Aquarium volume calculator
A tank sold as 300 litres rarely holds 300 litres of water. Work out what the glass holds from its size, then the net water volume that dosing, medicine and salinity sums need.
Formulas and sources last checked . How we check
Calculator
Result
Net water volume142 litres
- Display, filled
- 177 litres
- Less rock and sand
- 35 litres
- Plus sump, running
- 0 litres
- Net in UK gallons
- 31
- Net in US gallons
- 37
Use the net figure for dosing, medicines and salinity corrections; use the display figure for stocking and product labels that say tank size.
How do I work out net water volume?
Take the glass thickness off each side and the base, then the gap at the top, to get the water inside. Length times depth times water height in centimetres, divided by 1,000, gives litres. A bow front adds the curved slice at the front; a cylinder uses the area of a circle. Net water is that, less what rock and sand take up, plus the water in the sump while it runs.
litres = length x depth x water height (cm) / 1000bow front: add the circular segment with chord = length and height = bownet = display x (1 - rock and sand share) + sump
Worked example: a 90 x 45 x 50 cm tank
- Outside 90 x 45 x 50 cm with 8 mm glass: inside 88.4 x 43.4 cm, and 49.2 cm tall.
- Water 3 cm below the top: 46.2 cm deep. 88.4 x 43.4 x 46.2 / 1000 = 177 litres.
- Less 20 per cent for rock and sand: about 142 litres of net water. With a sump holding 40 litres while running: about 182 litres.
How much do rock and sand take up?
| Topic | What sources say | Our reading |
|---|---|---|
| Allowance for rock and sand |
| Use 20 per cent for a typical reef. Use 30 per cent before dosing anything that is dangerous in excess, such as carbon or medicine. |
What are the limits?
Other calculators
- Salt mixGrams of salt for a batch, and what your refractometer or hydrometer should read.
- Salinity correctionHow much water to swap, day by day, at a safe pace.
- Water change effectWhat a change, or a run of changes, does to a reading.
- ATO reservoir safetyHow far salinity falls if a stuck pump empties the reservoir.
- Dosing from a test resultMillilitres to bring alkalinity, calcium or magnesium back to target.
- Daily consumptionHow much your tank uses a day, and the daily dose to match.
- Unit converterdKH, meq/L, ppm, SG, ppt, litres and gallons, Celsius and Fahrenheit.
- Heater sizeWatts for a UK room, split across heaters so one stuck on is not a disaster.
- Light hours and PARPhotoperiod, ramps, daily light and PAR by coral group.
- Electricity running costWhat the tank costs to run at the current UK price cap.
- Stocking and compatibilityCheck a list of fish against each other, the tank and your corals.