Marginal cost
Learn what marginal cost is, how to calculate it and how to use it to price, plan output and grow profit.
Published Wednesday 30 September 2026
Table of contents
Key takeaways
- Marginal cost is the extra cost of producing one more unit, and it’s driven mainly by variable costs like materials and labour
- To calculate it, divide the change in total cost by the change in quantity, then compare the result with your selling price
- Each extra unit adds profit while marginal cost stays below your price, which helps you judge discounts and one-off orders
- Pair marginal cost with average cost and break-even analysis to plan pricing and capacity with confidence
What is marginal cost?
Marginal cost (biaya marginal) is the extra cost of producing one more unit of a product or delivering one more unit of a service. It tells you whether making more is worth it.
Say you run a candle business in Yogyakarta. Making 100 candles costs you Rp3.000.000 and making 101 costs Rp3.018.000, so the marginal cost of that 101st candle is Rp18.000.
Most of that Rp18.000 is direct cost: the wax and wick that go into one more candle. Because marginal cost looks only at the next unit, it’s the number to check before you change how much you produce.
Why marginal cost matters for your business
Knowing your marginal cost gives you a hard number to base everyday decisions on. It shapes how you price, spend, budget and plan production.
Set prices that protect your margins
When you know what one more unit costs, you can price with confidence. If an extra batch costs Rp12.000 per unit and you sell at Rp20.000, each additional sale leaves Rp8.000 towards fixed costs and profit.
That figure helps you weigh up a Lebaran promotion or a wholesale deal without guessing.
Spend where it counts
Marginal cost shows where your money goes as you scale. Say making 50 more units a week doubles your ingredient bill but lifts labour costs by only 10%. You’ll know to look for better supplier deals first.
Build more realistic budgets
Marginal cost data lets you estimate what your next production target will really cost, instead of relying on rough averages. That makes your budget and your cash flow projections more reliable.
Find your most efficient output
Every business reaches a point where producing more gets expensive. If your cost per extra unit climbs sharply after 500 units a month, that’s your cue to review your process or invest in more capacity.
What costs make up marginal cost?
Marginal cost is made up of the costs that change when your output changes. According to the Corporate Finance Institute (CFI), the calculation uses variable costs such as labour and materials, plus any expected increase in fixed costs.
Each type of cost behaves differently as you produce more.
- Variable costs, such as flour and packaging, rise with every unit and make up most of your marginal cost
- Fixed costs, such as rent and insurance, stay the same when you make one more unit, so they usually sit outside the calculation
- Semi-variable costs, such as an electricity bill with a fixed monthly charge, count only for the part that rises with usage
- Stepped costs, such as a second oven or an extra staff member, count only when extra output pushes you past a capacity limit
Marginal cost formula
The marginal cost formula divides the extra cost by the extra units. Here it is in plain terms.
Marginal cost = change in total cost ÷ change in quantity
Each part of the formula has a simple meaning.
- Change in total cost is your total cost at the new output level minus your total cost at the original level
- Change in quantity is the number of extra units you produce, whether that’s one unit or a batch of 100
Because fixed costs usually stay put, the change in total cost comes mostly from variable costs. The result is your cost per additional unit, ready to compare with your selling price.
How to calculate marginal cost in 3 steps
You need your total cost and output before a change and after it. Then follow these three steps.
- Subtract your original total cost from your total cost at the new output level to find the change in total cost
- Subtract your original quantity from your new quantity to find the change in quantity
- Divide the change in total cost by the change in quantity to get your marginal cost per unit
Say producing 200 units costs you Rp20.000.000 and producing 250 units costs Rp24.000.000. Your change in cost is Rp4.000.000 and your change in quantity is 50, so your marginal cost is Rp80.000 per unit.
If you charge Rp150.000 per unit, each extra unit leaves Rp70.000 to cover fixed costs and add to profit.
Marginal cost example
A worked example shows how the formula plays out in a real small business. Picture a sourdough bakery in Bandung.
Last month you baked 300 loaves for a total cost of Rp18.000.000. That’s Rp9.000.000 in fixed costs, like rent and equipment leases, and Rp9.000.000 in variable costs, like flour and packaging.
