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 Page 1


Microeconomics
Topic 8  “Apply principles of consumer/producer surplus
to explain efficient level of production and sales in a market.”
Reference: Gregory Mankiw’s Principles of Microeconomics, 2
nd
 edition, Chapter 7.
Consumer Surplus
Consumer surplus is the buyer's net gain from purchasing a good. Put another way, it is
the excess of what a buyer would have been willing to pay for a good over what he
actually had to pay for that good. Graphically, it is the area below the demand curve,
above the price, and left of the quantity bought.
For example, you might be willing, if you had to, to pay $20 to make an important phone
call. If the price you actually have to pay for that phone call is $1, then you earn
consumer surplus of $19 when you make the call.
Figure 2.1
 Price
Demand
Quantity
9
8
7
6
1 2 3 4
A: $3 of CS
B: $2 of CS
C: $1 of CS
D: $0 of CS
Total CS=3+2+1=$6
In figure 2.1, we suppose there are 4 consumers: A, B, C, and D. A is willing to pay $9
for one unit of the good. B will pay $8, C will pay $7, and D will pay $6. If the market
price of the good is $6 per unit, then A earns consumer surplus of $3, since he was
willing to pay $9, but only had to pay $6. Similarly, B earns $2 of consumer surplus
(CS), and C earns $1 of CS. Customer D, the so-called "marginal" consumer, is willing to
pay $6 for a unit, but since the market price is $6, D gets no consumer surplus.
In practice, consumer surplus is pictured as in figure 2.2, as the area below the demand
curve, above the price, and left of the quantity bought.
Page 2


Microeconomics
Topic 8  “Apply principles of consumer/producer surplus
to explain efficient level of production and sales in a market.”
Reference: Gregory Mankiw’s Principles of Microeconomics, 2
nd
 edition, Chapter 7.
Consumer Surplus
Consumer surplus is the buyer's net gain from purchasing a good. Put another way, it is
the excess of what a buyer would have been willing to pay for a good over what he
actually had to pay for that good. Graphically, it is the area below the demand curve,
above the price, and left of the quantity bought.
For example, you might be willing, if you had to, to pay $20 to make an important phone
call. If the price you actually have to pay for that phone call is $1, then you earn
consumer surplus of $19 when you make the call.
Figure 2.1
 Price
Demand
Quantity
9
8
7
6
1 2 3 4
A: $3 of CS
B: $2 of CS
C: $1 of CS
D: $0 of CS
Total CS=3+2+1=$6
In figure 2.1, we suppose there are 4 consumers: A, B, C, and D. A is willing to pay $9
for one unit of the good. B will pay $8, C will pay $7, and D will pay $6. If the market
price of the good is $6 per unit, then A earns consumer surplus of $3, since he was
willing to pay $9, but only had to pay $6. Similarly, B earns $2 of consumer surplus
(CS), and C earns $1 of CS. Customer D, the so-called "marginal" consumer, is willing to
pay $6 for a unit, but since the market price is $6, D gets no consumer surplus.
In practice, consumer surplus is pictured as in figure 2.2, as the area below the demand
curve, above the price, and left of the quantity bought.
Figure 2.2
Price
Quantity
Demand
$7
10
Consumer Surplus CS=(6x10)/2=$30
when price P=$7.
$13
$4
14
When price P falls to $4, existing
consumers gain (3x10)=$30 of CS
When price P falls to $4, new 
consumers gain (3x4)/2=$6 of CS
Figure 2.2 shows that when price is $7, consumer surplus is $30 (the yellow area). If the
price falls to $4, existing consumers save $3 per unit on the 10 units they were already
buying, for a gain of $30 (green) in consumer surplus. In addition, 4 more units are sold
to buyers who wouldn't have wanted the good for $7, but who do want it for $4. These
buyers gain $6 (blue) of consumer surplus when price falls from $7 to $4.
