Per-Unit Versus Lump-Sum Taxes on a Firm

A firm picks its output where marginal revenue equals marginal cost. A per-unit tax, like a dollar amount charged on every unit sold, gets added directly onto marginal cost, because producing one more unit now costs the original MC plus the tax. That shifts the MC curve up by the tax amount, so the quantity where price meets the new MC is smaller than before. The firm produces less and, in a competitive market, price to consumers tends to rise.

A lump-sum tax is a flat fee the firm owes no matter how much it makes, like an annual license fee or a fixed tax bill. It adds to total cost and average cost, since average cost is total cost divided by quantity, but it adds nothing to the cost of producing one more unit. Marginal cost is unaffected, so the quantity where MR equals MC never moves. The firm's profit falls by the amount of the tax, but its output stays exactly where it was.

This distinction matters because it shows which taxes distort market behavior and which ones just transfer money. A per-unit tax changes incentives at the margin and creates deadweight loss by shrinking output below the efficient level. A lump-sum tax raises revenue without touching the margin, so it does not distort the quantity decision, though it still cuts into the firm's profit and could push it to shut down if profit turns negative.