This month, a local café asks for an extra 50 loaves a month. Baking 350 loaves brings your total cost to Rp19.600.000: fixed costs stay at Rp9.000.000, while variable costs rise to Rp10.600.000.
Here’s how the numbers work out.
- The change in total cost is Rp19.600.000 minus Rp18.000.000, which equals Rp1.600.000
- The change in quantity is 350 minus 300, which equals 50 loaves
- The marginal cost is Rp1.600.000 ÷ 50, which equals Rp32.000 per loaf
- The new average cost is Rp19.600.000 ÷ 350, which equals Rp56.000 per loaf
If the café pays Rp55.000 a loaf, you earn Rp23.000 on each extra loaf, or Rp1.150.000 a month. That price sits below your average cost of Rp56.000, yet the order still adds profit because it’s above your marginal cost.
Your fixed costs are already paid, so the extra loaves only need to cover their own cost. For more scenarios, try this step-by-step marginal cost guide.
The marginal cost curve
Plot marginal cost against output and you’ll usually see a U-shaped curve. Knowing its shape helps you spot cost changes before they reach your bank account.
On the left of the curve, marginal cost falls as output grows. You buy materials in bigger, cheaper quantities and get more out of the equipment you already own.
At the bottom of the U, each extra unit costs the least. For a small business, this point often matches the capacity of your current equipment and team.
On the right, marginal cost climbs again because of diminishing returns. Overtime pay and rush fees from suppliers push up the cost of each unit.
This U shape describes the short run, when premises and equipment are fixed. In the long run, all costs, including capacity, can change, so a bigger oven or second shop can reset the curve.
Aim to operate near the bottom of the curve. Treat steadily rising unit costs as a signal to plan more capacity.
Marginal cost vs marginal revenue
Marginal revenue is the extra money you bring in from selling one more unit. Comparing it with marginal cost shows you the output level that maximises profit.
If you sell at a set price, your marginal revenue equals that price. Charge Rp20.000 per unit and each extra sale brings in Rp20.000.
Profit peaks where marginal cost equals marginal revenue. Every unit before that point adds profit; beyond it, each unit costs more to make than it earns.
Once marginal cost passes marginal revenue, you can scale back output or consider raising your prices. For a large discounted order, check that the discounted price still beats your marginal cost.
Marginal cost vs average cost
Total cost is everything you spend on all your units, fixed and variable. Average cost divides that total by the number of units, blending both types into one per-unit figure.
Marginal cost looks only at the next unit, and the two measures move together in a predictable way.
- When marginal cost is below average cost, each new unit pulls your average cost down
- When marginal cost equals average cost, your average cost is at its lowest point
- When marginal cost is above average cost, each new unit pushes your average cost up
Use average cost as your pricing floor, since you need to sell above it over time to cover all your costs. Use marginal cost to decide on one more order or batch.
Marginal cost vs stepped costs
Marginal cost assumes costs rise smoothly as you produce more. Stepped costs stay flat over a range of output, then jump when you cross a capacity limit.
Warehouse space is a good example. You pay the same rent whether you store 100 boxes or 900. Going past that capacity means renting a second unit and doubling the cost.
Other stepped costs show up as your business grows.
- Hiring another employee when your team reaches its workload limit
- Leasing a second machine when the first runs at full capacity
- Adding a delivery motorbike when orders exceed what one rider can handle
- Renting a bigger kitchen when you run out of prep space
Stepped costs change the maths. Your marginal cost might be Rp40.000 a unit, but if unit 501 triggers a Rp5.000.000 monthly machine lease, that one unit really costs Rp5.040.000.
Map these thresholds alongside your marginal costs, and time each step for when demand can carry the higher cost. A cash flow forecast helps you plan for the up-front payment.
Marginal costing vs absorption costing
Marginal costing and absorption costing are two ways to value your products and stock. The difference is how they treat fixed production overheads.
Marginal costing counts only variable production costs in the cost of each unit. Fixed production overheads, like factory rent, are expensed in the period they occur.
