Fixed vs variable costs in ROAS planning

Classify costs for the decision

A cost is variable for a model when it changes with the relevant activity, such as fulfilled orders. A fixed cost stays the same over the modeled range and period. Some costs step up when capacity is exceeded, and others have both a base fee and a usage component. The classification depends on the contract and planning horizon, not the label alone.

Calculate contribution before allocating overhead

Illustrative inputs: revenue of $100 per order and variable non-ad cost of $60. The order leaves $100 − $60 = $40 before ads. The variable-cost break-even ROAS is $100 ÷ $40 = 2.5×. Spending the entire $40 would leave nothing toward fixed costs.

Build a whole-period check

Assume 200 such orders and $2,000 of fixed costs for the period. Before ads and fixed costs, contribution is 200 × $40 = $8,000. After reserving $2,000, the total break-even advertising allowance is $6,000. Dividing by 200 orders gives $30 per order, and the modeled revenue-to-ad-spend requirement is $20,000 ÷ $6,000 ≈ 3.33×.

Do not treat an allocation as a new invoice

The $10 overhead allocation per order above is $2,000 ÷ 200. It depends on the assumed order volume. At 100 orders, the same fixed costs allocate to $20 per order. That does not mean each extra order causes a $20 fixed expense. Avoid using an average allocation as if it were the marginal cost of every new sale.

Use both views

Use incremental contribution to assess an additional order within existing capacity, and a whole-period model to assess whether the overall plan covers overhead. Include step changes in staffing, warehousing, or software if growth triggers them. A forecast that assumes order volume must also explain why that volume is plausible; the arithmetic does not supply demand.

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.