Tool
Tank Volume and Water Change Calculator
Most people do not know how much water is in their tank, which means they do not know how much a 25% change actually moves. Measure the glass, allow for what the substrate takes up, and the rest is arithmetic — printed on this page so you can check it.
Your tank
How do you want to give the size?
Inside measurements, in inches
Measure the water depth rather than the height of the glass — the few inches above the waterline are not in the tank.
Allowance for substrate, rock and equipment
A tank never holds its geometric volume. Ten percent is a common working allowance for a normal substrate bed and some decor.
Share of the water you are changing
Bucket size, in gallons
Live plants in the tank
Tank age
What that comes to
- Geometric volume
- 24.9 gal / 94.4 L
- Actual water, less 10%
- 22.4 gal / 85 L
- Water to change at 25%
- 5.6 gal / 21.2 L
- Buckets at 5 gal
- 2
Suggested starting point
Established without live plants: 25% weekly. That works out to about 5.6 gal weekly.
Why several small changes beat one big one
A single 25% change removes 25% of anything dissolved that the replacement water does not contain. Starting from a nitrate reading of 40 ppm, one change lands you at about 30 ppm, and a second identical change a week later takes it to about 22.5 ppm. Diluting is a curve, not a subtraction, which is why a steady schedule outperforms an occasional large effort.
Dosing
Dose your water conditioner for the volume you are adding, using the figure on the bottle in your hand. Concentrations differ enormously between products that look similar, and this calculator deliberately publishes no dosing numbers for anything.
The formulas, and what they ignore
What it calculates
- gallons = length × width × water depth ÷ 231
- liters = gallons × 3.785411784
- net gallons = gallons × (1 − displacement ÷ 100)
- change gallons = net gallons × percent ÷ 100
- buckets = ceiling(change gallons ÷ bucket size)
- remaining = before × (1 − percent ÷ 100)
What it does not account for
- Bow-front, corner, cylinder and hexagonal tanks. The dimension formula is for a rectangular box only.
- What is already in your tap water. A supply carrying nitrate puts a floor under what any water change can achieve.
- Glass thickness, which makes the true interior slightly smaller than an outside measurement suggests.
- Anything about your stocking, feeding or filtration, all of which move the right interval more than the arithmetic does.
- Conditioner strength. Every dose comes from the product label, never from here.
Why the number on the box is not the number
A tank sold as a twenty gallon does not hold twenty gallons of water. The label describes the box; the water stops several inches below the rim, and the substrate, rock, decor and equipment take up a share of what is left. Ten percent is a reasonable working allowance for a normal bed of gravel and some decor, and rather more than that if you have a deep substrate or a lot of stone.
This matters in two directions. It changes what a percentage water change is actually removing, and it changes every dose you measure by volume — which is one more reason those come from the product label rather than from a figure worked out here.
Common tank sizes, worked out
These are calculated with the same formula the tool uses, at typical dimensions and a ten percent allowance. Your own tank will differ; measure it rather than reading a row off this table.
| Sold as | Typical inside dimensions | Geometric volume | Water, less 10% | A 25% change |
|---|---|---|---|---|
| 10 gallon | 20 x 10 x 11 in | 9.5 gal | 8.6 gal | 2.1 gal |
| 20 gallon high | 24 x 12 x 15 in | 18.7 gal | 16.8 gal | 4.2 gal |
| 29 gallon | 30 x 12 x 17 in | 26.5 gal | 23.8 gal | 6 gal |
| 40 gallon breeder | 36 x 18 x 15 in | 42.1 gal | 37.9 gal | 9.5 gal |
| 55 gallon | 48 x 13 x 19 in | 51.3 gal | 46.2 gal | 11.5 gal |
| 75 gallon | 48 x 18 x 20 in | 74.8 gal | 67.3 gal | 16.8 gal |
Doing the change itself
- Unplug the heater and the filter at the outlet. Leave the heater unplugged until the glass is fully submerged again — a hot heater exposed to air can crack, and a cracked heater in water is an electrical hazard.
- Siphon from the substrate rather than from open water, working through a different section each week so the whole bed gets serviced over a month.
- Match the replacement water temperature by hand before it goes in. A large cold refill drops the tank several degrees and takes most of a day to recover.
- Treat the replacement water with a conditioner, dosed for the volume you are adding using the figure printed on that bottle.
- Refill gently so the substrate is not blasted into the water column, then plug the filter back in, then the heater.
- Clear the filter intake and squeeze the mechanical media in the bucket of water you just removed, while it is still there.
Where this fits
The water change is the single action behind almost every page on this site. Maintenance Schedule gives the intervals, The Nitrogen Cycle explains what a change does at each stage of a new tank, and Nitrate is the reading that tells you whether your interval is right.