If you only know your revenue and your total costs, you know almost nothing about whether the next order makes you money. The variable costs formula is the tool that fixes that. It isolates the expenses tied to volume, which is the first step toward knowing your true per-order profit.
This guide gives you the formula, walks a full numeric example, separates variable from fixed costs, covers the variable cost ratio, and then does the thing most explanations skip: connects variable costs to the profit on each sale.
What are variable costs?
A variable cost is any expense that changes in direct proportion to how much you produce or sell. Sell twice as many units and the total roughly doubles; sell nothing and it drops to zero. That volume-linked behavior is the whole definition.
According to the Corporate Finance Institute, variable costs scale with production output or volume in the current period — contrasted with fixed costs, which stay constant regardless of how much you make or sell.
Common variable costs for a physical-product or ecommerce business include:
- Raw materials or cost of goods sold (COGS)
- Shipping and fulfillment fees
- Payment-processing and transaction fees
- Per-order pick, pack, and fulfillment labor
- Sales commissions
- Paid-ad spend allocated per acquired order
For a deeper breakdown of what does and doesn't count, see our companion piece on what variable costs are. Here we focus on the math.
The variable costs formula
The core formula is simple:
Total variable cost = Quantity of output × Variable cost per unit
Two supporting formulas fill in the gaps depending on what you already know:
- Variable cost per unit = Total variable cost ÷ Output. Use this when you have a lump-sum cost and need the per-unit figure.
- Total variable cost = Total cost − Fixed cost. Use this when your books give you totals and you know your fixed base.
All three describe the same relationship from different angles. The one you reach for depends on which numbers you have in front of you. As 1800Accountant notes, both the output-times-unit-cost approach and the total-minus-fixed approach lead to the same result when the underlying data is accurate.
Worked example: calculating variable cost per unit
Say you run a print-on-demand apparel store, and you want the variable cost of one average order that sells for $40. Build it up line by line:
- Blank garment plus printing (COGS): $16.00
- Shipping (carrier): $5.00
- Payment processing: $1.46
- Pick and pack labor: $1.40
The payment-processing figure comes from a standard online card rate of 2.9% plus 30 cents per transaction — the widely published Stripe standard fee; on a $40 order that is (0.029 × $40) + $0.30 = $1.46.
Add the four lines: $16.00 + $5.00 + $1.46 + $1.40 = $23.86 variable cost per order.
Now scale it with the main formula. If you ship 1,000 of these orders in a month:
1,000 orders × $23.86 = $23,860 total variable cost.
Notice what did not appear in that number: rent, your software subscriptions, and your salary. Those are fixed, and mixing them in is the most common way sellers miscalculate their margins.
Fixed vs variable costs
The split between fixed and variable costs is what makes profit analysis possible. Get the line-drawing right and every downstream number behaves.
Variable costs scale with volume. In the example above, that is the $23.86 per order — it only exists when an order exists. According to American Express Business Insights, common variable expenses include raw materials, packaging, hourly labor, shipping, and sales commissions.
Fixed costs stay flat in the short run no matter how many units you move. Say your store carries $4,000 a month in rent, apps, and a base retainer. American Express notes that common fixed expenses include rent, insurance, loan payments, and salaries — that $4,000 is owed whether you sell one order or one thousand.
One nuance worth flagging: some "fixed" costs are really step costs. A second warehouse or an added shift is fixed within a range, then jumps when you cross a volume threshold. Treat those as fixed only inside the range you're currently operating in.
Average variable cost
Average variable cost (AVC) is the total variable cost spread across every unit produced.
Average variable cost = Total variable cost ÷ Output
From the example: $23,860 ÷ 1,000 = $23.86 per order. Here it matches the per-unit cost exactly, because every order carries the same variable load.
In the real world, AVC drifts as your inputs change. Bulk material discounts pull it down; a carrier rate hike or a spike in returns pushes it up. Watching AVC trend over time is often an earlier warning than watching total costs, because it strips out the noise of changing volume — a theme we expand on in projected cost vs actual cost. As 1800Accountant puts it, average variable cost "helps you understand the cost per unit over time" and is useful for "comparing performance month-over-month."
Variable cost ratio
A subtopic that many top-ranking results now cover — and that feeds directly into pricing decisions — is the variable cost ratio.
According to the Corporate Finance Institute, the variable cost ratio is a cost accounting tool used to express a company's variable production costs as a percentage of its net sales, calculated by dividing variable costs by net revenues.
Variable cost ratio = Total variable costs ÷ Net revenue
Using the worked example: $23,860 ÷ $40,000 (1,000 orders × $40) = 0.597, or roughly 60%. That means about 60 cents of every revenue dollar is consumed by variable costs before fixed costs or profit enter the picture. The Corporate Finance Institute notes this ratio "quantifies the relationship between a company's sales and the variable production costs tied to those sales" — a quick gut-check for whether pricing is sustainable.
A falling ratio over time means you're keeping more of each dollar; a rising one signals cost pressure that pricing or volume alone won't fix. This connects tightly to the net profit margin benchmark guide, which shows what healthy ratios look like by segment.
From variable costs to real profit
This is the part generic explanations leave out. Variable costs matter because they feed contribution margin — revenue minus all variable costs — which is the honest measure of what a sale contributes before fixed costs.
Take the example order again:
$40.00 revenue − $23.86 variable cost = $16.14 contribution margin per order.
