Marginal Revenue Constant: When Every Extra Unit Earns the Same

Think about a lemonade stand. On a hot day, you sell the first glass for ₹20. The second glass? Still ₹20. The third? ₹20 again. Every time you sell one more glass, your total revenue goes up by exactly ₹20. That ₹20 is your marginal revenue — and it stays the same no matter how many glasses you sell.

That is the simplest case of constant marginal revenue. But in economics, this situation is special, not automatic. Let's see why.

The Precise Meaning

Marginal Revenue (MR) is the change in total revenue when you sell one additional unit of output.

Where:

• $\Delta TR$ = change in total revenue
• $\Delta Q$ = change in quantity sold (usually 1 unit)

When MR is constant, it means every extra unit you sell adds exactly the same amount to your total revenue. The number does not change as you sell more.

When Does MR Stay Constant?

This happens only when the firm can sell any quantity at the same price — that is, when the firm is a price taker. In perfect competition, the market sets the price, and the individual firm cannot influence it. So:

The marginal revenue from selling one more unit is simply the market price. And since the price does not change with the firm's output, MR is constant.

Why It Matters

Constant MR changes how a firm thinks about profit. If MR is constant, the firm's total revenue is a straight line:

Every extra unit adds exactly $P$ to revenue. So the firm's decision to produce more depends entirely on whether the marginal cost of that unit is less than or greater than this constant MR.

The Contrast: When MR Is Not Constant

Most real-world firms face downward-sloping demand. To sell more, they must lower the price on all units. So the extra revenue from selling one more unit is less than the price — MR falls as output rises. That is the world of monopoly, monopolistic competition, and oligopoly. …