Producer Surplus
Producer surplus is the seller's net gain from selling a good. Alternatively, we could say it
is the excess of what a seller is paid for a good above what he would have barely been
willing to accept for it. Graphically, it is the area above the supply curve, below the price,
and left of the quantity sold.
Example: A seller sells a good for a price of $7. It costs him $4 to produce the good, so
$4 is the lowest price he would be willing to accept. He earns producer surplus of $3.
Page 3


Microeconomics
Topic 8  “Apply principles of consumer/producer surplus
to explain efficient level of production and sales in a market.”
Reference: Gregory Mankiw’s Principles of Microeconomics, 2
nd
 edition, Chapter 7.
Consumer Surplus
Consumer surplus is the buyer's net gain from purchasing a good. Put another way, it is
the excess of what a buyer would have been willing to pay for a good over what he
actually had to pay for that good. Graphically, it is the area below the demand curve,
above the price, and left of the quantity bought.
For example, you might be willing, if you had to, to pay $20 to make an important phone
call. If the price you actually have to pay for that phone call is $1, then you earn
consumer surplus of $19 when you make the call.
Figure 2.1
 Price
Demand
Quantity
9
8
7
6
1 2 3 4
A: $3 of CS
B: $2 of CS
C: $1 of CS
D: $0 of CS
Total CS=3+2+1=$6
In figure 2.1, we suppose there are 4 consumers: A, B, C, and D. A is willing to pay $9
for one unit of the good. B will pay $8, C will pay $7, and D will pay $6. If the market
price of the good is $6 per unit, then A earns consumer surplus of $3, since he was
willing to pay $9, but only had to pay $6. Similarly, B earns $2 of consumer surplus
(CS), and C earns $1 of CS. Customer D, the so-called "marginal" consumer, is willing to
pay $6 for a unit, but since the market price is $6, D gets no consumer surplus.
In practice, consumer surplus is pictured as in figure 2.2, as the area below the demand
curve, above the price, and left of the quantity bought.
Figure 2.2
Price
Quantity
Demand
$7
10
Consumer Surplus CS=(6x10)/2=$30
when price P=$7.
$13
$4
14
When price P falls to $4, existing
consumers gain (3x10)=$30 of CS
When price P falls to $4, new 
consumers gain (3x4)/2=$6 of CS
Figure 2.2 shows that when price is $7, consumer surplus is $30 (the yellow area). If the
price falls to $4, existing consumers save $3 per unit on the 10 units they were already
buying, for a gain of $30 (green) in consumer surplus. In addition, 4 more units are sold
to buyers who wouldn't have wanted the good for $7, but who do want it for $4. These
buyers gain $6 (blue) of consumer surplus when price falls from $7 to $4.
Producer Surplus
Producer surplus is the seller's net gain from selling a good. Alternatively, we could say it
is the excess of what a seller is paid for a good above what he would have barely been
willing to accept for it. Graphically, it is the area above the supply curve, below the price,
and left of the quantity sold.
Example: A seller sells a good for a price of $7. It costs him $4 to produce the good, so
$4 is the lowest price he would be willing to accept. He earns producer surplus of $3.
Figure 2.3
Price
Quantity
Supply
1
2
3
4
1
2 3
4
A: $3 of PS
B: $2 of PS
C: $1 of PS
D: $0 of PS
In Figure 2.3, we imagine 4 sellers: A, B, C, and D. A would barely be willing to sell a
unit of the good for $1, but if the going price of the good is $4, A earns producer surplus
(PS) of $3. B would sell for $2, but since he charges $4 he earns PS of $2. C earns PS of
$1, while D, the “marginal” seller, earns zero PS, since he is barely willing to sell for the
going price of $4.
Figure 2.4
Price
Quantity
8 12
7
10
Supply
Producer Surplus 
PS=(5x8)/2=$20 when
price P=$7
When Price rises to $10,
existing sellers gain
3x8=$24
When  Price rises to $10,
new sellers gain (3x4)/2=$6
2
In Figure 2.4, we see that at a price of $7, producer surplus is $20 (blue). If the price rises
to $10, then existing sellers gain (3x8)=$24 each (yellow), while new sellers gain
(3x4)/2=$6 (green).
