"How many gallons is my tank?" sounds like it has an easy answer: whatever the box said. But a tank sold as a 120 often holds closer to 100 gallons of water once you account for the glass, the gap at the top, and the gravel and rock inside. For everyday care that difference doesn't matter much. For medication, salt and water conditioner, it does. Dose a 100-gallon tank as if it held 120 and you've overdosed by 20%. Some treatments have a narrow margin between working and harming fish.
Here's how to work out the real volume, with formulas, a worked example, and the parts people forget: displacement, sumps and canister filters.
Why true volume matters
- Medications are dosed per gallon. Too little may not work. Too much can stress or kill fish, and some treatments also harm the bacteria in your biofilter.
- Salt for treatments is dosed as a concentration. A 20% error is a 20% error in the dose.
- Water conditioner has to cover the water you're treating. When you refill straight from a hose, that means dosing for the full tank volume. See our chloramine guide.
- Water-change planning. A "25% water change" only means something if you know what 100% is. Our nitrate guide shows how change size controls where nitrate settles.
How many gallons is my tank? The basic formula
A U.S. gallon is defined as exactly 231 cubic inches (NIST Handbook 44, Appendix C). So:
Gallons = length × width × water depth (all in inches) ÷ 231
In metric, liters = length × width × depth (in cm) ÷ 1,000. One U.S. gallon is 3.785 liters.
Measure the water, not the glass
Use inside dimensions and the actual water depth, not the outside measurements of the tank:
- Subtract the glass thickness on both sides for length and width.
- Measure depth to the water line, not the rim. In a bare tank, measure from the bottom glass. If there's gravel or sand, measure from the top of the substrate. Measuring from the glass counts the gravel as water, and that errs toward overdosing.
Worked example
A tank sold as about 120 gallons measures 48 × 24 × 24 inches outside:
- Box volume: 48 × 24 × 24 ÷ 231 = 119.7 gallons
- Inside the glass: about 47 × 23 inches (half-inch glass), with the water about 21.5 inches deep, after the bottom glass and a gap of about two inches at the top
- Actual water: 47 × 23 × 21.5 ÷ 231 = 100.6 gallons
That's about 16% less than the box volume, before any gravel or rock goes in.
Other tank shapes
| Shape | Formula (inches → gallons) |
|---|---|
| Rectangle or cube | L × W × depth ÷ 231 |
| Cylinder | 3.1416 × inside radius² × depth ÷ 231 (radius = half the inside diameter) |
| Quarter-circle corner tank | 3.1416 × radius² × depth ÷ 4 ÷ 231 |
| Regular hexagon | 2.598 × side² × depth ÷ 231 |
| Bowfront | Measure by filling (below). Or trace the inside footprint on graph paper or cardboard, find its area in square inches, and multiply by depth ÷ 231. |
Example: a 24-inch-diameter cylinder with 30 inches of water holds 3.1416 × 12² × 30 ÷ 231 ≈ 58.8 gallons.
What displaces water inside the tank
Everything solid in the tank takes up space that would otherwise be water:
- Substrate. Gravel and sand don't displace their full volume. Water fills the gaps between grains, so a bed of gravel displaces less than its measured bulk. It still adds up in a tank with a deep bed. If you measure water depth from the top of the substrate instead of the bottom glass, your estimate will run a little low, because it ignores the water between the grains. That errs toward underdosing rather than overdosing.
- Rock and wood. Large rockwork can take up a big share of a heavily aquascaped tank. Solid rock displaces its full volume. To measure a big piece, set it in a bucket or tub of water, mark the level, lift it out, and refill to the mark from a measuring jug.
- Equipment and large fish. Heaters, pumps and very large fish take up some space too. It's minor in most tanks, but not trivial in a crowded one.
The only way to know a scaped tank's volume for certain is to measure it.
The most accurate method: measure it
Next time you drain the tank for a big water change or a move, measure what goes back in:
- Count buckets. Mark a bucket at a known volume, such as 5 gallons, by filling it from a measuring jug. Count how many it takes to refill from your drain mark to the normal water line.
- Use an inline water meter on the hose. Garden-hose flow meters are inexpensive. Check yours once against a marked bucket, and it's accurate enough for this.
- Mark the glass. Once you know how many gallons a drop from the water line to a given mark represents, you can make the same size water change every time without measuring again.
If you only drained part of the tank, measure the drop in water depth and work the rest out from the formula.
Sumps, canisters and plumbing
For anything that circulates through the whole system, like medication, salt or conditioner, dose for the total system volume:
- Display tank, at its true volume
- Sump, at its running water level (with the pump on), not filled to the brim
- Canister filters and reactors, which can hold a gallon or more each on big systems
- Plumbing, which is usually small, but long runs of wide pipe add up
On sump systems, also check how much water drains down into the sump when the power goes off. Dose for the water that's actually in circulation. Many medications are removed by activated carbon, and some are broken down by UV, so follow the label's instructions about carbon and UV during treatment.
Dosing examples with true volume
Salt. A 0.1% salt solution is 1 gram per liter, or about 3.8 grams per U.S. gallon. In our example tank:
- Dosed for the box volume (119.7 gallons): about 453 grams
- Dosed for the true volume (100.6 gallons): about 381 grams
Dosing by the box overshoots by about 19%. Salt is used for several treatments, including protecting fish from nitrite. Aquaculture guidance is to keep chloride at least 10 times the nitrite-nitrogen level (SRAC 451). Our nitrite guide explains why. Always confirm the target concentration for the specific treatment.
Medication. If a label says "1 teaspoon per 10 gallons," a tank with 100 true gallons gets 10 teaspoons, not 12. Treatment timing can also depend on temperature. Ich, for example, cycles in about 3 to 6 days at 75°F to 79°F (UF/IFAS). See our aquarium temperature guide.
Emergency water changes. When ammonia spikes, the size of the water change matters. Knowing your true volume tells you how far to drain. See our ammonia guide.
Bonus: how heavy is your tank?
Fresh water weighs about 8.3 pounds per U.S. gallon. The USGS uses 8.34 as its standard figure, and warm aquarium water is a hair lighter (USGS). Our 100-gallon example holds roughly 840 pounds of water, before you add the glass, stand, substrate and rock. Large tanks belong on purpose-built stands. Before setting up a very large tank on an upper floor, check what the floor can carry.
FAQ
How do I calculate how many gallons my tank holds?
Multiply the inside length, width and water depth in inches, then divide by 231. For liters, multiply in centimeters and divide by 1,000.
Is the gallon size on the box accurate?
It's usually close to the outside box volume, rounded. The water inside is less. In our 120-gallon example above, it's about 16% less before any substrate or decor goes in, and more once they do.
Should I include my sump when dosing medication?
Yes. Medications, salt and conditioner circulate through the whole system, so dose for the display, the sump at running level and any canisters.
How do I find the volume of a bowfront or odd-shaped tank?
Measure it as you fill. Count known-volume buckets or use an inline water meter on the hose.
Does gravel count toward tank volume?
No. Substrate takes up space, though less than its full bulk because water fills the gaps between grains. Measuring water depth from the top of the substrate gives a slightly low estimate, which errs toward underdosing rather than overdosing.
Cover photo (cropped): Татьяна Максимова (Tatyana Maksimova) / iNaturalist, CC BY 4.0
