What it means
In economics, utility is a measure of the satisfaction or usefulness a buyer gets from something. Marginal utility is the extra satisfaction from one more unit.
The law says marginal utility falls as consumption rises, so the next unit is valued less than the one before. This is why a customer is willing to pay a lot for the first unit and far less for the second or third.
A thirsty runner will happily pay $4 for the first bottle of water but not much for the fourth. The falling willingness to pay gives the demand curve its downward slope, which links this law directly to the law of demand.
Businesses use the idea in several ways. Volume discounts, buy-one-get-one offers and tiered subscriptions all recognise that later units are worth less to the customer and must be priced lower to be sold.
Retailers also introduce variety, such as new flavours or colours, because variety restarts the satisfaction a repeated product has lost. Consumers use it, often without realising, to divide their money across different purchases.
A rational buyer keeps shifting spending towards whichever good gives the most extra satisfaction per dollar until the last dollar spent on each good delivers the same benefit. That balance explains why people do not spend their entire budget on a single product.
A useful caution is that utility is subjective and cannot be measured directly. Economists use numbers such as "utils" as a teaching device, but real businesses learn about it through willingness to pay, survey data and sales response.
Some goods, such as collectables or addictive products, can appear to break the law over a short range.
In practice
Real-world examples.
Example
A streaming service finds that subscribers watch a great deal in the first month and far less by the sixth month. The satisfaction from each extra hour of the same library shrinks, so cancellations rise. The product team commissions new shows to renew the interest of existing subscribers.
Example
A fast-food chain offers a second burger at half price. Customers value the second burger less than the first, so the lower price is enough to persuade many to buy it. The chain earns extra revenue it would not have made at full price.
Example
A software company sells five user licences per account at full price and then offers a 30% discount on each seat beyond the first five. Managers at the customer value each additional seat a little less, because fewer staff need the tool. The discount keeps the buyer expanding rather than stopping at five.
Formula
Calculation
Marginal utility = change in total utility / change in units consumed
Suppose a customer values pizza slices as follows: the first slice gives 20 utils (units of satisfaction), the second 14, the third 8, the fourth 3 and the fifth 0. Total utility is 20, then 34, then 42, then 45, then 45. If each util is worth $0.50 to the customer, the slices are worth $10, $7, $4, $1.50 and $0. With slices priced at $4 each, the customer buys three slices, because the third is worth exactly $4 and the fourth is worth only $1.50, which is less than the price.Case study
Seen in the real world.
Sunrise Snacks is an illustrative, fictional vending business that placed machines selling the same chocolate bar in a large office building. In the first week the machines sold 600 bars a day, but sales drifted down to 380 a day by the third month as staff tired of the same choice.
The operations manager noticed that rotating the stock to include a fruit bar and a nut bar lifted sales to 540 a day within two weeks. Offering three bars for $5 instead of $2 each for singles then lifted the average basket without hurting profit. The illustrative lesson is that variety and bundling are practical responses to a customer's falling appetite for the same thing.
Watch out
Common mistakes.
- Thinking diminishing utility means total satisfaction falls, when total utility keeps rising until the marginal utility reaches zero.
- Assuming every customer feels the same drop in satisfaction, when tastes and circumstances differ widely between buyers.
- Treating utils as measurable amounts a business can observe, rather than a teaching device behind real willingness to pay.
Questions
People also ask.
Why do bundles and multi-buy offers rely on this law?
Customers value later units less, so a lower price on the second or third unit matches what they would actually pay and draws out extra sales.
Can marginal utility ever be negative?
Yes. After the point of satiation, an extra unit can cause discomfort, such as a sixth slice of pizza making someone feel unwell.
Does the law apply to money itself?
Often yes, because an extra $1,000 matters more to a person on a modest income than to a person who is very wealthy.
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