Absorption costing spreads a share of fixed production overheads across every unit, including stock you haven’t sold yet. That’s why the two methods can show different profits for the same month.
Marginal costing suits internal decisions, like pricing a one-off order. For external financial statements, the IFRS Foundation explains that under International Accounting Standard 2 (IAS 2), inventory cost includes purchase costs plus conversion costs, which cover direct labour and production overheads.
Reporting requirements vary by business, so check with your accountant or advisor which approach applies to you.
Contribution margin and break-even point
Contribution margin is what each sale leaves after variable costs, ready to cover fixed costs and then add profit. Break-even is the sales volume where total contribution equals your fixed costs.
Two formulas do the work.
Contribution per unit = selling price minus variable cost per unit
Break-even units = fixed costs ÷ contribution per unit
When variable cost per unit stays constant, it equals your marginal cost. In that case, contribution is simply price minus marginal cost.
Go back to the bakery. You sell loaves at Rp60.000, variable costs are Rp30.000 a loaf and fixed costs are Rp9.000.000 a month.
Each loaf contributes Rp30.000, so you break even at 300 loaves (Rp9.000.000 ÷ Rp30.000). If you usually sell 400 loaves, the extra 100 are your buffer above break-even, known as your margin of safety.
Benefits of understanding marginal cost
Tracking marginal cost gives you a clearer basis for everyday decisions. Here’s what it helps you do.
- Find the output level where each extra unit costs the least
- Price with confidence, including volume discounts and promotions
- Spot rising costs early, such as a supplier price increase that lifts your unit cost for three months in a row
- Direct spending towards the inputs that do most to lower your unit costs
- Judge whether a large order or new product variation will add profit
- Decide when to invest in capacity to grow your profits
Limitations of marginal cost
Marginal cost works best alongside other measures. Keep these limits in mind when you use it.
- It assumes your variable cost per unit stays stable, even though supplier prices and wastage can shift it
- It leaves out fixed costs, so it suits short-term decisions better than long-term planning
- It can mislead you on price, because selling at marginal cost for long periods won’t cover your overheads
- It can understate the true cost of one extra unit when that unit triggers a stepped cost
- It’s designed for internal decisions, while external financial statements usually need absorption costing
Track your costs with Xero
Marginal cost is only as reliable as the cost data behind it. Xero keeps your figures current with automated bank feeds and easy document uploads, so your variable costs are ready when you need them.
Turn those figures into clear financial reports and spot cost trends early to make confident pricing decisions. Try Xero today and get one month free.
FAQs on marginal cost
Here are quick answers to common questions about marginal cost.
What causes marginal cost to increase?
Rising input prices are a common cause, such as a flour supplier lifting prices or a weaker rupiah making imported materials dearer. Extra wastage or rework as you rush to meet demand also adds to the cost of each unit.
Can marginal cost be negative?
It’s possible in theory if producing more lowers your total cost, for example, when a larger order unlocks a big enough supplier discount. In practice, it’s rare for small businesses.
How does marginal cost relate to supply and demand?
As demand rises and you produce more, marginal cost usually climbs once you pass your most efficient output. That rising cost limits how much you’ll supply at a given price, which is why supply curves slope upwards.
How does marginal cost apply to service businesses?
For a service business, the unit is usually one more client or project. The marginal cost might be extra software licences or contractor hours, and knowing it helps you set fees and decide when to hire.
What is the difference between marginal cost and incremental cost?
Incremental cost is the total extra cost of a whole decision, such as taking on a big contract, while marginal cost covers one unit. Incremental cost can include new fixed costs, like extra equipment.
What is the difference between short-run and long-run marginal cost?
Short-run marginal cost assumes your premises and equipment are fixed, so it rises steeply as you near capacity. Long-run marginal cost includes the cost of adding capacity, so it’s the better guide when you’re weighing expansion.
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Disclaimer
This glossary is for small business owners. The definitions are written with their requirements in mind. More detailed definitions can be found in accounting textbooks or from an accounting professional. Xero does not provide accounting, tax, business or legal advice.