Page 4


Microeconomics
Topic 8  “Apply principles of consumer/producer surplus
to explain efficient level of production and sales in a market.”
Reference: Gregory Mankiw’s Principles of Microeconomics, 2
nd
 edition, Chapter 7.
Consumer Surplus
Consumer surplus is the buyer's net gain from purchasing a good. Put another way, it is
the excess of what a buyer would have been willing to pay for a good over what he
actually had to pay for that good. Graphically, it is the area below the demand curve,
above the price, and left of the quantity bought.
For example, you might be willing, if you had to, to pay $20 to make an important phone
call. If the price you actually have to pay for that phone call is $1, then you earn
consumer surplus of $19 when you make the call.
Figure 2.1
 Price
Demand
Quantity
9
8
7
6
1 2 3 4
A: $3 of CS
B: $2 of CS
C: $1 of CS
D: $0 of CS
Total CS=3+2+1=$6
In figure 2.1, we suppose there are 4 consumers: A, B, C, and D. A is willing to pay $9
for one unit of the good. B will pay $8, C will pay $7, and D will pay $6. If the market
price of the good is $6 per unit, then A earns consumer surplus of $3, since he was
willing to pay $9, but only had to pay $6. Similarly, B earns $2 of consumer surplus
(CS), and C earns $1 of CS. Customer D, the so-called "marginal" consumer, is willing to
pay $6 for a unit, but since the market price is $6, D gets no consumer surplus.
In practice, consumer surplus is pictured as in figure 2.2, as the area below the demand
curve, above the price, and left of the quantity bought.
Figure 2.2
Price
Quantity
Demand
$7
10
Consumer Surplus CS=(6x10)/2=$30
when price P=$7.
$13
$4
14
When price P falls to $4, existing
consumers gain (3x10)=$30 of CS
When price P falls to $4, new 
consumers gain (3x4)/2=$6 of CS
Figure 2.2 shows that when price is $7, consumer surplus is $30 (the yellow area). If the
price falls to $4, existing consumers save $3 per unit on the 10 units they were already
buying, for a gain of $30 (green) in consumer surplus. In addition, 4 more units are sold
to buyers who wouldn't have wanted the good for $7, but who do want it for $4. These
buyers gain $6 (blue) of consumer surplus when price falls from $7 to $4.
Producer Surplus
Producer surplus is the seller's net gain from selling a good. Alternatively, we could say it
is the excess of what a seller is paid for a good above what he would have barely been
willing to accept for it. Graphically, it is the area above the supply curve, below the price,
and left of the quantity sold.
Example: A seller sells a good for a price of $7. It costs him $4 to produce the good, so
$4 is the lowest price he would be willing to accept. He earns producer surplus of $3.
Figure 2.3
Price
Quantity
Supply
1
2
3
4
1
2 3
4
A: $3 of PS
B: $2 of PS
C: $1 of PS
D: $0 of PS
In Figure 2.3, we imagine 4 sellers: A, B, C, and D. A would barely be willing to sell a
unit of the good for $1, but if the going price of the good is $4, A earns producer surplus
(PS) of $3. B would sell for $2, but since he charges $4 he earns PS of $2. C earns PS of
$1, while D, the “marginal” seller, earns zero PS, since he is barely willing to sell for the
going price of $4.
Figure 2.4
Price
Quantity
8 12
7
10
Supply
Producer Surplus 
PS=(5x8)/2=$20 when
price P=$7
When Price rises to $10,
existing sellers gain
3x8=$24
When  Price rises to $10,
new sellers gain (3x4)/2=$6
2
In Figure 2.4, we see that at a price of $7, producer surplus is $20 (blue). If the price rises
to $10, then existing sellers gain (3x8)=$24 each (yellow), while new sellers gain
(3x4)/2=$6 (green).
