Calufrax was an oblate spheroid planet with a diameter of about 14,000 kilometres. It was the last world captured by Zanak and reduced to a trophy the size of a football. Its metricized structure, having a variable atomic weight, betrayed its origin as not a natural planet, but the second segment of the Key to Time, and it was recovered by the Fourth Doctor and Romana.

Prior this adventure, the Doctor had visited Calufrax, which he found to be uninhabited and ice-coated. Perhaps due to this, he considered Calufrax "dull, boring, and tedious". and was immediately disappointed that the tracer indicated that one of the segments of the Key to Time was there. (DW: The Pirate Planet)

It was unrelated to Callufrax Minor. (IDW: The Forgotten)