Marginal Utility is the importance of the use enabled by an additional unit, or forgone when a unit is lost. In the Austrian account it concerns the ranking of needs, not measuring feelings. First establish the relevant unit, whether units can substitute for each other, and the person’s available options.
Imagine three identical containers of drinking water, each sufficient for one of Peter’s plans: drinking, cooking and watering, in that order of importance. One breaks. Peter can reallocate the remaining water, keeping the more important uses and giving up watering. It does not matter whether the broken container was labelled “drinking”. What matters is the use actually lost after reallocation. This connects with Subjective Value Theory: value relates a good to a person, their needs and substitutes. [1] [2]
In our example, water from any container can serve the same purposes as water from the others. If one held water unsafe to drink, Peter could not freely allocate it among all three uses. Water at the cottage would not quench his thirst on a distant hike if he could not access it. In Principles of Economics, Carl Menger stresses the capacity to meet the relevant need. Check quality, availability and actual interchangeability before counting units. [1]
If Peter gets another comparable container and his plans remain unchanged, the most important uses are already covered. The extra water can serve a less urgent purpose. This is the idea of diminishing marginal utility: under unchanged conditions, additional units serve lower-ranked uses. It does not say every purchase automatically brings less pleasure. Visitors arriving at the cottage or a change in weather alter the needs and situation being compared. [2]
Consider a torch that needs two identical batteries. If Peter has only one, the second enables light; an extra spare serves a different purpose. This is not a mechanical comparison of individual pieces regardless of circumstances. The batteries complement each other in use, and the working pair matters for the intended service. Before drawing conclusions about another “unit”, establish whether the choice concerns a separate item, spare supplies or a functioning set. [2]
The first bite often feels different from the last to a hungry person. That can help intuition, but the explanation does not rest on physical satiation. In Human Action, Mises examines choices between uses, not a laboratory measure of pleasure. Peter can allocate tools or next week’s supplies without consuming anything now. The question remains which plan a portion of the supply enables, and which plan he would forgo without it. [2]
Peter ranks drinking above watering without knowing exactly how much more important it is. Ordinal Utility means this ranking of alternatives. Rank numbers are not points to add into total satisfaction. Nor can multiplying one container’s importance give the value of the whole supply. Losing all the water also sacrifices uses more important than those lost with one container. The object of the decision has changed. [2] [3]
Marginal utility helps explain why Peter wants another container of water or can do without that extra amount in a particular situation. It does not reveal the price tag. Sellers’ offers, buyers’ demand and the circumstances of exchange matter too. Two people can pay the same amount while attaching different importance to the purchase. Explaining a particular market price needs further information; one person’s ranking of needs does not measure everyone else’s benefit. [1] [3]
Bitcoin can fund a payment, remain in reserve or be exchanged. People usually consider the amount needed for a particular purpose, not an isolated smallest unit. Fees and what else the funds could buy can change the decision. Marginal utility helps compare these options. It does not imply that each additional unit of bitcoin must have a lower market price. Nor does it provide a formula for future prices. Personal priorities and market outcomes remain distinct questions. [2] [3]
For the clearest picture, read this entry together with Subjective Value Theory, Ordinal Utility, Carl Menger, Opportunity Cost, Time Preference, Bitcoin. The reverse links also lead from Time Preference, Subjective Value Theory, Ordinal Utility, Carl Menger.