Aquarium volume calculator

Outer dimensions give the gross volume — but the glass, the substrate bed and the gap below the rim all eat into the water you actually hold. This works out the real, net litres (plus US and UK gallons).

Manfred · the honest-litres guy

The number on the box is the outer box. Manfred subtracts the glass walls, the substrate bed and the few centimetres you never fill — so your dosing and stocking are based on the water that's really in there, not the label.

Tank dimensions

LengthWidthHeight

Measure the outer edges in centimetres. Don't subtract anything yet — the glass and substrate come off below.

Real-water adjustments

Glass thicknessEach pane removes twice its thickness from length and width. 6 mm is typical for tanks up to about 120 cm.Substrate depthThe bed below the water line. Roughly half its height displaces water — gravel and sand are porous.Gap below the rimHow far below the top edge you fill. 2 cm is a common safe margin.

Enter length, width and height to see the net and gross volume.

How the net volume is worked out

Gross volume is simply length × width × height. The net water volume subtracts the glass walls from length and width, the gap below the rim and the bottom pane from the height, and about half the substrate depth for displacement — then converts cm³ to litres (1 L = 1000 cm³).

net L = (L − 2·g) × (W − 2·g) × (H − fill − g − 0.5·sub) ÷ 1000

Treat the result as a close estimate. Substrate porosity, rockwork, large décor and a sump or external filter all shift the true water volume — subtract hardscape displacement separately if you run a lot of stone.

In the app, Stormy uses your net volume for every dose and water-change suggestion, so the maths matches your real tank.

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