Count the lost net benefit over the wrong quantity interval.
Deadweight loss is the reduction in net social benefit when output differs from the social optimum. With overproduction, it is the excess of social cost over social benefit on the extra units. With underproduction, it is the beneficial net output that is forgone. For straight marginal curves, the gap forms a triangle. Do not confuse this area with all external damage, total spending or government revenue.
- Meaning
- Deadweight loss (DWL) is net social benefit lost because output differs from the social optimum. It includes harmful extra units or worthwhile units that are not produced.
- Triangle rule
- When the marginal benefit and cost curves are straight, DWL = half x quantity gap x marginal dollar gap. The gap between the curves is zero at the optimum and widens towards the actual quantity.
- Not the same
- Total external damage counts all harm from the activity. DWL compares the net outcome with the best output, allowing for benefits lost and costs saved when quantity changes. Government spending is a different measure too.
| Case | Private Q | Social Q | DWL at private Q |
|---|---|---|---|
| MEC 4; no external benefit | 80 | 60 | 40 |
| MEB 4; no external cost | 80 | 100 | 40 |
Worked example: The difference between damage and avoidable loss
In the negative-externality model, private quantity is 80, social quantity is 60 and constant MEC is $4.
- The relevant output error is 20 units, from 60 to 80. At 60, MSB and MSC meet; at 80, the social cost-benefit gap is $4.
- DWL = 0.5 x 20 x 4 = $40 per day. The triangle's base is 20 deliveries per day and its height is $4 per delivery, so the area is dollars per day. It measures the net gain available from removing the excessive units.
- Total external damage at 80 is 80 x $4 = $320. That is not the $40 DWL: reducing production also forgoes some benefits and saves private costs.
- The efficient quantity need not eliminate all external damage. At 60, remaining damage is balanced against the benefits and resource costs of the activity in this model.
Watch out for this
All pollution damage is deadweight loss.
DWL compares net social benefit with the best feasible output. Total damage is only one part of that comparison.
Check your understanding
The market produces 30 units beyond the optimum. The social cost-benefit gap rises linearly from zero to $6. What is DWL?
- $180
- $90
- $6