What is marginal cost?

Published Thursday 23 July 2026

Table of contents

• What is marginal cost?
• Why marginal cost matters for small businesses
• How to calculate marginal cost
• Marginal cost examples
• Marginal cost vs average cost
• Marginal cost vs marginal revenue
• Marginal cost and economies of scale
• Track your business costs with Xero
• FAQs on marginal cost

Key takeaways

• Marginal cost is the additional expense of producing 1 more unit of a product or service, and it helps you set prices, plan production, and protect your profit margins.
• You can calculate marginal cost with a simple formula: divide the change in total cost by the change in quantity produced (MC = ΔTC / ΔQ).
• Marginal cost often falls as you produce more units, thanks to economies of scale, but it can rise again once you hit capacity limits or need to invest in new resources.
• Comparing marginal cost to marginal revenue shows you the point where producing more units stops being profitable.

What is marginal cost?

Marginal cost is the additional expense you incur when you produce 1 more unit of a product or service. It tells you how much your total costs go up each time you increase output by a single unit.

To understand marginal cost, it helps to know the difference between fixed costs and variable costs. Fixed costs stay the same regardless of how much you produce; think rent, insurance, or overhead costs like equipment leases. Variable costs change with your output level; these include raw materials, packaging, and direct labour. You can learn more about tracking these in your cost of goods sold.

Marginal cost focuses on variable costs because your fixed costs don't change when you produce 1 extra unit. If you run a bakery and bake 1 more loaf, your rent stays the same, but you'll use more flour, energy, and labour. That extra spend is your marginal cost.

Understanding this number gives you a clearer picture of whether it's worth increasing your output, and at what point the extra cost outweighs the extra revenue.

Why marginal cost matters for small businesses

Knowing your marginal cost helps you make smarter decisions about pricing, production, and where to invest your resources. Here are 4 areas where it makes a real difference.

• Pricing decisions: your marginal cost sets a floor price. If you charge less than it costs to produce 1 more unit, you're losing money on every additional sale.
• Production planning: marginal cost helps you find the sweet spot for output. Producing too few units means you're not making the most of your capacity; producing too many can push costs up.
• Resource allocation: when you know which products or services have the lowest marginal costs, you can direct your time and budget towards the ones that deliver the best return.
• Profitability analysis: tracking marginal cost over time shows you whether scaling up is helping or hurting your bottom line.

Labour is one of the biggest variable costs for many businesses. According to Xero Small Business Insights, wages for Australian small business employees grew by 2.0% year-on-year in the December quarter of 2025, based on data from 520,000 businesses. Tracking these cost movements helps you understand whether your marginal costs are rising or staying stable.

How to calculate marginal cost

The marginal cost formula is straightforward. You divide the change in total cost by the change in the quantity produced.

Marginal cost = change in total cost / change in quantity

Here's how to work it out step by step. For a more detailed walkthrough, see the full guide on how to calculate marginal cost.

1. Determine the change in quantity. Work out how many additional units you're producing compared to your previous output level.
2. Calculate the change in total cost. Add up the extra costs you've incurred to produce those additional units. Include materials, labour, energy, and any other variable costs.
3. Divide the cost change by the quantity change. The result is your marginal cost per unit.

For example, if producing 10 extra units adds $150 to your total costs, your marginal cost is $150 / 10 = $15 per unit.

Marginal cost examples

Seeing the formula in action makes it easier to apply to your own business. Here are 3 worked examples across different industries.

Bakery

A bakery currently produces 100 loaves a day at a total cost of $500. The owner decides to bake 120 loaves, and total costs rise to $580.

• Change in quantity: 120 - 100 = 20 loaves
• Change in total cost: $580 - $500 = $80
• Marginal cost: $80 / 20 = $4 per loaf

If each loaf sells for $7, the bakery earns $3 in extra profit per loaf. Scaling up makes sense here.

Trades business

An electrician typically completes 15 jobs a week at a total cost of $3,000. Taking on 3 extra jobs pushes costs to $3,720 because of overtime pay and additional materials.

• Change in quantity: 3 jobs
• Change in total cost: $3,720 - $3,000 = $720
• Marginal cost: $720 / 3 = $240 per job

If the electrician charges $350 per job, the margin is still healthy. But if overtime rates push the marginal cost above $350, those extra jobs aren't worth taking on.

Online store

An online retailer sells 500 units a month at a total cost of $5,000. A bulk order of 600 units brings total costs to $5,800, thanks to a volume discount on materials.

