Q.Find the consumers' surplus for the demand function $p = 25 - x - x^2$ when $p_0 = 19$.

Consumer Surplus: The Extra You Get for Free

Imagine you walk into a shop to buy a phone. You were willing to pay up to ₹30,000 for it — that's the maximum you'd give. But the shopkeeper has priced it at ₹22,000. You pay ₹22,000 and walk out with the phone. How much did you gain from this deal?

You gained ₹8,000. That's the difference between what you were willing to pay (₹30,000) and what you actually paid (₹22,000). That extra ₹8,000 is your consumer surplus.

The Intuition: Why It Exists

Markets don't charge each person their personal maximum. Instead, they set a single price for everyone. If you value the good more than that price, you get a "bonus" — you paid less than it was worth to you. If you value it less, you simply don't buy.

Think of it like this: you'd have been happy paying ₹30,000. Paying only ₹22,000 feels like getting ₹8,000 for free. That feeling of "extra value" is consumer surplus.

The Precise Statement

For a single unit, consumer surplus is:

For the whole market, it's the sum of all individual surpluses. Graphically, it's the area below the demand curve and above the market price, up to the quantity bought.

A Simple Example

Suppose the demand for apples is:

If the market price is ₹6, then:

• The first buyer (willing to pay ₹10) gets surplus = ₹10 − ₹6 = ₹4
• The second buyer (willing to pay ₹8) gets surplus = ₹8 − ₹6 = ₹2
• The third buyer (willing to pay ₹6) gets surplus = ₹6 − ₹6 = ₹0

Total consumer surplus = ₹4 + ₹2 + ₹0 = ₹6.

Why It Matters

Consumer surplus measures how much buyers benefit from participating in a market. A high consumer surplus means buyers are getting a great deal relative to their valuation. Policymakers use it to evaluate the impact of taxes, subsidies, or price controls — a tax reduces consumer surplus, while a subsidy increases it.

In short: consumer surplus is the hidden profit you earn every time you buy something for less than it's worth to you.