication dosing. Enter your tank dimensions below to calculate volume in gallons, liters, and cubic inches.
Tank Dimensions
Reduces usable water volume. Uses same unit as above.
Common Tank Sizes Reference
| Size | Dimensions (in) | Liters | Water Weight | Heater |
|---|---|---|---|---|
| 5 gal | 16 × 8 × 10 | 19 L | 42 lbs | 25W |
| 10 gal | 20 × 10 × 12 | 38 L | 83 lbs | 50W |
| 20 gal | 24 × 12 × 16 | 76 L | 167 lbs | 100W |
| 29 gal | 30 × 12 × 18 | 110 L | 242 lbs | 150W |
| 40 gal | 36 × 18 × 16 | 151 L | 334 lbs | 200W |
| 55 gal | 48 × 13 × 20 | 208 L | 459 lbs | 275W |
| 75 gal | 48 × 18 × 20 | 284 L | 626 lbs | 2×200W |
| 90 gal | 48 × 18 × 24 | 341 L | 751 lbs | 2×250W |
| 125 gal | 72 × 18 × 22 | 473 L | 1,043 lbs | 2×300W |
How to Use the Fish Tank Volume Calculator
Knowing the exact volume of your aquarium is the foundation of successful fishkeeping. Every decision you make — from how many fish to add, to what size heater and filter to buy, to how much water conditioner or medication to dose — depends on an accurate fish tank volume calculation. This calculator supports five common tank shapes and gives you volume in gallons, liters, and cubic inches, along with weight estimates and equipment recommendations.
Step 1: Select Your Tank Shape
Choose the shape that best matches your aquarium from the dropdown. Rectangular tanks are the most common and use a simple length times width times height formula. Cylindrical tanks use diameter and height with pi. Bow-front tanks add extra curved volume to a rectangular base. Hexagonal tanks use a specialized formula based on side length. L-shaped tanks are calculated as two rectangular sections combined.
Step 2: Enter Dimensions
Measure the interior dimensions of your tank — not the outside. Glass thickness can add a half inch or more on each side, which significantly affects the calculation for smaller tanks. Select inches or centimeters depending on your preference. For bow-front tanks, the bow depth is the additional distance the curved glass extends beyond where a flat front panel would sit.
Step 3: Account for Substrate
Most aquariums have gravel, sand, or planted substrate on the bottom. Enter your substrate depth to see the usable water volume after displacement. A typical aquarium has 1 to 3 inches of substrate, which can reduce water volume by 5 to 15 percent depending on tank dimensions. This adjusted volume is what you should use for stocking calculations and medication dosing.
Step 4: Review Weight and Equipment Estimates
Water weighs 8.34 pounds per US gallon. The calculator estimates total weight including water and the tank itself so you can verify your stand and floor can handle the load. A filled 55-gallon tank weighs over 500 pounds — always place larger tanks on a dedicated aquarium stand on a ground floor. The heater recommendation follows the standard 5 watts per gallon rule for tropical tanks. The filter flow rate suggestion of 4 to 6 times the tank volume per hour ensures adequate biological and mechanical filtration.
Step 5: Use the Reference Table
The common tank sizes table below the calculator lists standard aquarium dimensions for 5 through 125 gallon tanks. Use it to verify your measurements against industry-standard sizes, or to quickly look up heater wattage and water weight without entering dimensions. These are nominal sizes — actual water volume may be slightly less due to glass thickness and the curved top edges of some tanks.
Tips for Accurate Measurements
Always measure the inside of the tank. If your tank is already filled, measure the water line height rather than the tank's full height. Remember that decorations, rocks, driftwood, and internal filters all displace water and reduce the actual volume available to your fish. For the most accurate stocking decisions, subtract about 10 percent from your calculated volume to account for decorations and equipment displacement.
Frequently Asked Questions
Is this fish tank volume calculator free?
Yes, this fish tank volume calculator is completely free with no signup or account needed. All calculations run locally in your browser, so your tank dimensions are never sent to any server.
Is my data private?
Absolutely. Everything runs in your browser using JavaScript. Your measurements and calculations never leave your device. No cookies, no tracking, no server-side storage.
How do I measure my fish tank dimensions?
Measure the inside dimensions of your tank, not the outside. Use a tape measure to get the interior length, width, and height in inches or centimeters. For bow-front tanks, measure the bow depth from the front glass curve to where a flat front would be.
Why is my usable volume less than the total volume?
Substrate, decorations, rocks, driftwood, and equipment like filters and heaters all displace water. A typical tank with 2 inches of substrate loses about 10-15% of its total volume. This calculator lets you subtract substrate depth to get a more accurate usable volume.
How much does a full aquarium weigh?
Water weighs 8.34 pounds per gallon. A filled 55-gallon tank holds about 459 pounds of water alone. Add another 50-100 pounds for glass, substrate, and decorations. Always place aquariums on a sturdy stand rated for the total weight, and on a ground floor if possible.
What size heater do I need for my fish tank?
The general rule is 5 watts per gallon for tropical aquariums. A 20-gallon tank needs a 100-watt heater, while a 55-gallon tank needs 275 watts. For tanks over 75 gallons, consider using two smaller heaters for more even heat distribution and as a backup if one fails.
How do I calculate filter flow rate for my aquarium?
A good filter should turn over your tank's volume 4 to 6 times per hour. For a 30-gallon tank, you want a filter rated for 120 to 180 gallons per hour. Heavily stocked tanks and tanks with messy fish like goldfish should aim for the higher end of that range.
How do I calculate the volume of a hexagonal fish tank?
A regular hexagonal tank's volume is calculated using the side length and height. The formula is (3 times the square root of 3, divided by 2) times the side length squared, times the height. This calculator handles the math automatically — just enter the side length and height.