Calculate the estimated weight and water displacement of rocks and driftwood in your aquarium setup.
Rock and driftwood transform a glass box into an aquascape, but every kilogram of hardscape displaces water, shifts the weight load on the cabinet, and changes the effective volume used for fertilizer and medication dosing. This calculator takes the type, count, and size of your rocks and woods, returns total weight, water displacement, and a corrected net water volume so that dosing, stocking, and stand-load calculations remain accurate. It is invaluable when building Iwagumi, jungle, or reef rockwork where hardscape can easily occupy 10-25% of the tank.
Each rock type has a characteristic specific gravity: lava rock is highly porous at roughly 2.0 g/cm3, slate sits near 2.8 g/cm3, and dragon stone (Ohko) averages around 2.3 g/cm3. The calculator estimates volume per piece from S/M/L size categories, multiplies by density to obtain mass, and computes displacement from the submerged volume. Driftwood is treated separately because it is buoyant when dry but absorbs water over the first weeks; once saturated it displaces roughly its submerged volume regardless of mass.
Select the type of rock and enter the approximate size and count of rocks and driftwood. Different rock types have different densities.
Water displacement shows how much less water your tank will hold due to the hardscape. This helps determine your actual net water volume.
Calculated weights are estimates based on average densities. Actual weight may vary based on the specific shape and porosity of your materials.
A typical aquascape can add 20–40 kg of rock to a 75 L tank. Combined with water, that load is concentrated on the tank stand and floor. Apartment floors are usually rated for 200 kg/m²; large tanks with heavy rockwork can exceed this and need spread support.
Lava rock is the lightest at about 1.5 g/cm³. Seiryu, dragon stone, and most landscape stones are around 2.7 g/cm³. Slate and granite are denser at 2.8–3.0 g/cm³. Always weigh dense rocks before adding them to large rockscapes.
Hardscape pushes water out, reducing actual water volume. A heavy aquascape can displace 10–20% of the tank volume, which affects medication dosing, fertilizer amounts, and stocking calculations. Use the displacement figure as your true water volume.
Driftwood is much less dense (about 0.6–0.9 g/cm³) and may even float until waterlogged. It still displaces water by its full submerged volume. Soak new driftwood for 1–2 weeks or boil it to make it sink and to leach excess tannins.
Total load is tank + water + rock + stand. A 200 L tank with 30 kg of rock and a steel stand can exceed 280 kg. Spread the load across joists with a wide stand or place the tank against a load-bearing wall, parallel to the joists below.