But you haven't paid to acquire that order yet. Ad spend is also variable — it rises with the orders you chase. Say you allocate $10 in ad spend per order. Then:
$16.14 − $10.00 = $6.14 in profit per order after acquisition.
That $6.14 is the number that actually determines whether scaling helps or hurts. A sale can look great at the revenue line and still leave you almost nothing once every variable cost — including ads — is netted out. As InvoiceFly points out, if your selling price is lower than your variable cost per unit, "each sale increases losses, even if revenue is rising." If you want to push further into how each extra order compounds, our incremental profit formula guide picks up exactly here.
Controlling variable costs is also one of the fastest levers on checkout completion rate decisions — when your margin per order is tight, high-friction checkout flows that lose even a fraction of buyers cost more than they appear to.
Break-even: where variable costs meet fixed costs
Once you know contribution margin per unit, break-even falls out directly.
Break-even units = Fixed costs ÷ Contribution margin per unit
Using the after-acquisition margin of $6.14 and $4,000 in monthly fixed costs: $4,000 ÷ $6.14 = 652 orders to cover your fixed base. Every order past 652 is profit; every order short of it is a loss you're funding from somewhere else.
Lower your variable cost per unit — cheaper fulfillment, a better processor rate, tighter ad efficiency — and contribution margin rises, which drops your break-even. That single lever is why disciplined variable-cost tracking beats chasing top-line revenue. For practical ways to reduce the per-order acquisition cost side of the equation, see our guide on CRO techniques and our walkthrough of increasing AOV with AI — both raise contribution margin without touching your cost of goods.
Pricing strategy and variable costs
Knowing your variable cost per unit is the floor for any pricing decision. According to Indeed's career advice resource, "you need to understand your costs before you can identify the best pricing strategy" — whether you price on a margin percentage or a fixed dollar amount above cost per item.
For print-on-demand sellers, three pricing approaches are most common:
- Cost-plus pricing: Variable cost per unit + target markup. Simple but ignores what the market will bear.
- Margin-based pricing: Set a target contribution margin percentage and back-calculate the required price. More useful when ad costs fluctuate.
- Competitive pricing: Anchor to market prices and work backward to see whether your cost structure supports that price profitably.
All three require an accurate variable cost figure as the starting point — which is exactly what the formula above provides. For POD sellers running Meta campaigns, the Facebook Ads for Shopify guide shows how ad spend integrates into per-order economics.
Where PodVector fits
Calculating variable cost per order by hand is fine once. Doing it live, across hundreds of orders, as your product costs, shipping, fees, and ad spend all move — that's where a spreadsheet quietly falls behind.
PodVector connects your Shopify, Meta Ads, Google Ads, Printify, and Printful accounts and computes the true per-order economics for every sale — COGS, shipping, fees, and allocated ad spend already netted out. Victor, PodVector's AI employee, reads that live data, proposes a typed action with its rationale and expected effect, and — with your approval — executes the move on the Shopify side. He can reprice products to a target margin, adjust discounts, raise your free-shipping threshold, and more. He reads your ad accounts and proposes changes, but all writes happen on Shopify. PodVector is built specifically for print-on-demand sellers on Shopify who fulfill through Printify or Printful.
See your true per-order profit with PodVector →
Knowing your real margin per order is also what makes retention worth chasing — repeat buyers carry no acquisition cost, so their contribution margin runs higher. That connection is spelled out in our guide on how to improve customer retention rate. And for POD sellers exploring whether to expand channels, see can I dropship from Etsy to Shopify — variable costs behave differently across fulfillment models.
FAQs
What is the variable costs formula?
Total variable cost = Quantity of output × Variable cost per unit. If you already have a lump-sum figure, you can also work backward: Variable cost per unit = Total variable cost ÷ Output. A third version — Total variable cost = Total cost − Fixed cost — is useful when your books give you totals and you know your fixed base. All three describe the same relationship between volume and cost.
What is the difference between fixed and variable costs?
Variable costs change with how much you produce or sell — materials, shipping, payment fees, per-order labor, and ad spend. Fixed costs stay constant in the short run regardless of volume, such as rent, salaries, and software subscriptions. The clean split between the two is what lets you calculate contribution margin and break-even.
What is the variable cost ratio?
The variable cost ratio is total variable costs divided by net revenue. It expresses what percentage of each revenue dollar is consumed by variable costs. A lower ratio means more revenue is available to cover fixed costs and generate profit. According to the Corporate Finance Institute, it is "a key factor in assessing a company's overall profitability."
Is labor a fixed or variable cost?
It depends on the type. Hourly pick-and-pack labor that scales with order volume is variable, because you pay more of it when you ship more. A salaried manager whose pay doesn't move with volume is a fixed cost. The test is always whether the expense rises and falls with output.
How do variable costs affect profit?
They set your contribution margin — revenue minus all variable costs — which is what each sale leaves toward covering fixed costs and profit. In the example above, a $40 order carries $23.86 in variable costs, leaving $16.14 before ads and $6.14 after a $10 acquisition cost. Cutting variable cost per unit raises margin and lowers your break-even point.
Is average variable cost the same as variable cost per unit?
They're the same only when every unit carries an identical variable load. Average variable cost is total variable cost divided by output, so it smooths across all units. Variable cost per unit can differ order to order once bulk discounts, returns, or shipping surcharges enter, at which point AVC and the per-unit figure diverge.