How to Calculate Your Aquarium’s Real Water Volume

Measure the space that actually holds water

Measure inside the glass, up to your normal running water level. Outside measurements include the glass, and the gap above the waterline holds no water. Use centimetres throughout. The calculator works for rectangular tanks, so a bow-front or unusual shape needs its own estimate rather than its outside measurements forced into this formula.

Be clear about what your measurement includes. If your all-in-one’s footprint already includes its rear chamber, don’t add that chamber again. If you measure the display only, add the chamber’s running water separately and note that you did. Leave your freshwater top-off reservoir out of the saltwater total.

Worked example: 160 litres of space, 170 litres in the system

Take a made-up rectangular tank measuring 100 cm long, 40 cm wide and 40 cm deep to the running waterline inside. Say its rock and sand push out 20 L, and the sump holds 30 L once its own contents are allowed for. These are example numbers, not a standard allowance.

Rock and sand: an estimate you can explain

The calculator doesn’t turn kilos of rock or sand into litres. Shape, material and how they sit in the tank all matter, and one percentage for everyone would hide that. Note whether your figure came from a measured fill, another method you can explain, or a rough guess.

If you’re working from a fill record, think about water that was already in the system. A note saying “added 140 L today” doesn’t prove the system holds 140 L if a chamber or the plumbing already had water in it. Note what was empty and what was already full before treating the litres you added as the volume.

A range is more honest than extra decimal places. In the example, if the rock and sand push out 15–25 L and the sump stays at 30 L, the system holds 175–165 L. Keep that range next to your planning figure if you’re not sure.

Use your sump’s running level, not its full capacity

The sump figure is the water in it at its normal running level, after allowing for what’s inside it. A sump’s full size includes space that’s normally empty. Checking that your sump can hold the water that drains back when the pump stops is a separate job; this calculator doesn’t measure spare capacity or confirm that it’s safe.

This simple model leaves out the water in your pipes unless you add a separate allowance for it. Just don’t count it twice. Note exactly what your estimate includes, so you can repeat it when your equipment or water level changes.

Use your real volume to plan water changes

For a 10% water change, the 170 L example needs a 17 L batch. The unadjusted 160 L display would give 16 L, and a 200 L label would give 20 L. Those are just sums to compare, not advice on how much or how often to change water.

Save your result in this browser and open the saltwater planner. When it loads a saved volume, it uses your Companion tank volume first if you have one, then a saved volume from this calculator. Check which number it loaded; saving here doesn’t update a tank you’ve already set up in the Companion. Update that yourself if you need to.

Your real volume is useful for planning salt and water costs. It doesn’t replace the instructions for any product that needs a confirmed volume or other conditions. Keep your measurements, assumptions and the date in your tank notes, and check them again when your setup changes.

Should I use the tank’s advertised capacity?

Use it as a reference, and check what it actually measures. Your real water depth, rock and sand, and sump can make your system volume quite different.

Does the calculator work out rock and sand automatically?

No. You enter your own measured or estimated figure. The tool doesn’t assume a percentage or convert rock weight into water.

Should I include my top-off reservoir?

No, leave a separate freshwater top-off reservoir out. Count only the water actually in your running saltwater system.