Budgeting & pacing
Underspend
Underspend is spend running behind the approved plan, caused by narrow audiences, low bids, restrictive cost targets, disapproved creative, tracking or billing issues, low demand or budgets spread too thinly to exit learning. Discovered late, it forces compressed spend at higher marginal cost.
In practice
Underspend looks less urgent than overspend and often costs more, because the recovery happens in the worst buying conditions of the flight.
Some underspend should be accepted: spending the full budget is not the objective if the available inventory cannot produce value at an acceptable cost. Reallocate early rather than force the original line.