Fish Tank Gallon Calculator
Fish Tank Gallon Calculator: Accurately Measure Your Aquarium Volume
Wondering how many gallons your fish tank holds? Whether you’re setting up a new aquarium or upgrading your current setup, knowing the exact water volume is essential. Our Fish Tank Gallon Calculator takes the guesswork out of it—just enter your tank’s dimensions, and let the math work for you.
Why does this matter? Because your fish’s health depends on it. From choosing the right filter to avoiding overcrowding, calculating your aquarium’s capacity is a vital first step in smart fishkeeping.
Why You Need to Know Your Fish Tank Gallons
Accurate water volume helps ensure your aquarium ecosystem runs smoothly. Here’s why:
Proper Equipment Sizing: Filters and heaters are rated for specific gallon ranges. Using the wrong size can harm water quality or stress your fish.
Avoid Overcrowding: Overstocked tanks cause stress, aggression, and illness. Gallon count helps you determine a healthy stocking level.
Precise Water Treatment Dosing: Most treatments require specific dosages per gallon. Guessing can lead to under-treatment or harmful overdoses.
Smart Planning: When you know your tank’s gallons, you can better plan for upgrades, water changes, and fish species compatibility.
With a quick calculation or a simple tool, you can avoid many common mistakes.
How to Calculate the Gallons in a Fish Tank
You can calculate tank gallons by dimensions using this simple formula:
Tank Volume (US gallons) = Length × Width × Height ÷ 231
Note: All dimensions must be in inches for this formula to work.
This formula works best for rectangular tanks. For example, a tank that’s 30″ long, 12″ wide, and 18″ tall holds:
Make sure you’re measuring inside dimensions—excluding glass thickness or rim space. For tanks measured in centimeters, convert to inches first:
Need it faster? Use our Fish Tank Gallon Calculator above to skip the math and get instant results.
Manual Gallon Calculation Formulas by Tank Shape
Not all tanks are rectangular! Here’s how to convert other shapes using a fish tank size calculator approach:
Rectangular Tank
Formula:Length × Width × Height ÷ 231
Example:
36″ × 18″ × 18″ ÷ 231 = ~50.5 gallons
Cylindrical Tank
Formula:π × radius² × height ÷ 231
Example:
Radius = 10″, Height = 20″
3.14 × 10² × 20 ÷ 231 = ~27.2 gallons
Bow Front or Irregular Tanks
Formula (approximate):Average Width × Length × Height ÷ 231
Tip: Use this formula if the tank has curved glass. For the most accurate result, use our online calculator that accounts for various shapes.
Common Fish Tank Sizes and Their Gallons
Here’s a quick fish tank dimensions to gallons guide to help you visualize:
| Tank Size (L × W × H in inches) | Gallons | Good For |
|---|---|---|
| 20 x 10 x 12 | ~10 | Bettas, shrimp |
| 30 x 12 x 18 | ~20 | Small community fish |
| 36 x 18 x 18 | ~40 | Goldfish, tropical setups |
| 48 x 13 x 21 | ~55 | Cichlids, reef aquariums |
Tips for Accurate Gallon Calculation
Measure water-holding space only: Ignore rims or outer walls that don’t hold water.
Subtract for substrate & décor: Gravel, rocks, and driftwood reduce usable volume.
Be precise: Small rounding errors can add up, especially for larger tanks.
Stick to one unit: Don’t mix inches and centimeters in the same formula.
Benefits of Using Our Online Fish Tank Gallon Calculator
Why struggle with math when you can use our free tool?
Beginner-Friendly: No math skills required.
Works for all tank shapes: Rectangular, cylindrical, bow front, and more.
Instant Results: Get your tank size in seconds.
Helpful for Planning: Perfect for water changes, equipment upgrades, and stocking levels.
Let the aquarium size to gallon converter do the heavy lifting for you.
Frequently Asked Questions
How many gallons is my fish tank in liters?
Use this conversion:1 US gallon = 3.785 liters.
Multiply your gallons by 3.785 to convert.
Is the calculator accurate for curved tanks like bow fronts?
Yes! Just use the average width when entering dimensions. The tool gives a good estimate.
Can I use centimeters instead of inches?
Yes—just convert first:Centimeters ÷ 2.54 = Inches
Why is my result different from the tank label?
Manufacturers often list the total volume, not the usable water volume. Decorations, substrate, and glass thickness can reduce actual capacity.
Know Your Aquarium Volume the Smart Way
Whether you’re adjusting water treatments or choosing new fish, knowing your tank’s size in gallons makes all the difference. You can manually calculate it—or make life easier with our tool.
👉 Try our Fish Tank Gallon Calculator now and take the guesswork out of your fishkeeping!