How to calculate cost per unit and contribution margin

How to calculate cost per unit

Cost per unit = Total costs for the batch ÷ Units in the batch

For an online store, "total costs" means landed cost: everything you pay to get the stock to your warehouse, ready to sell.

Worked example: landed cost for a batch of 500 unitsExample numbers

Cost	Amount
Supplier price (500 × $8.00)	$4,000
Inbound freight	$600
Import duties and customs fees	$300
Packaging	$250
Total landed cost	$5,150

Cost per unit = $5,150 ÷ 500 = $10.30

The real cost is about 29% higher than the $8.00 supplier price. Using the supplier price alone would overstate your margin on every sale.

Recalculate cost per unit for each new batch. Freight and duty costs change, and an old cost per unit quietly inflates your margins.

Contribution margin per unit

Cost per unit tells you what the product costs. Contribution margin tells you what each sale actually leaves after all the costs that come with it.

Contribution margin = Price − Variable costs per unit Contribution margin ratio = Contribution margin ÷ Price

Variable costs

costs that rise with each unit sold: landed cost, outbound shipping, payment and marketplace fees, pick-and-pack

Continuing the example, with the product selling for $29:

1. Variable costs = $10.30 (landed cost) + $5.00 (shipping) + $1.20 (fees) = $16.50
2. Contribution margin = $29.00 − $16.50 = $12.50 per unit
3. Contribution margin ratio = $12.50 ÷ $29.00 = 43.1%

That $12.50 is what each sale contributes toward fixed costs (apps, staff, rent) and profit.

Contribution Margin per SKU CalculatorWhat a single SKU actually contributes after COGS, shipping, and fees.Open calculator

Comparing SKUs side by side

The real value comes from putting every product in one table:

SKU B looks fine on gross margin: ($18 − $7.40) ÷ $18 = 59%. But shipping costs nearly as much for a cheap item as for an expensive one, so its contribution is under half of SKU A's. Options for B include a higher price, selling it only as an add-on with other items, or bundling it.

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Fixed vs variable costs

Contribution margin only works if you split costs correctly:

The test is simple: if selling one more unit makes the cost go up, it's variable.

Once you know contribution per unit, you can work out how many units cover your fixed costs:

Break-even units = Monthly fixed costs ÷ Contribution margin per unit

With $6,000 of monthly fixed costs and the $12.50 contribution from the example above, you need to sell 6,000 ÷ 12.50 = 480 units a month to break even. Every unit after that adds $12.50 of profit.

Allocating shared freight across products

When one shipment carries several products, the freight bill has to be split between them. The two common methods give very different answers:

Worked example: splitting a $1,200 freight billExample numbers

Product	Value in shipment	Weight	Freight by value	Freight by weight
A: Water bottles	$6,000	200 lb	$900 (75%)	$300 (25%)
B: Yoga mats	$2,000	600 lb	$300 (25%)	$900 (75%)
Total	$8,000	800 lb	$1,200	$1,200

Allocate the way the freight was actually charged. If the carrier priced the shipment by weight or volume, split by weight or volume; otherwise heavy, cheap products look more profitable than they are. Duties are usually charged on value, so split them by value.

Gross margin vs contribution margin

The guide to gross profit margin covers the first; the calculator above covers the second.

How to lower cost per unit

• Order in larger batches where the volume discount outweighs the extra holding cost. The economic order quantity calculator helps find the balance.
• Consolidate freight into fewer, fuller shipments.
• Review duty classifications with a customs broker; the wrong code can mean paying more than you need to.
• Simplify packaging without hurting the unboxing experience.
• Negotiate with suppliers using your order history and forecasts.

To see how unit costs roll up into your store's overall margin, see the guide to gross profit margin. For standard vs actual costing, see the guide to product costing.