Marginal Product of Labor: The Number Behind Every Hiring Decision

You run a small warehouse fulfilling online orders. Today you have six packers and they move 720 boxes a shift. A seventh worker is available for $160 a day. Should you hire them?

The honest answer is not "yes if we're busy" or "no if money's tight." It is a number, and the number you need is the marginal product of labor. Get comfortable with it and nearly every staffing decision a firm faces stops being a gut call and becomes arithmetic.

Two measures that look alike but are not

Let's start with the distinction that trips people up most.

The marginal product of labor, or MPL, is the extra output you get from adding exactly one more worker, with everything else held constant: the equipment, the floor space, the hours. It answers one question: what does the next person actually add?

The average product of labor, or APL, is total output divided by the number of workers. It answers a different question, how much each worker produces on average across the whole team.

These are not the same, and the gap between them is where the money is. As the Concise Encyclopedia of Economics explains in its treatment of marginalism, the value of what the last worker adds, not the average worker, governs whether hiring them pays.¹

Walk through the warehouse

Let's run the numbers, worker by worker. The building, the conveyor, and the packing stations are all fixed; only headcount changes.

Trace the two right columns. Early on, MPL rises: workers specialize, one staging boxes while another tapes and labels, and the team finds a rhythm. MPL peaks at the fifth worker (140 extra boxes) and falls after that, the classic signature of diminishing returns against fixed equipment.

Now watch the relationship between the two columns. Through worker five, MPL sits above APL and the average climbs. At worker six, MPL (120) exactly equals APL (120), and that is precisely where average product hits its maximum at 120 boxes per worker. From worker seven on, MPL drops below APL and the average starts falling.

This crossing is not a coincidence; it is a mathematical certainty. Whenever the next item you add is above the running average, the average rises. When it falls below, the average falls. They can only be equal at the average's peak. It is the same reason a basketball player whose next game beats their season average pulls that average up.

The hiring decision, made concrete

Now shift to the part that actually costs or earns money. Knowing output is not enough; you need the value of that output and the cost of the worker. Suppose each box nets the firm $1.40 in contribution, meaning the price received minus the materials and shipping cost for that box. The value of a worker's marginal product is simply MPL times $1.40.

The rule a profit-maximizing firm follows: keep hiring as long as the value of the next worker's marginal product exceeds the wage, and stop when they are equal. Here the sixth worker still clears the bar, $168 of value for a $160 wage, a net $8. The seventh does not: they add $128.80 of value while costing $160, losing the firm $31.20 a day.

So the answer to the question we opened with is no. Six packers is the profit-maximizing crew at this wage and margin. This is the foundation of what economists call marginal productivity theory, the standard account of why a competitive firm's demand for labor traces out the declining portion of its marginal-product curve.¹

Change one thing and watch the answer shift

The answer is not fixed. It moves with wages and margins, which is exactly why this framework stays useful.

If the daily wage falls to $120 (say, a slack local labor market), the seventh worker now clears the bar: $128.80 of value beats a $120 wage, so you'd hire seven. A cheaper workforce justifies pushing further into diminishing returns.

If instead the margin per box rises to $1.80 (you raise prices or cut shipping cost) while the wage stays at $160, the seventh worker's value becomes 92 times $1.80, which is $165.60, now above $160. Again you'd hire the seventh. Higher revenue per unit makes each additional worker worth more.

This is why the same factory hires aggressively when demand and prices are strong and sheds workers when margins compress: the marginal calculation shifts, even though the underlying production function has not changed at all. The broad pattern shows up in national data. The U.S. Bureau of Labor Statistics tracks output per hour² precisely because the relationship between labor input and output is what ultimately anchors how much that labor can be paid, a relationship the Fed's industrial capacity data³ reflects in aggregate every month as firms across the economy decide, worker by worker, exactly how far to push.

Labor productivity figures confirm this at scale. FRED's nonfarm business output-per-hour series⁴ shows the long-run climb in what each hour of work produces, driven by capital deepening and technology, but the short-run swings follow exactly the logic above: firms cut workers faster than output falls in a downturn, pushing MPL back up, and add them slowly at first in a recovery until diminishing returns set in.⁵

The trap of managing by the average

Overall, here is where most managers go wrong. A manager looking at the table might say, "Our packers average 116 boxes each with seven workers. That's great, hire more." But the average is hiding the margin.

The seventh worker dragged the average down and lost money. The healthy-looking average of 116 is the residue of the strong earlier hires, not evidence that the next hire pays. Every real hiring, ordering, and capacity decision lives at the margin, meaning the next unit, the next worker, the next shift, not in the comfortable average of everything that came before.⁶

The broader lesson reaches well past warehouses. Whenever you are deciding whether to add one more of anything (a salesperson, a server, a machine, an hour), the question is never "how productive is the team on average?" It is "what will the next one add, and is that worth what it costs?"

That is the marginal product of labor doing its quiet, decisive work. The firms that have internalized it hire with precision and cut early; the ones that manage by averages are always a little surprised when the next quarter comes in light.