The Efficient Level of Production in a Market
The efficient level of production is the quantity of output that results in the highest total
surplus, which is the sum of consumer surplus and producer surplus.
Figure 2.5
Price
Quantity
A
B
C
D
Supply
Demand
$12
6 10
15
$7=value to last buyer
$16=cost to last seller
F
E
Deadweight loss from
underproduction=B+D
Deadweight loss from
overproduction=E+F
Figure 2.5 shows the relative efficiency of different levels of production. At the
equilibrium output of 10 units, consumer surplus is given by areas A + B (green + blue).
Producer surplus is given by areas C+D (yellow + purple).
Now suppose that output is reduced from 10 to 6 units, while the price remains the same.
Consumer surplus is reduced to area A. (Recall that CS is the area below demand, above
price, and left of the quantity bought.) Similarly, producer surplus is reduced to area C.
Thus, total surplus is reduced to area A+C, and society suffers a deadweight loss equal to
B+D (blue+purple), which is the loss in total surplus when production falls below the
equilibrium quantity.
Conclusion: the equilibrium output level is economically superior to any lower output
level. Producing less than the equilibrium quantity is inefficient because total surplus is
reduced. Put differently, at output levels below equilibrium, consumers’ willingness to
pay for an extra unit of the good exceeds the sellers’ cost of producing an extra unit of the
good. So from a welfare standpoint, more of the good should be produced.
It is also possible to produce too much. Suppose that output rises from 10 to 15. At Q=15,
the height of the demand curve is $7, while the height of the supply curve is $16. This
means that it costs $16 to produce a unit that buyers value at only $7. The 15
th
 unit thus
created a loss of (16-7)=$9. Consumer surplus falls by the area F --where the price of $12
Page 5


Microeconomics
Topic 8  “Apply principles of consumer/producer surplus
to explain efficient level of production and sales in a market.”
Reference: Gregory Mankiw’s Principles of Microeconomics, 2
nd
 edition, Chapter 7.
Consumer Surplus
Consumer surplus is the buyer's net gain from purchasing a good. Put another way, it is
the excess of what a buyer would have been willing to pay for a good over what he
actually had to pay for that good. Graphically, it is the area below the demand curve,
above the price, and left of the quantity bought.
For example, you might be willing, if you had to, to pay $20 to make an important phone
call. If the price you actually have to pay for that phone call is $1, then you earn
consumer surplus of $19 when you make the call.
Figure 2.1
 Price
Demand
Quantity
9
8
7
6
1 2 3 4
A: $3 of CS
B: $2 of CS
C: $1 of CS
D: $0 of CS
Total CS=3+2+1=$6
In figure 2.1, we suppose there are 4 consumers: A, B, C, and D. A is willing to pay $9
for one unit of the good. B will pay $8, C will pay $7, and D will pay $6. If the market
price of the good is $6 per unit, then A earns consumer surplus of $3, since he was
willing to pay $9, but only had to pay $6. Similarly, B earns $2 of consumer surplus
(CS), and C earns $1 of CS. Customer D, the so-called "marginal" consumer, is willing to
pay $6 for a unit, but since the market price is $6, D gets no consumer surplus.
In practice, consumer surplus is pictured as in figure 2.2, as the area below the demand
curve, above the price, and left of the quantity bought.
Figure 2.2
Price
Quantity
Demand
$7
10
Consumer Surplus CS=(6x10)/2=$30
when price P=$7.
$13
$4
14
When price P falls to $4, existing
consumers gain (3x10)=$30 of CS
When price P falls to $4, new 
consumers gain (3x4)/2=$6 of CS
Figure 2.2 shows that when price is $7, consumer surplus is $30 (the yellow area). If the
price falls to $4, existing consumers save $3 per unit on the 10 units they were already
buying, for a gain of $30 (green) in consumer surplus. In addition, 4 more units are sold
to buyers who wouldn't have wanted the good for $7, but who do want it for $4. These
buyers gain $6 (blue) of consumer surplus when price falls from $7 to $4.
