😊 Consumer Surplus
Definition: Consumer surplus is the 'bonus' that buyers get when they pay less than they were prepared to.
Where is it on the diagram?
Consumer surplus is the triangle between the demand curve and the equilibrium price line, from Q = 0 to Qₑ. The demand curve shows what consumers are willing to pay; the market price is what they actually pay. The gap is their surplus.
- Area ABOVE the price line and BELOW the demand curve
- The first units have the most surplus (buyers valued them highest)
- The last unit at Qₑ has zero surplus (the buyer's willingness to pay equals the price)
Think of it this way: if you would have paid $10 for a coffee but it only costs $4, your consumer surplus on that coffee is $6. On the diagram, that $6 is part of the triangle above the price line.
🏭 Producer Surplus
Definition: Producer surplus is the 'bonus' that sellers get when they sell for more than their minimum acceptable price.
Where is it on the diagram?
Producer surplus is the triangle between the equilibrium price line and the supply curve, from Q = 0 to Qₑ. The supply curve shows the minimum price producers need; the market price is what they actually receive.
- Area BELOW the price line and ABOVE the supply curve
- The first units have the most surplus (cheapest to produce, sold at market price)
- The last unit at Qₑ has zero surplus (cost of production equals the price)
Consumer surplus = triangle ABOVE price, BELOW demand. Producer surplus = triangle BELOW price, ABOVE supply. Together they form the total welfare in the market.
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🔄 How Changes Affect Surplus
When price rises
- Consumer surplus DECREASES (buyers pay more, fewer can afford it)
- Producer surplus INCREASES (sellers receive more per unit)
- Some surplus transfers from consumers to producers
When price falls
- Consumer surplus INCREASES (buyers pay less, more can buy)
- Producer surplus DECREASES (sellers receive less per unit)
- Some surplus transfers from producers to consumers
Community surplus and efficiency
Community surplus (or total welfare) = consumer surplus + producer surplus. At the free-market equilibrium, community surplus is maximised — this is called allocative efficiency.
Why this matters: When government intervenes (price controls, taxes), it usually creates a deadweight loss — a reduction in community surplus. This concept connects to Topics 2.7 and 2.8.