• Change in quantity: 600 - 500 = 100 units
• Change in total cost: $5,800 - $5,000 = $800
• Marginal cost: $800 / 100 = $8 per unit

The marginal cost of $8 is actually lower than the average cost of $10 per unit ($5,000 / 500). The volume discount is reducing costs at the margin, which is a sign that scaling up is working in the retailer's favour.

Marginal cost vs average cost

Marginal cost and average cost measure different things, and both are useful for making business decisions.

Marginal cost is the cost of producing the next unit. Average cost is the total cost divided by the total number of units produced. In simple terms, marginal cost looks forward at the next unit, while average cost looks back at all units so far.

Here's a quick comparison using the bakery example. If you've produced 100 loaves at a total cost of $500, your average cost is $5 per loaf. But your marginal cost for the next 20 loaves is $4 each. The marginal cost is lower because your fixed costs (rent, equipment) are already covered.

When marginal cost is below average cost, producing more units pulls your average cost down. When marginal cost rises above average cost, your average cost starts climbing. This relationship helps you judge whether expanding output is improving your overall cost efficiency.

Marginal cost vs marginal revenue

Marginal revenue is the additional income you earn from selling 1 more unit. Comparing it to marginal cost shows you the profit-maximisation point for your business.

The general rule is this: keep producing as long as marginal revenue is greater than marginal cost. Each extra unit sold adds more to your revenue than it adds to your costs, so your profit margin grows.

The profit-maximisation point is where marginal cost equals marginal revenue (MC = MR). At this point, the last unit you produce earns just enough to cover its cost. Beyond this point, marginal cost exceeds marginal revenue, and every additional unit reduces your total profit.

For a small business, this doesn't have to be a precise calculation. It's a way of thinking: if taking on 1 more job, producing 1 more batch, or fulfilling 1 more order costs more than you'll earn from it, it's time to stop and reassess.

Marginal cost and economies of scale

Marginal cost doesn't stay flat as your output grows. It typically follows a U-shaped pattern, falling at first and then rising again.

In the early stages of scaling up, marginal cost tends to decrease. You're spreading your fixed costs over more units, getting better prices on bulk materials, and your team becomes more efficient through repetition. This is the economies of scale phase, where producing more actually costs less per unit.

At some point, though, marginal cost starts to rise. You might run out of workspace, need to pay overtime rates, or hit the limits of your equipment. This is the diseconomies of scale phase, where adding more output becomes increasingly expensive.

There are also stepped costs to consider. These are costs that stay fixed within a range of output but jump up once you pass a threshold. For example, a trades business might handle 20 jobs a week with its current team, but job 21 requires hiring another employee, which pushes costs up significantly.

Hiring is one of the most common stepped costs for growing businesses. According to Xero Small Business Insights, jobs growth across Australian small businesses reached 3.4% year-on-year in the December quarter of 2025, the strongest result in 2 years, based on data from 520,000 businesses. This suggests many businesses are reaching the point where they need to invest in additional employees to keep up with demand.

Understanding where you sit on the cost curve helps you decide whether it's the right time to expand, or whether you're better off optimising what you already have.

Track your business costs with Xero

Keeping a close eye on your costs is the first step towards understanding your marginal cost and making better pricing, production, and growth decisions. Xero's online accounting software gives you real-time visibility into your expenses, so you can spot changes in your cost structure as they happen.

With features like automated bank reconciliation, customisable reporting, and smart financial insights, Xero helps you stay on top of the numbers without spending hours on manual bookkeeping. 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: marginal cost = change in total cost / change in quantity produced. It tells you how much your total costs increase for each additional unit you produce.

What is the difference between marginal cost and average cost?

Marginal cost is the expense of producing 1 additional unit, while average cost is the total cost divided by all units produced. Marginal cost looks at the next unit; average cost looks at the overall picture.

Why does marginal cost increase?

Marginal cost rises when you hit capacity limits, such as needing to pay overtime, buy more equipment, or hire additional staff. These stepped costs push your per-unit expense higher once you pass certain output thresholds.

How can small businesses use marginal cost?

Small businesses can use marginal cost to set prices above their production floor, decide whether to take on extra work, and judge when scaling up is no longer profitable. It's a practical tool for protecting your margins.

What are variable costs in marginal cost?

Variable costs are expenses that change with your output level, such as raw materials, direct labour, packaging, and energy. Marginal cost is driven by these variable costs because fixed costs stay the same regardless of how many units you produce.