Formula Used
Rectangular tank: Volume = Length × Width × Height.
Cylindrical tank: Volume = π × Radius² × Height.
Regular hex tank: Volume = ((3√3) ÷ 2) × Side² × Height.
US gallons: Cubic inches ÷ 231.
Liters: US gallons × 3.785411784.
Water weight: US gallons × 8.345404 pounds, or liters × 1 kilogram.
Usable water: Filled volume − substrate volume − decor displacement.
Water change: Usable water × water change percent.
How to Use This Calculator
- Select the tank shape.
- Choose the unit used for your measurements.
- Enter the needed dimensions for that shape.
- Add the fill level, substrate depth, and decor displacement.
- Enter the water change percent and treatment values.
- Press calculate to see the result below the header.
- Use the CSV or PDF buttons to save the result.
Example Data Table
| Tank Type |
Dimensions |
Fill |
Substrate |
Decor |
Estimated Usable Water |
| Rectangular |
36 × 12 × 18 in |
90% |
2 in |
8% |
About 25.75 US gal |
| Cylindrical |
18 in diameter × 24 in high |
85% |
1.5 in |
6% |
About 20.64 US gal |
| Regular hex |
10 in side × 20 in high |
88% |
2 in |
10% |
About 25.33 US gal |
Plan Aquarium Water With Better Numbers
A fish tank may look simple, but its true water amount changes after setup. Glass thickness, gravel, rocks, ornaments, and an unfilled top space all reduce the usable volume. This calculator helps you move from outside dimensions to a closer working estimate. It supports common rectangular tanks, round cylinders, and regular hex tanks. It also estimates water weight, change volume, conditioner needs, salt amount, and filter flow targets.
Why Usable Volume Matters
Usable volume is more helpful than the number printed on a tank box. Fish waste, medication dose, plant fertilizer, and water conditioner are usually based on actual water. A tank filled below the rim may hold much less than its rated size. Deep substrate can remove several gallons from a large aquarium. Heavy decorations can also displace water. These details matter when you plan safe maintenance.
Daily Care Benefits
Better volume estimates make routine work easier. You can measure a water change before moving buckets. You can size a filter by turnover per hour. You can estimate the full tank weight before choosing a stand. Water is heavy, so a small tank can still need strong support. The tool gives weight in pounds and kilograms for quick planning.
Using the Outputs
Start with inside dimensions when possible. Measure length, width, and height from the water area, not the outside trim. Choose the tank shape. Then enter fill level, substrate depth, and decoration displacement. Leave salt at zero unless you are following a trusted aquarium plan. Use conditioner values from the product label. The result should be treated as a planning estimate, not a veterinary instruction.
Good Maintenance Habits
Keep records after each water change. Download the results and save them with the date. Check fish behavior after any treatment. Avoid sudden changes in temperature, hardness, salinity, or pH. Test water regularly. Stable conditions are usually better than perfect numbers chased too quickly. When unsure, make gradual changes and seek advice from a qualified aquatic specialist.
Reading Results Carefully
Round each output to a practical measure. Buckets, test kits, and product caps are not exact tools. Use the closest safe amount. Recheck measurements after adding new gravel, rocks, driftwood, breeding boxes, or equipment.
FAQs
1. What does this calculator measure?
It estimates gross tank volume, usable water, water weight, water change volume, conditioner amount, salt amount, and filter flow needs.
2. Should I use inside or outside tank dimensions?
Inside dimensions are better. Outside measurements include glass, trim, and empty space that may not hold water.
3. Why is usable volume lower than gross volume?
Usable volume is reduced by fill level, substrate, rocks, plants, ornaments, filters, and other displaced space.
4. Is the water weight the full aquarium weight?
No. It is water weight only. Add glass, stand, substrate, rocks, equipment, and decorations for a full load estimate.
5. How should I set decor displacement?
Use a small value for light decorations. Use a larger value for dense rocks, large wood, caves, or heavy aquascapes.
6. Is conditioner calculated for the full tank?
This tool calculates conditioner for the replacement water volume. Some products may require different dosing, so check the label.
7. Should I add salt to every aquarium?
No. Leave salt at zero unless your fish, plants, and treatment plan require it. Follow trusted species guidance.
8. Can this replace professional aquarium advice?
No. It is a planning tool. For illness, sensitive species, or unusual water chemistry, ask a qualified aquatic specialist.