Producer Surplus
Producer surplus is the seller's net gain from selling a good. Alternatively, we could say it
is the excess of what a seller is paid for a good above what he would have barely been
willing to accept for it. Graphically, it is the area above the supply curve, below the price,
and left of the quantity sold.
Example: A seller sells a good for a price of $7. It costs him $4 to produce the good, so
$4 is the lowest price he would be willing to accept. He earns producer surplus of $3.
Figure 2.3
Price
Quantity
Supply
1
2
3
4
1
2 3
4
A: $3 of PS
B: $2 of PS
C: $1 of PS
D: $0 of PS
In Figure 2.3, we imagine 4 sellers: A, B, C, and D. A would barely be willing to sell a
unit of the good for $1, but if the going price of the good is $4, A earns producer surplus
(PS) of $3. B would sell for $2, but since he charges $4 he earns PS of $2. C earns PS of
$1, while D, the “marginal” seller, earns zero PS, since he is barely willing to sell for the
going price of $4.
Figure 2.4
Price
Quantity
8 12
7
10
Supply
Producer Surplus 
PS=(5x8)/2=$20 when
price P=$7
When Price rises to $10,
existing sellers gain
3x8=$24
When  Price rises to $10,
new sellers gain (3x4)/2=$6
2
In Figure 2.4, we see that at a price of $7, producer surplus is $20 (blue). If the price rises
to $10, then existing sellers gain (3x8)=$24 each (yellow), while new sellers gain
(3x4)/2=$6 (green).
The Efficient Level of Production in a Market
The efficient level of production is the quantity of output that results in the highest total
surplus, which is the sum of consumer surplus and producer surplus.
Figure 2.5
Price
Quantity
A
B
C
D
Supply
Demand
$12
6 10
15
$7=value to last buyer
$16=cost to last seller
F
E
Deadweight loss from
underproduction=B+D
Deadweight loss from
overproduction=E+F
Figure 2.5 shows the relative efficiency of different levels of production. At the
equilibrium output of 10 units, consumer surplus is given by areas A + B (green + blue).
Producer surplus is given by areas C+D (yellow + purple).
Now suppose that output is reduced from 10 to 6 units, while the price remains the same.
Consumer surplus is reduced to area A. (Recall that CS is the area below demand, above
price, and left of the quantity bought.) Similarly, producer surplus is reduced to area C.
Thus, total surplus is reduced to area A+C, and society suffers a deadweight loss equal to
B+D (blue+purple), which is the loss in total surplus when production falls below the
equilibrium quantity.
Conclusion: the equilibrium output level is economically superior to any lower output
level. Producing less than the equilibrium quantity is inefficient because total surplus is
reduced. Put differently, at output levels below equilibrium, consumers’ willingness to
pay for an extra unit of the good exceeds the sellers’ cost of producing an extra unit of the
good. So from a welfare standpoint, more of the good should be produced.
It is also possible to produce too much. Suppose that output rises from 10 to 15. At Q=15,
the height of the demand curve is $7, while the height of the supply curve is $16. This
means that it costs $16 to produce a unit that buyers value at only $7. The 15
th
 unit thus
created a loss of (16-7)=$9. Consumer surplus falls by the area F --where the price of $12
is above their willingness to pay for an extra unit of the good-- and producer surplus falls
by the area E --where the price of $12 lies below their cost of producing an extra unit of
the good. The deadweight loss of the excess production is measured by areas E+F
(orange), which is the loss in total surplus when production rises above the equilibrium
quantity.
Conclusion: the equilibrium output level is economically superior to any higher output
level. Producing more than the equilibrium quantity is inefficient because total surplus is
reduced. Put differently, at output levels above equilibrium, the sellers’ cost of producing
an extra unit of the good exceeds consumers’ willingness to pay for an extra unit of the
good. So from a welfare standpoint, less of the good should be produced.