Deadweight Loss from Any Inefficient Quantity

Efficiency happens where marginal social benefit equals marginal social cost. Every unit below that quantity would have added more benefit than it cost to make, so skipping it throws away surplus. Every unit beyond that quantity costs more than it's worth, so making it also throws away surplus. Underproduction and overproduction are the same mistake in opposite directions: both leave units traded that shouldn't be, or missing that should exist, and both waste value. That's why deadweight loss cares only about distance from the efficient quantity, not which direction you're off.

Picture the gap between the efficient quantity and the actual quantity. For each unit in that gap, the vertical distance between MSB and MSC is the surplus lost on that one unit. Stack those losses across the whole gap and you get a triangle: its height is the MSB-MSC gap at the actual quantity, its base is the quantity gap itself. Area equals one half times base times height, so deadweight loss grows with the square of how far output strays from efficient. Move twice as far away, lose four times as much surplus.

The same triangle appears everywhere a wedge separates benefit from cost. A tax wedges a gap between what buyers pay and what sellers receive, cutting quantity below efficient. A monopoly restricts output to raise price, same shape. A negative externality lets producers ignore costs they impose on others, pushing quantity above efficient. Different causes, identical geometry. Later topics in this unit just ask what's causing the wedge and how to close it.