TOOL 01
Tank Volume
Calculator.
Enter your aquarium dimensions to calculate actual water capacity. Glass thickness and substrate reduce real volume — we apply a 90% fill factor for a realistic estimate.
Volume = L × W × H × 0.9 (fill factor). Water change range is 20–30% of actual capacity, changed weekly.
ABOUT THIS TOOL
Tank Volume
Calculator.
The Tank Volume Calculator helps you determine the actual water capacity of your aquarium based on its physical dimensions. Many aquarists mistakenly calculate volume using the external tank measurements without accounting for glass thickness, substrate, decorations, and the fact that tanks are never filled to the brim. This tool applies a 90% fill factor to give you a realistic estimate of how much water your tank actually holds.
Knowing your true water volume is essential for every other aquarium calculation — from dosing medications and fertilisers to sizing your heater and planning water changes. An overestimate can lead to overdosing; an underestimate can leave your tank under-filtered. This tool gives you a safe, practical number to work with.
The calculator also provides conversions to US gallons and Imperial gallons, so you can follow care guides from any region. Additionally, it suggests a weekly water change range (20–30%) based on your actual capacity, so you know exactly how much new water to prepare each week.
HOW TO USE
Using the
Volume Calculator.
- Measure your tankUsing a tape measure, measure the internal length, width, and height of your aquarium in centimetres. Measure from inside the glass for accuracy, and measure only up to the typical water line (not the tank rim).
- Enter the dimensionsInput the length, width, and height values into the three fields above. The calculator updates instantly as you type — no need to press a button.
- Read the actual capacityThe primary result shows your tank's estimated real water volume in litres, with a 90% fill factor already applied to account for substrate, decorations, and the unfilled top portion.
- Check the gallon conversionsUS gallons and Imperial gallons are shown for reference. Use the unit that matches your local equipment specs or care guides.
- Note the water change rangeThe calculator shows how much water to replace weekly at 20% (minimum) and 30% (maximum). Use this to plan your maintenance routine.
- Account for displacementIf your tank has heavy rockwork or thick substrate, your actual volume may be slightly lower. Consider measuring water added during setup for the most precise figure.
FAQ
Common
Questions.
Why is the calculated volume lower than the tank's stated size?
Manufacturers often list the external dimensions or the theoretical maximum fill. In reality, glass thickness, substrate, decorations, and the fact that you don't fill to the brim all reduce actual water volume. Our 90% fill factor accounts for this. If you have heavy rockwork or deep substrate, your real volume may be even lower.
Should I measure internal or external dimensions?
Internal dimensions are more accurate because they exclude glass thickness. For small tanks (under 50L), the difference can be 5–10%. For larger tanks, the percentage difference is smaller but still worth measuring correctly. If you only have external measurements, the 90% fill factor helps compensate.
What is the 90% fill factor and why does it exist?
The 90% factor accounts for three things: (1) tanks are typically filled to about 5cm below the rim, (2) substrate and hardscape displace water, and (3) equipment like filters and heaters take up space. This gives a practical, real-world estimate rather than a theoretical maximum.
Which gallon conversion should I use?
Use US gallons if you are following American care guides, buying equipment from US retailers, or reading forums where most users are American. Use Imperial gallons for UK sources. 1 US gallon = 3.785 litres; 1 Imperial gallon = 4.546 litres. The difference is significant, so always check which unit a guide is using.
How accurate is the water change recommendation?
The 20–30% weekly range is a widely accepted guideline for most community aquariums. However, heavily stocked tanks or those with sensitive species may need larger or more frequent changes. Always monitor your water parameters with test kits and adjust your routine based on actual ammonia, nitrite, and nitrate readings.
Can I use this for bow-front or corner tanks?
This calculator works for rectangular and square tanks. Bow-front, hexagonal, corner, and cylindrical tanks have different volume formulas. For bow-front tanks, you can approximate by using the average of the front and back widths. For other shapes, search for a tank-specific volume calculator.