Aquarium Volume Calculator
From your tank dimensions, get the gross volume, the realistic usable water volume after substrate and hardscape, the filled weight for fresh or salt water, plus recommended heater wattage, filter flow rate, and a rough fish stocking estimate. Supports rectangular, cylindrical, bow-front, hexagonal, half-cylinder, and corner tanks. Switch between imperial (inches) and metric (centimetres) with one click.
How aquarium volume is calculated
For a rectangular tank the calculation is simple: length x width x height gives the volume in cubic inches, and dividing by 231 converts to US gallons (one gallon equals 231 cubic inches). Cylindrical tanks use the formula for a circular area (pi x radius squared) multiplied by height. Bow-front tanks are approximated by subtracting the elliptical segment removed from the front of a rectangular footprint. Hexagonal tanks use the regular hexagon base area formula ((3 x sqrt(3) / 2) x side squared) multiplied by height. In every case the result is the gross geometric volume, which is always larger than the amount of water you actually put in.
Why usable volume matters
The gross volume of a tank is rarely what you fill with water. Substrate (gravel, sand, or soil) typically sits 1-3 inches deep, removing 10-20% of the gross volume before a drop of water goes in. Rocks, driftwood, filters, heaters, and powerheads displace another 5-15%. Most keepers also leave a 1-2 inch gap at the top for splashing and evaporation. After all those deductions a 55-gallon tank may hold only 42-48 gallons of actual water. That real figure is the number you should use when dosing medications, calculating salt mixes for marine tanks, or sizing a heater.
Sizing your heater
A heater rated at 5 watts per gallon handles a temperature rise of about 10 degrees Fahrenheit (roughly 6 degrees Celsius) above room temperature. If your room is cold, say 60 F, and your tropical fish need 78 F, you need an 18 F rise, which calls for roughly 9 watts per gallon. This calculator scales wattage linearly with the temperature rise you enter and rounds up to the nearest 25 W to match standard heater sizes on the market. For tanks larger than 100 gallons it is often better to use two heaters of half the total wattage for even heat distribution.
Sizing your filter and stocking your tank
Filter manufacturers rate their products in gallons per hour (GPH). The general guideline is to turn over the entire tank volume 4 times per hour for community tropical fish, 3 times for heavily planted tanks (where strong flow can damage delicate plants), and 6-10 times for goldfish, cichlids, and other messy feeders. These are starting points: if the water stays cloudy or nitrates rise fast, move to a higher-rated filter. The 1-inch-per-gallon stocking rule is a rough and imperfect guide designed for small, slender community fish. Larger, fuller-bodied, or more territorial fish have higher bioload per inch of body length, so always research the specific species you plan to keep.
Common aquarium sizes reference
| Size | Dimensions (L x W x H) | US gallons | Liters |
|---|---|---|---|
| 2.5 gal | 12 x 6 x 8 in | 2.5 | 9.5 |
| 5 gal | 16 x 8 x 10 in | 5.0 | 18.9 |
| 10 gal | 20 x 10 x 12 in | 10.0 | 37.9 |
| 20 gal (long) | 30 x 12 x 12 in | 20.0 | 75.7 |
| 29 gal | 30 x 12 x 18 in | 29.0 | 109.8 |
| 40 gal (breeder) | 36 x 18 x 16 in | 40.0 | 151.4 |
| 55 gal | 48 x 13 x 21 in | 55.0 | 208.2 |
| 75 gal | 48 x 18 x 21 in | 75.0 | 283.9 |
| 90 gal | 48 x 18 x 24 in | 90.0 | 340.7 |
| 125 gal | 72 x 18 x 22 in | 125.0 | 473.2 |
| 180 gal | 72 x 24 x 24 in | 180.0 | 681.4 |
Standard U.S. tank sizes and their gross volumes. Actual usable water is typically 10-20% less after substrate and equipment.
Frequently asked questions
Why is the usable water volume less than the tank size?
The advertised gallon rating of a tank is its geometric capacity when filled to the brim. In practice you lose volume to substrate (gravel or sand), rocks and decorations, equipment like filters and heaters, and the air gap you leave at the top. Deductions of 10-20% are common, so a 55-gallon tank typically holds 42-48 gallons of water once everything is set up.
How much does a filled aquarium weigh?
Freshwater weighs about 8.34 pounds per gallon (1 kg per liter). Saltwater is slightly heavier at roughly 8.55 lb per gallon because of dissolved salts. Add the weight of the glass or acrylic tank itself (commonly 1-2 lb per gallon of gross capacity for glass), plus substrate and decorations, and a fully set-up 55-gallon freshwater tank can weigh 600 to 700 pounds. Always verify your floor can handle the load before filling.
How many watts does my heater need to be?
The standard starting point is 5 watts per gallon of usable volume for a 10-degree Fahrenheit temperature rise above room temperature. If your room runs colder, scale up proportionally: a 20-degree rise needs about 10 watts per gallon. For large tanks over 100 gallons, split the load between two heaters for more even heating and redundancy.
What filter flow rate do I need?
A common guideline is 4 times the tank volume per hour for community tropical fish. So a 50-gallon usable volume calls for a filter rated at 200 GPH. Goldfish and cichlids are messier and benefit from 6-10x turnover. Planted tanks can use 3x to avoid blasting delicate plants. Check the filter rating at the head height it will actually operate at, not the maximum printed on the box.
Is the 1-inch-per-gallon rule accurate?
It is a rough beginner rule that works reasonably well for small, slender tropical fish like tetras and danios in the 1-3 inch range. It breaks down quickly for large or wide-bodied fish: a 6-inch oscar has far more bioload than six 1-inch neon tetras. Use it as a very conservative ceiling and research the specific waste output and territorial needs of every species you plan to keep.
How do I calculate the volume of a non-standard shaped tank?
The best approach is to break the tank into standard shapes (rectangles, half-cylinders, etc.), calculate each section separately, and add the results. For example, a bow-front tank can be approximated as a rectangle minus the elliptical bite taken out of the front. This calculator handles six common shapes directly. For highly irregular tanks, filling with water and counting the gallons from the tap meter is the most accurate method.
Gallons or liters - which should I use?
Either works as long as you are consistent. US hobbyists typically size equipment in gallons, while the rest of the world uses liters. Product labels in most countries show both. When buying salt mix for a marine tank, check whether the bag directions are per gallon or per liter to avoid under- or over-salting.