ACOS vs ROAS: formulas and conversion

Use the same sales and spend

Illustrative inputs: spend $800 and attributed sales $4,000. ACOS is $800 ÷ $4,000 × 100% = 20%. ROAS is $4,000 ÷ $800 = 5×. Converting the ACOS gives 100 ÷ 20 = 5. If ACOS is stored as a fraction, use 1 ÷ 0.20 instead.

Connect ACOS to your margin

Assume, for illustration, that an order produces $100 of revenue and $65 of non-ad variable costs. The remaining $35 is 35% of revenue: $35 ÷ $100 × 100%. Spending all $35 on ads corresponds to 35% break-even ACOS and $100 ÷ $35 ≈ 2.86× break-even ROAS. This covers the included variable costs only. Fixed expenses or a profit requirement would need a lower ACOS and higher ROAS.

Do not confuse attributed sales with total sales

Using total store sales in the denominator changes the measure. It can be useful to compare total sales with total ad spend, but label that blended calculation separately. A channel-specific ACOS cannot be converted into a store-wide ROAS by simply taking its reciprocal; the underlying revenue scopes differ.

Watch the edge cases

When attributed sales are zero and spend is positive, ROAS is zero and ACOS is undefined. If spend is zero and sales are positive, ACOS is zero and ROAS is undefined. Missing inputs should remain missing rather than being silently converted to zero. Refunds and reporting windows must also match before you compare either metric across campaigns.

Source: Amazon Ads’ ACOS guide documents the ACOS and ROAS formulas. The cost example and margin derivation above are original illustrative calculations.

Examples are illustrative, not client results or benchmarks. The calculations use the inputs shown; real-world decisions depend on your records and assumptions. Report a correction.