Media buying

Break-even ROAS for ecommerce: calculate your minimum return

By Mirow · September 15, 2026 · 6 min read

Break-even return on ad spend (ROAS) equals 1 divided by your contribution margin before advertising. If 45% of net sales remains after product costs and other variable order costs, your break-even ROAS is 1 ÷ 0.45 = 2.22x. That leaves nothing for fixed overhead or profit. A useful spending target needs room for both.

The calculation is straightforward. The difficult part is deciding which costs belong in the margin and whether the revenue in your ad dashboard matches the revenue in your accounts.

Start with what each order leaves you

For this calculation, net sales means product sales after discounts and returns, excluding sales tax. Include product costs and the other costs that change with orders: payment fees, pick-and-pack, packaging, and shipping paid by the business after customer shipping charges. Include expected return-related costs that have not already been counted.

Use one consistent treatment of shipping and returns. Deducting a refund from sales and also deducting the same refunded revenue as a cost counts it twice. Return handling, lost shipping, and nonrecoverable inventory are separate costs when they apply.

Shopify's gross-profit reports calculate gross margin from net sales and recorded product cost; discounts and refunds affect the result. Product cost data also needs to have been recorded when the product sold. Check which costs your export includes before treating its margin as money available for advertising. Shopify profit reports.

Here is a hypothetical month for a generic ecommerce store. These are illustrative assumptions, not Mirow client results.

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This store has $36,000 available for advertising, fixed overhead, and profit. Its 60% product gross margin would overstate the money available for those purposes because it leaves out the other $12,000 of variable costs.

Calculate the floor, then set a working target

Let c be contribution margin before advertising, expressed as a decimal:

Break-even ROAS = 1 ÷ c

At a 45% margin, $1 of sales leaves $0.45 before advertising. Spending that entire $0.45 on media produces $1 ÷ $0.45 = 2.22x ROAS and zero contribution after media. Treat 2.22x as the approximate threshold; do not round it down when setting a spending limit.

To retain a chosen share of net sales after advertising, include that share in the formula:

Required ROAS = 1 ÷ (c − desired contribution margin after media)

Suppose this store wants to retain 12% of net sales after media. Its working target becomes 1 ÷ (0.45 − 0.12) = 3.03x. At $80,000 of net sales, the corresponding maximum media budget is $80,000 × 33% = $26,400, leaving $9,600 before fixed overhead.

The 12% is a planning assumption, not an industry benchmark. Set it from the overhead, cash requirements, and profit the business needs to fund. The formula only works when the desired remaining margin is below the margin available before advertising.

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Check what your reported ROAS measures

The formula assumes the revenue basis used in ROAS matches the margin calculation. A dashboard that includes tax, ignores refunds, or uses a different attribution window is measuring something different.

Google Ads optimizes against the conversion values you provide. Those values can represent revenue or profit-related values, so inspect the conversion setup before interpreting the ratio. Google Ads conversion values.

Write down the value definition, attribution window, reporting dates, and included conversion actions beside your channel ROAS. Avoid adding platform-attributed revenue across channels and treating that sum as store sales. The same order may receive credit in more than one system. Shopify's explanation of attribution discrepancies.

For a business-level check, calculate contribution from reconciled store sales and all media spend. If you also show total store revenue divided by total media spend, label it a blended revenue-to-spend ratio. It includes sales from returning customers and other demand sources; it does not establish what advertising caused.

A lower ratio can still leave more dollars

Compare two hypothetical operating scenarios using the same 45% contribution margin before media:

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The higher-spend scenario leaves $3,500 more contribution, while the remaining margin falls by 2.3 percentage points. It clears the illustrative 12% requirement in both dollars and margin terms. It also requires $10,000 more media cash and more inventory to support the additional sales.

This table tests the economics of an assumption. It does not prove that another $10,000 of advertising will cause $30,000 of additional sales. Before increasing spend, state the expected sales and margin, choose a budget limit, and decide when enough orders and returns will have arrived to assess the result.

Refresh the calculation if a promotion, product mix, shipping expense, or return rate changes. A constant-margin model becomes misleading when the next orders are materially different from the previous ones.

Put three numbers in the next budget review

Bring contribution margin before media, contribution dollars after media, and contribution margin after media. Keep channel ROAS beside them as a diagnostic.

Start with one recent period whose returns are reasonably settled. Reconcile its sales and variable costs, subtract the full media bill, then calculate the remaining margin the next budget must protect. If cost or return data is incomplete, show a reasonable range and label the estimate.

That calculation gives your creative, media, conversion, and loyalty teams a shared constraint. The broader paid acquisition system explains how those teams can work against the same target.