How heavy is a Companion Cube?

Neither game gives the cube a weight, so this is our estimate. It starts from the size: about 76 cm (2.5 ft) on each side in Portal and 69 cm (2.25 ft) in Portal 2, the smaller cube of the sequel. That gives a volume of 0.76 × 0.76 × 0.76 ≈ 0.44 cubic metres. Solid steel that size would weigh about 3.4 tonnes, and even solid plastic would top 500 kg, far more than Chell could ever haul around a test chamber.

So we treat it as a hollow shell: plastic panels set in a metal frame with chunky corners, which is how the model looks. Six faces of 0.76 × 0.76 m make about 3.5 square metres of skin. At 30 kg, that is roughly 8.6 kg per square metre, about the same as 1 mm steel sheet, or plastic panels around 7 mm thick. That is a sturdy crate one adult could lift, but not casually.

The same build at Portal 2’s 69 cm has about 2.9 square metres of skin, which brings it down to about 25 kg.

The closest thing to an official number comes from the Educational Version of Portal 2, made for teachers. Its Puzzle Maker has a Contraption Cube whose mass you can set anywhere from 1 to 85 kg, with a default of 40 kg. The heavier you make it, the bigger the sphere shown inside it.

It is not the Companion Cube, and the Combine OverWiki treats it as non-canon, but it shows the kind of weight Valve had in mind for a cube you carry around a test chamber. Our 30 kg sits a little below it.

Apart from the pink hearts, the Companion Cube is an ordinary Weighted Storage Cube, so it weighs the same. Valve’s Puzzle Maker notes say it has no special properties at all. Its job is to hold down the Heavy Duty Super-Colliding Super Button, which a cube, a person standing on it or a Frankenturret can all press.

It only got the hearts because playtesters kept leaving their cube behind in Test Chamber 17. Marking it as special told them to bring it along, and having GLaDOS make them incinerate it at the end gave the chamber its sting.