What this calculator does
Consumer surplus is the gap between what buyers were willing to pay for something and what they actually paid, summed across every unit sold. If someone would have paid $50 for a product but the market price is $30, they have gained $20 of surplus value on that purchase, even though no money changed hands to reflect it.
Consumer surplus is easy to confuse with deadweight loss, which is a different concept entirely. Consumer surplus measures value gained by buyers at the actual market outcome; deadweight loss measures value lost to everyone, buyers and sellers combined, when a market is distorted away from its efficient outcome by a tax, a price control or another restriction. One is about the size of a benefit, the other about the size of a loss.
The formula
With a simple linear demand curve, consumer surplus is the triangular area above the market price and below the demand curve, out to the quantity actually sold. That area works out to one-half times the gap between the maximum price buyers would pay (where the demand curve meets zero quantity) and the actual market price, times the quantity sold.
| Term | Meaning |
|---|---|
| Willingness to pay | The highest price at which quantity demanded falls to zero, sometimes called the choke price. |
| Market price | The price buyers actually pay for the good. |
| Quantity | The number of units sold at the market price. |
The inputs explained
| Field | What to enter |
|---|---|
| Maximum willingness to pay (choke price) ($) | The maximum price the most eager buyer would pay, or the price at which demand for the good disappears entirely. |
| Actual market price ($) | The actual price buyers are paying in the market. |
| Quantity sold at this price | How many units are being sold at that market price. |
When to use it
Illustrating a demand curve in an economics course
Consumer surplus is the standard textbook example of the triangular area under a demand curve, and working it from actual numbers makes the geometry concrete rather than abstract.
Evaluating a price cut
Lowering price expands both the quantity sold and the gap between willingness to pay and price, so recalculating surplus before and after a price change shows how much extra value shifts to buyers.
Comparing buyer benefit against seller revenue
Consumer surplus and total revenue answer different questions about the same sale, and looking at both shows how the value created by a transaction splits between the two sides.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How does consumer surplus change as the market price falls?
The same demand ceiling and quantity, at a range of market prices.
| Market price | Consumer surplus | Gap between willingness to pay and price |
|---|---|---|
| $10 | $4,000.00 | $40.00 |
| $20 | $3,000.00 | $30.00 |
| $30 | $2,000.00 | $20.00 |
| $35 | $1,500.00 | $15.00 |
| $40 | $1,000.00 | $10.00 |
| $45 | $500.00 | $5.00 |
Questions
How is consumer surplus different from deadweight loss?
Consumer surplus is the value buyers gain at the market outcome that actually happens. Deadweight loss is value lost to the whole market, buyers and sellers together, when something (a tax, a quota, a price ceiling) pushes the market away from its efficient quantity. They are measured from different triangles on a supply-and-demand diagram.
What if I only know price and quantity, not willingness to pay?
Willingness to pay has to come from somewhere, usually the price intercept of an estimated demand curve. Without an estimate of what buyers would pay at very low quantities, consumer surplus cannot be calculated exactly.
Does a lower price always increase total consumer surplus?
For a normal downward-sloping demand curve, yes: a lower price both increases the per-unit gap for existing buyers and brings in new buyers who now find the price worth paying, so total surplus rises.
Is this calculator only for a straight-line demand curve?
Yes. It treats consumer surplus as the area of a triangle, which holds exactly only when demand is linear over the range being considered. A curved demand function needs integration for an exact figure, though the triangle is a reasonable approximation over a small price range.
For the related concept of value lost rather than value gained, see the deadweight loss calculator.