Aquarium volume calculator
Your tank does not hold width × height × depth. Substrate and hardscape displace water, so the volume you should dose against is smaller — typically 15–25% smaller on a planted tank. This calculator gives you both numbers: the gross litres and the net water volume that fertilizer, conditioner and medication amounts should be based on.
The formula, exactly
Dimensions are internal, in centimetres. The result is in litres, because 1000 cm³ is one
litre — the /1000 in each line is that conversion and nothing else.
These are the formulas AquaGuru uses internally for every tank, unchanged — the app just does the arithmetic for you and reuses the net volume for dosing, water changes and AI context.
Worked example: an ADA 60P
Internal dimensions 60 × 36 × 30 cm, 5 cm of aqua soil, medium hardscape:
| Step | Calculation | Result |
|---|---|---|
| Gross | 60 × 36 × 30 ÷ 1000 | 64.8 L |
| Substrate | 60 × 30 × 5 ÷ 1000 | − 9.0 L |
| Hardscape | 64.8 × 0.055 | − 3.6 L |
| Net | 64.8 − 9.0 − 3.6 | 52.2 L |
| Weekly change | round(52.2 × 0.30) | 16 L |
| Dechlorinator | 16 ÷ 40 | 0.4 ml |
| Reef salt (if marine) | 16 × 35 | 560 g |
The label on the bottle says 60 litres. The tank holds 52. A 5 ml dose calculated on gross volume is a 10% overdose, every single day — which is exactly how planted tanks end up with algae problems that no one can explain.
Why net volume matters
- Dosing — fertilizer and liquid carbon dissolve in water, not in gravel. Net volume sets the concentration.
- Water changes — the litres you remove and replace are a share of net volume, so 30% of 52 L is 16 L, not 18 L.
- Dechlorinator — dosed against replacement water, so it follows the water change.
- Salt mix — grams per litre of new saltwater; over-mixing pushes salinity above 35 ppt.
- Medication and CO₂ — both are concentration-sensitive; gross volume over-doses both.
Imperial units are a conversion, not a different formula: 1 inch = 2.54 cm, and 1 US gallon = 3.78541 litres.
Questions
Is a 60×36×30 cm tank really 64.8 litres?
64.8 litres is the gross volume. With 5 cm of substrate and medium hardscape you have about 52 L of water. Dose against 52.
Do substrate and hardscape reduce the volume?
Yes — permanently. Substrate takes width × depth × substrate depth; hardscape is estimated at 3%, 5.5% or 9% of gross volume depending on how much rock and wood is in the tank.
Should I use gross or net volume for dosing?
Net. Using gross volume over-doses by 15–25% on a typical planted tank.
How much water should I change weekly?
30% of net volume, rounded to whole litres, is a solid starting point — 16 L for a 52 L net tank. Heavily stocked or high-tech tanks often need more.
How much dechlorinator do I need?
This tool uses 1 ml per 40 L of replacement water, so a 16 L change needs 0.4 ml. Check your bottle — concentrations vary a lot, and some are dosed per total tank volume instead.
How much salt for a reef water change?
About 35 g per litre, so 560 g for 16 L. That lands near 35 ppt (SG ≈ 1.025) with most reef salts, but mix to your refractometer, not to the number.
AquaGuru stores these numbers per tank — net volume, dosing in millilitres, weekly change — and hands them to AquaAI so advice uses your real numbers.