Aquarium weight and volume: what it really weighs
A 300 litre tank is 300 litres on the outside only. Water, glass and substrate add up to a figure the stand has to carry.
Outside volume and inside volume
A tank that measures 120 × 50 × 50 cm on the outside holds 300 L. Inside there is less, for three reasons: the water stops short of the rim, the substrate takes up part of the height, and the glass eats about a centimetre on every side.
Work from the internal dimensions instead: 118 × 48 × (50 − 1 − 5) ≈ 249 L. That is roughly 17% below the figure on the box. The gap matters for fish and for fertilizers, because doses are calculated from the water you actually have, not from the label.
Two tanks can carry the same label and hold different amounts of water. Glass thickness, substrate depth and the water line all move the real figure, and the real figure is the one worth using.
What the glass weighs
A useful rule of thumb: 2.5 kg per m² at 1 mm of thickness. For the same tank:
- bottom, 1.2 × 0.5 m at 10 mm: 15 kg;
- walls, two at 1.2 × 0.5 m and two at 0.5 × 0.5 m, 8 mm: about 34 kg;
- glass in total: about 49 kg.
The bottom alone is 15 kg, and it is usually the thickest piece in the tank.
What the substrate weighs
Substrate volume is bottom area times layer depth. With a 5 cm layer: 1.18 × 0.48 × 0.05 ≈ 28 L.
| Substrate | Bulk density | 28 L gives |
|---|---|---|
| Expanded clay | ≈ 0.6 kg/L | ≈ 17 kg |
| Quartz sand | ≈ 1.6 kg/L | ≈ 45 kg |
The spread is almost threefold, so "substrate" on its own says nothing. Sand and nutrient soils are heavier; expanded clay and lava rock are lighter. Sand can weigh nearly as much as the glass, and that changes what the stand has to carry.
Why a 10% margin
The stand calculation adds 10% to the weight of water, glass and substrate for everything that is not in the geometry: a filter full of water, the light, the lid, a CO₂ cylinder, driftwood, stones, decor.
The total for the example
| Part | Weight |
|---|---|
| Water, 249 L | 249 kg |
| Glass | 49 kg |
| Substrate (expanded clay, 5 cm) | 17 kg |
| Total | 315 kg |
| With a 10% margin | ≈ 347 kg |
That is the figure the stand calculation uses. Swap the expanded clay for sand and the total grows by another 28 kg, which puts the build in a different class altogether.
What is not counted
- the lid and the light, often 3 to 10 kg;
- an external filter with water in it;
- driftwood and stones;
- the weight of fish and plants, usually negligible.
None of these is large against 315 kg. Together they are why the calculation adds a margin instead of trusting the geometry alone, and why big stones still get counted separately.
Calculator
The calculator takes your dimensions and works out glass, weight, stand and doses.
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