Market demand and consumers surplus

Suppose that the market price of a cup of coffee is K£4 but the consumer was willing to pay £9 for the first unit, £8 for the second, £7 for the third, £6 for the fourth, £5 for the fifth and £4 for the sixth.

However, he pays the market price for all the six cups.  The consumer thus earns a surplus on the first five units consumed i.e.

A measure of the difference between the value that consumers place on their total consumption of some commodity and the amount they actually pay for it.

For continuous demand curves, consumer's surplus can be measured by the area under the demand curve and above the price.

NB:  The shaded area represents utility which the consumers received but did not pay for i.e. consumer surplus.

Mathematically it can be calculated as follows:

£5 + £4 + £3 + £2 + £1 = £15

*Weaknesses of cardinalist approach