Marginal Cost, Revenue and Product: Calculate the Change

Subtract adjacent totals and divide by the change in quantity.

Read Two Incremental Comparisons

Choose the Change Formula

MC=Δ TC/Δ Q, MR=Δ TR/Δ Q, MP_L=Δ Q/Δ L, and marginal utility is Δ TU/Δ Q. If quantity changes by more than one, divide by that change. Write which unit is being added. The change from four to five workers gives the fifth worker’s MP. It is not output per five workers.

Separate Output Choice from Profit

At a smooth interior profit maximum, MR=MC is a necessary condition; identifying a maximum also requires the appropriate revenue-cost pattern and checks on feasible boundaries and shutdown. The equality does not calculate profit. Profit is TR-TC or (P-ATC)Q. A firm can satisfy MR=MC while earning profit, breaking even, or minimizing loss.

Calculate an Interval Change

A marginal comparison asks what an additional unit contributes; observations of past changes can help estimate that contribution. Begin with two adjacent total observations, subtract the earlier total from the later total, and divide by the number of units added. The order matters. If total cost rises from $410 at 20 units to $446 at 23 units, then MC=(446-410)/(23-20)=$12 per unit. Reporting $36 describes the total added cost of three units, not the average additional cost per unit over the interval. The interval result need not equal the cost of every individual added unit.

Distinguish an Average from an Addition

Tables often tempt students to divide a total by the new quantity. That produces an average. If five workers make 140 units and six make 164, the sixth worker’s marginal product is 24 units. Average product with six workers is 164/6. Both are legitimate numbers, but they answer different questions.

Keep the Numerator and Denominator Straight

Choose Among Discrete Output Levels

The smooth interior condition MR=MC is not a promise that a discrete schedule will contain an equality. Evaluate each successive block’s contribution to profit. In the example, unit four adds $15 of revenue at a $13 cost, whereas unit five adds $15 at an $18 cost. If earlier units are worthwhile and later units also add more cost than revenue, output should end at four, subject to comparing operation with shutdown. Only a separate total or per-unit calculation reveals the resulting profit.

Apply Incremental Thinking Beyond Firms

Marginal thinking also applies outside firms. A consumer buys another slice if marginal benefit covers price. A government expands pollution reduction while marginal social benefit exceeds marginal social cost. A student studies another hour if the expected score benefit outweighs the best alternative use of that hour. The labels change, but the incremental comparison remains.

Check Direction, Units and Interval Sizes

Use three checks. Identify the exact unit added, preserve the direction of subtraction, and attach units. A negative marginal product can occur if added labor reduces total output, but a negative MC in a standard production problem is suspicious. Finally, an interval average over unequal quantity blocks must weight each block by its quantity change; a simple unweighted mean can be misleading. A table may conceal changing increments.

Annotate the Interval Before Calculating

A final exam habit is to annotate each table interval before touching the options. Write “unit 6” beside the change from five to six, and circle any interval where quantity rises by more than one. Then estimate the direction of the total: positive MC means TC must rise, positive MR means TR rises, and diminishing MP means successive output additions shrink. These annotations expose distractors built from reversed subtraction, skipped rows, or a missing divisor. They also make the explanation reproducible instead of dependent on recognizing a familiar number.

Check the Meaning and Assumptions

OpenStax’s choice lesson introduces marginal analysis within limited resources. A marginal comparison evaluates a feasible adjustment, not every consequence of the entire activity.

The short-run cost lesson distinguishes total dollars from dollars per unit and explains sunk costs. An old cost matters to the next choice only through consequences still affected by that choice; an irrecoverable payment itself cannot be changed.

The competitive-firm output lesson connects additional revenue and cost to changes in profit. An interior equality is not enough by itself: compare feasible alternatives and shutdown, and use total revenue minus total economic cost to determine the profit level.

Weight Unequal Intervals Correctly

Total cost is $100 at 10 units, $118 at 12 units and $178 at 17 units. Calculate the additional cost per unit in each block and across the whole increase from 10 to 17 units.

Try Three Marginal-Value Questions

Connect Changes, Averages and Decisions

Compare marginal, average and total values, work with per-unit and per-dollar ratios, and practice interpreting economic calculations. Continue through the CLEP Microeconomics learning hub to connect the models.