Desk utilisation is the share of your desks actually used over a period. If you have 200 desks and an average of 90 are occupied, utilisation is 45 percent. It is the single most useful number for deciding how much office you need.
Measure it from booking and check-in data rather than badge swipes alone: a person who badges in once can sit anywhere, but a checked-in booking ties a person to a desk. Most hybrid offices discover their true peak is far below their desk count.