Use the following formula for calculating marginal utility:
Marginal Utility = Change in total utility / Change in Quantity
Here, total utility is the total satisfaction derived by consuming specific units of a commodity. You should have appropriate data of total utility and total consumed quantity to calculate marginal utility.
Take this example to calculate marginal utility. Suppose total utility increases from 15 to 21 units when consumption of chocolate increases from five to seven units. Here, change in total utility is six and change in quantity consumed is two. Therefore, marginal utility is six divided by two, which equals three. This way you can calculate marginal utility.
Put your data in the formula and calculate the result. However, marginal utility is not always the same. As you consume more chocolates, your marginal utility will decrease. This is the law of diminishing marginal utility.
Calculate the marginal utility again for additional units consumed. Suppose an increase in consumption of chocolate from seven to nine units increases total utility from 21 to 25. Here, change in total utility is four units and change in quantity demanded is two; marginal utility will be four divided by two, which equals two. Marginal utility previously calculated was three, which has now decreased to two. This is because of the law of diminishing marginal utility.