Break-even ACOS equals your pre-advertising profit margin. Subtract COGS, Amazon referral fees, FBA fulfilment, storage, and a returns allowance from your selling price; divide the remainder by the selling price. If a $30 product leaves $9.60 after all costs, your margin is 32% and your break-even ACOS is 32%.
A worked example
Selling price $30.00. COGS $8.00. Referral fee at 15% is $4.50. FBA fulfilment $5.50. Storage and returns allowance $2.40. Total costs $20.40, leaving $9.60.
$9.60 ÷ $30.00 = 32%. At a 32% ACOS you make nothing on an advertised sale. At 20% you keep $3.60 per advertised unit.
The costs people forget
Returns are the most commonly omitted line. A 5% return rate on a category where returns are unsellable is a real 5% off your margin, not a rounding error.
Long-term storage fees, inbound shipping, and coupon or Subscribe & Save discounts all belong in the calculation. Leaving them out produces a break-even figure that is optimistic by several points, which is exactly the margin you are trying to protect.
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