In departmental accounts, rent is apportioned on the basis of

In departmental accounts, shared expenses that cannot be traced to a single department must be apportioned (shared out) between departments on some fair, logical basis that reflects how each department actually causes or benefits from the cost.

Rent is a charge for occupying physical space, so the fairest basis for sharing it out is the amount of floor area each department occupies. A department that takes up more space is naturally responsible for a larger share of the rent, regardless of how much it sells, purchases, or how many staff it employs.

Volume of sales, purchases, and number of personnel are appropriate bases for apportioning other kinds of expenses (for example, selling expenses might follow sales volume, and staff-related costs might follow personnel numbers), but none of these measures how much space a department occupies, so none of them fairly reflects the burden of a rent charge.

When apportioning any shared expense, always match the basis to what actually drives that cost: for rent and similar occupancy costs, that driver is floor area.