Aquarium Volume Calculator – Gallons & Liters
Knowing your aquarium’s exact volume is essential for correct doses of water treatments and medications, and understanding how many fish your tank can support.
Aquarium Volume Calculator
Common Aquarium Sizes
| Tank | Dimensions (L×W×H in) | Volume |
|---|---|---|
| 5 Gallon | 16×8×10″ | 5 gal / 19 L |
| 10 Gallon | 20×10×12″ | 10 gal / 38 L |
| 20 Gallon Long | 30×12×12″ | 20 gal / 76 L |
| 55 Gallon | 48×13×21″ | 55 gal / 208 L |
How to Calculate Aquarium Volume
Tank volume is length x width x height, converted into gallons or liters. The conversion factor depends on the units you measured in.
| If you measured in | To get US gallons | To get liters |
|---|---|---|
| Inches | L x W x H / 231 | L x W x H / 61.02 |
| Centimetres | L x W x H / 3,785 | L x W x H / 1,000 |
Worked example: a standard 36 x 18 x 16 inch tank
- Volume in cubic inches: 36 x 18 x 16 = 10,368
- US gallons: 10,368 / 231 = 44.9 gallons
- Liters: 10,368 / 61.02 = 169.9 liters
That is the classic “45 gallon” tank. Notice the marketing name is rounded, which is normal across the hobby.
Gross Volume vs Actual Water Volume
The number above is the tank’s gross volume: what it would hold if filled to the brim with nothing inside. Your real water volume is always lower, and for dosing medication or fertiliser the real number is the one that counts.
| What reduces volume | Typical reduction |
|---|---|
| Freeboard (water sits below the rim) | 5 to 10 percent |
| Substrate, 2 inches of gravel or sand | 10 to 15 percent |
| Rocks, driftwood and hardscape | 2 to 10 percent |
| Equipment inside the tank | 1 to 3 percent |
Stocking: How Many Fish Fit?
The old “one inch of fish per gallon” rule is a rough starting point for small community fish only. It breaks down badly for larger or messier species, because waste output scales with body mass rather than length.
| Tank size | Suitable for |
|---|---|
| 5 to 10 gallons | A single betta, or a small shrimp colony |
| 20 gallons | A school of 8 to 10 small tetras or rasboras |
| 40 to 55 gallons | A community tank, or a pair of medium cichlids |
| 75 gallons and up | Larger species, goldfish, or a serious aquascape |
Filtration turnover matters as much as volume. Aim for a filter rated at four to six times your actual water volume per hour.
Water Changes and Heater Sizing
Once you know your real water volume, two routine jobs become straightforward:
- Water changes. A standard weekly change is 20 to 25 percent. On 36 gallons of actual water that is roughly 8 gallons.
- Heater wattage. Budget about 3 to 5 watts per gallon. A 36 gallon water volume wants a heater in the 100 to 150 watt range, at the higher end if the room runs cold.
Common Aquarium Volume Mistakes
- Dosing to the tank’s advertised size. Medications are the most dangerous case, since the safety margin on many treatments is small.
- Measuring outside dimensions. Glass thickness on a large tank can be 10 to 12 mm per side. Measure the inside.
- Ignoring the sump. On a marine setup the sump can add 20 percent or more to the total system volume, which works in your favour for stability but must be counted when dosing.
Frequently Asked Questions
How do I calculate my aquarium volume in gallons?
Multiply length x width x height in inches, then divide by 231. For example, a 36 x 18 x 16 inch tank is 10,368 cubic inches, which divided by 231 gives about 44.9 US gallons. If you measured in centimetres instead, divide by 3,785 for gallons or by 1,000 for liters.
Why is my actual water volume less than the tank size?
Because substrate, rocks, driftwood, equipment and the empty gap between the water line and the rim all take up space. A typical planted aquarium holds only about 75 to 85 percent of its rated volume in real water, so a 45 gallon tank often contains closer to 36 gallons.
Does the one inch per gallon rule actually work?
Only as a rough guide for small community fish. Waste output scales with body mass rather than length, so a single six inch goldfish produces far more waste than six one inch tetras. Use it as a starting point for small species and research the specific needs of anything larger.
How much water should I change each week?
A standard weekly water change is 20 to 25 percent of your actual water volume. On a tank holding 36 gallons of real water that is roughly 8 gallons per week. Heavily stocked tanks may need more, lightly stocked planted tanks often need less.
What size heater do I need for my tank?
Budget about 3 to 5 watts per gallon of actual water volume. A tank holding 36 gallons wants a heater somewhere between 100 and 150 watts. Choose the higher end if the room gets cold in winter, or use two smaller heaters for redundancy on larger tanks.
Should I use gross volume or actual water volume for dosing?
Always use actual water volume. Dosing medication or fertiliser to the tank’s advertised size will overdose the tank, typically by 15 to 25 percent. This matters most with medications, where the margin between an effective dose and a harmful one can be narrow.