Tank volume calculator
Calculate your aquarium's true water volume for any shape. Adjust fill level, substrate depth, rock displacement and filter volume to get the exact figure you need for CO₂, dosing and medication calculations.
Why accurate volume matters for your planted tank
Tank manufacturers size their products by nominal capacity — the volume a tank would hold if filled to the brim with no substrate, no hardscape, and no fill-level shortfall. In practice, a tank marketed as "90 litres" often contains 15–25% less actual water than that figure suggests.
Glass thickness reduces the internal footprint. A typical 6mm pane on each wall of a 90 × 45 × 45 cm tank removes over 3 litres before you add anything. Five centimetres of substrate across the full footprint removes another 18 litres. Fill the tank to 90% rather than the brim and you lose another 8 litres. The final net water volume can easily be 25–30 litres below the nominal figure.
This matters because CO₂ dosing, liquid fertiliser doses, and medication are all per-litre calculations. Dosing against an inflated nominal volume leads systematically to understrength treatment — plants get less fertiliser than calculated, CO₂ targets are reached at a lower bubble rate than expected, and any medication prescribed "per litre" is diluted by the inaccuracy.
The nominal tank size is always an overestimate
Glass thickness, substrate depth, fill level and hardscape displacement all reduce actual water volume. A 90-litre tank with 6 cm of substrate, filled to 90%, typically contains 60–70 litres of water — not 90. Use the calculator above and enter your real dimensions.
Hardscape and the displacement effect
Rocks displace a meaningful volume of water. The exact amount depends on the density of the stone — denser rocks displace less water per kilogram than porous ones. Seiryu stone, one of the most popular aquascaping stones, has a density of roughly 2.7 g/cm³, meaning each kilogram displaces about 0.37 litres. A 10 kg Seiryu scape displaces roughly 3.7 litres of water.
Dragon stone (Ohko) is significantly more porous, with a density around 1.5–1.8 g/cm³ depending on the piece. A 10 kg dragon stone arrangement displaces 5.5–6.5 litres. For heavily scaped tanks, particularly iwagumi styles with substantial stone masses, hardscape displacement can account for 5–10 litres of reduced water volume.
The calculator includes density data for common stone types sold by specialist UK retailers including Horizon Aquatics and Aquarium Gardens. Enter the total weight of your hardscape in kilograms and select the stone type — the displacement is calculated automatically. For custom or unusual materials, you can enter the displacement in litres directly.
True system volume vs net water volume
Your external filter holds water too — water that participates in the system and is replaced during water changes. The Eheim Classic 2217 holds approximately 3 litres of water in its canister and hoses. The Fluval 307 holds around 2.5 litres. For a 60-litre net tank volume, a 3-litre filter represents a 5% addition to the system — significant for CO₂ concentration calculations.
For water change percentage calculations, the net water volume in the tank is what you drain and refill, so use the net water volume. For CO₂ dosing and fertiliser target concentrations, the true system volume (tank + filter) gives a more accurate target, because CO₂ and dissolved nutrients distribute throughout the entire water volume including the filter circuit.
Send to dashboard
The button above passes your volume directly to the pH Monitor tab in the AquaCalc dashboard. Use the true system volume if you've entered hardscape and filter data — it gives the most accurate CO₂ and dosing calculations.
Once you know your tank volume, use the substrate calculator to find out how many bags of aquasoil or gravel your footprint needs at your target depth. Then use the CO₂ cylinder runtime estimator to find out how long your cylinder will last based on your tank's CO₂ demand.