Contribution Margin vs Gross Margin: Which Costs Belong in Your ROAS Math?
Ask ten store owners for their margin and you will hear ten different numbers. Most quote the figure from their accounting software, which is measured before shipping, card fees and returns. They then plug it into an ad calculator and wonder why a campaign that looks profitable on paper keeps draining the bank account. The gap almost always comes down to one decision: which costs you count before you judge your ads.
What Is Contribution Margin for Ecommerce?
Contribution margin for ecommerce is the share of each order left after every cost that rises with that order. Start with average order value. Subtract product cost, shipping and fulfilment, payment fees, the revenue lost to returns, and any other per-order cost such as packaging. Divide what remains by order value. That percentage is what you have available to pay for ads, rent, salaries and, finally, profit.
Contribution Margin vs Gross Margin
Gross margin usually stops at product cost. Contribution margin keeps going, and that is where stores separate. Take two examples from our calculator. A supplements store with a $60 order, light parcels and 2% returns keeps 59.5% of each sale. A home goods store with a $140 order, a 45% product cost and $22 shipping keeps 29.1%. Their break-even ROAS is 1.68 and 3.43 respectively. Same formula, completely different bar to clear.
Which Costs Belong in the Calculation?
Include a cost only if it rises with each order:
- Product cost, packaging, and pick and pack
- Postage, taken from carrier or 3PL invoices divided by orders shipped
- Payment processing, such as 2.9% + 30¢ on Shopify Payments Basic in the US, or 2.25% + 30¢ on Plus
- Returns, entered as the value you actually lose
Leave out rent, salaries, software subscriptions and agency retainers. They are paid from contribution, not subtracted before it. Ad spend stays out too, because it is the cost your ROAS has to clear.
Two Inputs That Go Stale
Shipping and returns move during the year, so check them often. USPS Ground Advantage prices rose about 7.8% on average in January 2026, and a temporary increase averaging 8.0% on competitive products followed in April. A postage figure from last winter is already too low.
On returns, US retailers expected 19.3% of online sales to come back in 2025, according to NRF and Happy Returns. Use a full year of refunds rather than a quiet month, and count only what you lose. An unworn item that resells at full price costs you return postage and handling, not its whole price.
A Worked Example
Take a $100 apparel order: $100 − $35 product − $9 shipping − $3.20 payment fees − $8 returns − $1 other leaves $43.80. That is a 43.8% margin, so break-even ROAS is 1 ÷ 0.438, or 2.28.
Basket size can shift this without touching your ads. At an athletic apparel brand we work with, orders averaged about 2.2 units in June 2026, up from about 1.9 a year earlier. The same pick, pack and postage now spread over more revenue, so margin rose and break-even fell.
Want to test your own numbers? Our break-even ROAS calculator builds this margin from your real costs.
Conclusion
Your ads can only be judged against an honest margin. Count every per-order cost, refresh shipping and returns data regularly, and keep fixed overheads out of the sum. Once you have a real contribution margin, every ROAS target becomes simple arithmetic. If you would like help setting those targets across channels, our paid media management service starts from exactly this calculation.
