Method 1: weigh the water
Water weighs 8.34 pounds per US gallon (1 kilogram per litre). If you can weigh the tank empty and again full, the difference in pounds divided by 8.34 is the gallons. Practical for nano tanks on a kitchen scale; for anything larger, weigh the buckets you fill it from.
Example: a bowl weighs 2.2 lb empty and 21.6 lb full. 19.4 ÷ 8.34 = 2.3 gallons.
Method 2: count buckets
Fill the tank from a container of known volume and count. Mark the container at exactly one gallon or five litres with a measuring jug first; "5 gallon" buckets usually hold more than five gallons to the brim. Fill to your normal water line, not the rim.
Example: 9 full 5-gallon marks plus about half a bucket is 47.5 gallons.
Method 3: back-calculate from the pump
If you know your return pump's real flow rate and have timed how long the sump takes to drain or refill a known depth, you can work back to volume. More useful: time how long it takes to fill the tank with a hose, then measure the hose's flow into a bucket. Flow rate times time equals volume.
Example: the hose fills a 2-gallon jug in 20 seconds (6 gallons per minute) and the tank takes 9 minutes 10 seconds: 55 gallons.
Method 4: look it up
Most framed tanks are standard sizes. Measure the outside length with anything, even a sheet of paper, and match it to the volume by size table or the brand table. A 48-inch tank with a 13-inch width is a 55; with an 18-inch width it is a 75 or 90 depending on height.
Which method to use
- Nano or desktop tank: weigh it.
- Setting up a new tank: count buckets, it is free and exact.
- Established tank you cannot drain: time a water change, then scale up.
- Standard framed tank: look it up and confirm with the rectangular calculator.