Marginal factor cost (MFC) is the extra cost a firm takes on when it hires one more unit of a productive input, whether that’s an hour of labor, an acre of land, or a piece of equipment. It’s calculated by dividing the change in total spending on that input by the change in the quantity of the input used. The concept is central to how firms decide the right amount of any resource to employ.
How MFC Works in Practice
The logic behind MFC is straightforward: every time a business decides to use a little more of something, it pays a price for that addition. MFC captures that price, not as a simple per-unit cost, but as the actual change in the firm’s total bill for that input. In many situations those two things are identical. But in others, they diverge sharply, and that divergence is where MFC becomes most useful.
Think of it as a question the firm asks before every hiring or purchasing decision: “If I bring on one more worker (or buy one more machine), how much more will I actually spend in total?” The answer depends on the type of market the firm operates in.
MFC in a Competitive Market
When a firm is one of many buyers competing for workers or materials, it has no power to influence the going price. It can hire as many workers as it wants at the market wage without pushing that wage up. In this situation, the cost of hiring one more worker is simply the wage itself. MFC equals the wage rate at every quantity, producing a flat, horizontal MFC curve.
This also means MFC equals the average factor cost (the total spent on labor divided by the number of workers). Because every worker costs the same amount, the marginal and the average never pull apart. The supply curve of labor, the average factor cost curve, and the MFC curve all sit on top of each other as one horizontal line.
MFC in a Monopsony
Things change dramatically when a firm is the dominant (or only) buyer of an input. A single large employer in a small town, for instance, faces the entire upward-sloping labor supply curve on its own. To attract one more worker, it must offer a higher wage. But here’s the critical part: it typically has to pay that higher wage to all its existing workers too, not just the new hire.
This creates a compounding effect. Suppose a firm employs one worker at $3 per hour and wants a second. The second worker requires a wage of $4 per hour, but now the first worker must also be paid $4. Total cost jumps from $3 to $8, making the marginal factor cost of that second worker $5, not $4. The MFC rises faster than the wage because each new hire raises the pay for the entire workforce.
Graphically, the MFC curve in a monopsony sits above the labor supply curve and climbs at roughly twice its slope. This gap between what the last worker costs and what you actually pay in wages is the defining feature of monopsony power.
How Firms Use MFC to Maximize Profit
Every input a firm uses generates revenue. The additional revenue produced by one more unit of that input is called the marginal revenue product (MRP). A profit-maximizing firm keeps hiring as long as each new unit of input brings in more revenue than it costs. The optimal stopping point is where MRP equals MFC.
In a competitive labor market, this rule simplifies to: hire until the marginal revenue product of a worker equals the market wage. In a monopsony, the rule is the same in structure (MRP = MFC), but because MFC exceeds the wage, the firm hires fewer workers and pays them less than a competitive market would. Workers in a monopsony earn a wage set by the supply curve at the chosen quantity, which is lower than the MFC at that point.
MFC and Average Factor Cost
The relationship between MFC and average factor cost follows a pattern familiar from other areas of economics. When MFC is greater than the average, it pulls the average up. When MFC equals the average, the average stays flat. When MFC is less than the average, the average falls.
For competitive input markets, MFC and average factor cost are equal at every quantity, so the average never changes. For monopsonies and other firms with buying power, MFC sits above average factor cost at every quantity, which means average factor cost continuously rises. This is a visual signal on a graph that the firm has market control over its inputs.
How Minimum Wage Changes the MFC Curve
A minimum wage creates an interesting effect on MFC in monopsony markets. When the government sets a wage floor, the firm no longer needs to raise wages to attract additional workers (up to a point). It can hire at the fixed minimum wage, making MFC equal to that wage for a range of employment levels. The MFC curve effectively becomes flat at the minimum wage until the firm needs to hire beyond what the floor attracts.
A Congressional Budget Office analysis found that a moderate minimum wage in a monopsony market can actually increase both wages and employment. Because the wage floor eliminates the firm’s ability to suppress pay, MFC drops to the level of the minimum wage, making additional hires cheaper than before. The firm responds by hiring more. A minimum wage set too high, however, pushes MFC above the level where hiring is profitable, and employment falls. The outcome depends entirely on where the floor lands relative to the competitive wage.
Calculating MFC
The basic formula is:
MFC = Change in total factor cost ÷ Change in quantity of the factor
If hiring a third worker raises your total labor bill from $16 to $27, the MFC of the third worker is $11. In calculus terms, MFC is the first derivative of the total factor cost function with respect to the quantity of the input. For a linear supply curve, this derivative produces a line with twice the slope of the supply curve, confirming the graphical relationship seen in monopsony models.
In competitive markets, total factor cost is simply the wage multiplied by the number of workers (a linear function), so its derivative is a constant equal to the wage. That’s why MFC is flat in competition and rising in monopsony.