What is marginal cost?

Published Thursday 23 July 2026

Table of contents

• What is marginal cost?
• Why marginal cost matters
• How to calculate marginal cost
• Marginal cost vs average cost
• Marginal costs vs stepped costs
• Track your business costs with Xero
• FAQs on marginal cost

Key takeaways

• Marginal cost is the cost of producing one more unit of a product or service.
• You work it out by dividing the change in total cost by the change in quantity.
• Knowing your marginal cost helps you set prices and decide how much to produce.
• Marginal cost rises when variable input costs go up and when you push production near capacity.

What is marginal cost?

Marginal cost is the cost of producing one more unit of a product or service. It's most commonly used in manufacturing, where it's called the marginal cost of production.

In other words, it tells you precisely how much more you'd spend to make one more product, or to deliver a service one more time.

Why marginal cost matters

Knowing the cost of one more unit helps you make better pricing and production decisions. When you can see how much an extra sale actually costs, you can judge whether it's worth making and at what price.

The incremental cost is often quite small, because you've already paid for the tools and systems needed to make the item or deliver the service. A small marginal cost means you can afford to increase production, and possibly revenue, and it gives you room to offer customers a better price.

Your marginal cost moves with variable input prices, so it's worth recalculating before you set prices. According to Xero Small Business Insights, UK small business margins are under pressure from rising fuel and energy costs in 2026. When those variable costs shift, the cost of your next unit shifts with them, which can affect your profitability if your prices stay the same.

How to calculate marginal cost

You can work out marginal cost with a simple formula, then apply it to a real production change. The formula is: marginal cost = change in total cost / change in quantity.

Here's how to calculate marginal cost step by step.

1. Record your total cost at the current level of output.
2. Increase your output by a set number of units and record the new total cost.
3. Subtract the old total cost from the new total cost to find the change in cost.
4. Divide the change in cost by the change in quantity to get your marginal cost per unit.

Say you run a bakery. Making 100 cakes costs you £1,000 in total. Baking the 101st cake takes your total cost to £1,005, so the change in cost is £5 across 1 extra unit. That gives a marginal cost of £5 for that cake.

For a fuller walk-through, including how variable costs feed into the maths, see our guide on how to calculate marginal cost.

Marginal cost vs average cost

Marginal cost and average cost answer different questions, so it helps to keep them apart. Average cost is your total cost divided by the number of units, while marginal cost is the cost of the next unit alone.

When your marginal cost sits below your average cost, producing more pulls the average down. Your profit-maximising point is where marginal cost meets marginal revenue, so tracking both alongside your profit margins shows you when an extra unit still adds to profit.

Marginal costs vs stepped costs

Marginal cost behaves differently once you run out of spare capacity, and that's where stepped costs come in. Understanding the difference stops you from underpricing when growth suddenly gets more expensive.

You can often raise output by simply buying more raw materials, and while you're doing that, your direct costs rise in small, predictable steps. There comes a point where capacity is maxed out, though, and you can't produce more without a bigger investment.

Those bigger investments, like new premises, more equipment, or additional staff, are stepped costs. You'll typically need to sell a lot more products or services to pay back the investment a stepped cost involves.

Track your business costs with Xero

Marginal cost only guides good decisions when your numbers are clear and current. Keeping your costs organised in one place makes it easier to recalculate the moment input prices move.

Xero accounting software brings your income and expenses together so you can watch your margins and your gross profit margin without the manual admin. See your costs update in real time when you get one month free.

FAQs on marginal cost

Here are answers to frequently asked questions about marginal cost.

What is the marginal cost formula?

The marginal cost formula is the change in total cost divided by the change in quantity. It gives you the cost of producing one more unit.

What is an example of marginal cost?

If a workshop's total cost rises from £2,000 to £2,020 when it makes 5 more items, the change in cost is £20 across 5 units. That works out to a marginal cost of £4 per item.

What is the difference between marginal cost and variable cost?

Variable cost is the total cost that changes with output, such as materials and hourly labour. Marginal cost is the cost of just the next single unit you produce.

What causes marginal cost to increase?

Marginal cost rises when the price of variable inputs like materials or energy goes up. It also climbs as you near capacity and need overtime, extra shifts, or new equipment.