How could you calculate unrealised profit on closing stock?

An unrealised profit in stock only arises in the context of group financial statements.
When group companies sell goods among each other at a profit, it is proper that each group company incorporates such profits in its own financial statements.
However, if these goods are not eventually sold to third parties outside of the group, all related profits are unrealised at the group level.
This follows from the principle that all group companies are regarded as one economic entity