# How much is the marginal benefit?

Computes marginal benefit, the change in total benefit divided by the change in quantity, and compares it with marginal cost to say whether one more unit is worth it.

- Page: https://www.acalculator.org/finance/marginal-benefit-calculator
- JSON spec: https://www.acalculator.org/finance/marginal-benefit-calculator.json
- Version: ef6dd14a638d

## Default answer

Example with the default inputs (Starting quantity 2, Total benefit at the start 32, New quantity 3, Total benefit at the new quantity 42): From a quantity of 2 to 3, total benefit changes by 10: a marginal benefit of 10 per unit.

## Inputs

| Key | Label | Description |
| --- | --- | --- |
| q1 | Starting quantity | Units of the good or activity at the start. |
| b1 | Total benefit at the start | The total benefit of the starting quantity, in dollars or any unit such as points. |
| q2 | New quantity | Units of the good or activity after the change. |
| b2 | Total benefit at the new quantity | The total benefit of the new quantity, in the same unit. |
| mc | Marginal cost of each extra unit | What each extra unit costs, in the same unit as the benefit, to compare with the marginal benefit. Optional. |

## Outputs

| Key | Label | Description |
| --- | --- | --- |
| mb | Marginal benefit | Change in total benefit ÷ change in quantity: the benefit of each extra unit. |
| dBenefit | Change in total benefit | New total benefit − starting total benefit. |
| dQuantity | Change in quantity | New quantity − starting quantity. |
| net | Marginal benefit − marginal cost | The net gain from each extra unit, when a marginal cost is given. |
| decision | Marginal decision rule | More when marginal benefit is above marginal cost, less when below, stop where they are equal. |

## Method

Marginal benefit = (TB₂ − TB₁) ÷ (Q₂ − Q₁), where TB is total benefit at quantity Q. With a marginal cost MC: do more when MB > MC, do less when MB < MC, and stop where MB = MC.

## Assumptions

- Total benefit is in one unit throughout, such as dollars of willingness to pay or points.
- Over more than one unit, the result is the average marginal benefit of each extra unit in that range.
- Arithmetic is exact on the typed decimals; results show up to 6 decimals (6 significant figures below 0.0001 in size).

## Worked examples

1. q1 = 2, b1 = 32, q2 = 3, b2 = 42 gives mb = 10, dBenefit = 10, dQuantity = 1. Source: Principles of Economics v2.0 (Saylor Academy open textbook, CC BY-NC-SA 3.0), 6.1 The Logic of Maximizing Behavior (marginal benefit is the amount by which an additional unit of an activity increases its total benefit; Laurie Phan’s economics study: total benefit 18, 32 and 42 points after 1, 2 and 3 hours, 50 after 5; marginal benefits 18, 14 and 10 points; net benefit is largest where marginal benefit equals marginal cost). https://saylordotorg.github.io/text_principles-of-economics-v2.0/s09-01-the-logic-of-maximizing-behavi.html: 42 − 32 = 10 points.
2. q1 = 1, b1 = 18, q2 = 2, b2 = 32 gives mb = 14. Source: Principles of Economics v2.0 (Saylor Academy open textbook, CC BY-NC-SA 3.0), 6.1 The Logic of Maximizing Behavior (marginal benefit is the amount by which an additional unit of an activity increases its total benefit; Laurie Phan’s economics study: total benefit 18, 32 and 42 points after 1, 2 and 3 hours, 50 after 5; marginal benefits 18, 14 and 10 points; net benefit is largest where marginal benefit equals marginal cost). https://saylordotorg.github.io/text_principles-of-economics-v2.0/s09-01-the-logic-of-maximizing-behavi.html: 32 − 18 = 14 points.
3. q1 = 3, b1 = 42, q2 = 5, b2 = 50, mc = 6 gives mb = 4, net = -2, decision = Do less: each extra unit costs more than the benefit it adds.. Source: Principles of Economics v2.0 (Saylor Academy open textbook, CC BY-NC-SA 3.0), 6.1 The Logic of Maximizing Behavior (marginal benefit is the amount by which an additional unit of an activity increases its total benefit; Laurie Phan’s economics study: total benefit 18, 32 and 42 points after 1, 2 and 3 hours, 50 after 5; marginal benefits 18, 14 and 10 points; net benefit is largest where marginal benefit equals marginal cost). https://saylordotorg.github.io/text_principles-of-economics-v2.0/s09-01-the-logic-of-maximizing-behavi.html.
4. q1 = 1.5, b1 = 12.25, q2 = 4, b2 = 20.5, mc = 3.3 gives mb = 3.3, net = 0, decision = Stop here: marginal benefit equals marginal cost, where net benefit is largest.. Source: Principles of Economics v2.0 (Saylor Academy open textbook, CC BY-NC-SA 3.0), 6.1 The Logic of Maximizing Behavior (marginal benefit is the amount by which an additional unit of an activity increases its total benefit; Laurie Phan’s economics study: total benefit 18, 32 and 42 points after 1, 2 and 3 hours, 50 after 5; marginal benefits 18, 14 and 10 points; net benefit is largest where marginal benefit equals marginal cost). https://saylordotorg.github.io/text_principles-of-economics-v2.0/s09-01-the-logic-of-maximizing-behavi.html.

## FAQ

### What is marginal benefit?

Marginal benefit is the extra benefit from one more unit of a good or an activity: the amount by which total benefit rises when the quantity goes up by one. It usually falls as you have more, because the first units meet the most urgent wants.

### How do I calculate marginal benefit?

Subtract the starting total benefit from the new total benefit, and divide by the change in quantity. A student who expects 32 points from 2 hours of study and 42 points from 3 hours gains (42 − 32) ÷ (3 − 2) = 10 points from the third hour.

### How does marginal benefit relate to marginal cost?

Compare them. If the marginal benefit of one more unit is above its marginal cost, doing more adds to net benefit; if it is below, doing less does. Net benefit is largest where marginal benefit equals marginal cost: the marginal decision rule.

### What if the quantity changes by more than one unit?

The calculator divides by the whole change, so the result is the average marginal benefit of each extra unit in the range. Going from 3 to 5 hours adds 8 points, or 4 points an hour.

### What units does marginal benefit use?

The units of total benefit per unit of quantity. Benefit is often measured in dollars, as the most someone would pay; it can be any measure, such as exam points or hours saved, as long as both totals use the same one.

### Can marginal benefit be negative?

Yes. When one more unit makes you worse off, such as a meal too many, total benefit falls and the marginal benefit is below zero.

## Sources

- Principles of Economics v2.0 (Saylor Academy open textbook, CC BY-NC-SA 3.0), 6.1 The Logic of Maximizing Behavior: the marginal benefit is the amount by which an additional unit of an activity increases its total benefit, and it equals the slope of the total benefit curve; Laurie Phan expects 18, 32 and 42 points from 1, 2 and 3 hours of economics study and 50 points from 5, so the first three hours add 18, 14 and 10 points; if the marginal benefit exceeds the marginal cost the activity should be increased, and net benefit is maximized where marginal benefit equals marginal cost. https://saylordotorg.github.io/text_principles-of-economics-v2.0/s09-01-the-logic-of-maximizing-behavi.html (retrieved 2026-10-19)
- OpenStax, Principles of Microeconomics 3e, 6.1 Consumption Choices: the marginal value of one more unit is the change in the total divided by the change in quantity (marginal utility = change in total utility ÷ change in quantity), and it usually diminishes. https://openstax.org/books/principles-microeconomics-3e/pages/6-1-consumption-choices (retrieved 2026-10-19)