Budget your ad spend from margin, not hope
Most stores set an ad budget by gut ("let's try $5k this month") and then hope the sales show up.
The disciplined way runs backward: your margin, and how long you'll wait to break even, set a hard ceiling on what you can spend to acquire a customer. That ceiling sets the budget.
Fastest path: one prompt, end to end
🤖 AI prompt — paste into ChatGPT / Claude
You are a DTC growth-finance analyst. Use MY numbers only; invent nothing.
Contribution margin per order: [$] (see contribution-margin if unknown)
Repeat purchases in 6 months (avg): [#]
Payback tolerance: [first order only / 90 days / 6 months]
Target new customers this month: [#]
Current blended CAC: [$]
Do this, showing the math:
1. Affordable CAC = contribution margin over my payback window (1st order = per-order margin; longer = margin x expected orders in the window).
2. Budget ceiling = affordable CAC x target new customers.
3. Compare current CAC to affordable CAC: am I under, at, or over the line?
4. Give the scale rule: only raise budget while actual payback stays within tolerance.
For the ROAS floor / break-even ROAS, tell me to see break-even-roas under marketing; do not compute ROAS here.
If a number is missing, ask; do not assume. If you cannot browse a benchmark, use only my numbers.
Output: affordable CAC + monthly budget ceiling + under/over verdict + scale rule.
Or do it in 4 steps
- Start from contribution margin, not revenue. The dollars each order leaves after all variable costs (see contribution-margin) is what you have to spend on acquisition. Revenue and gross margin will both lie to you here.
- Set your payback tolerance, then compute affordable CAC. Breaking even on the first order means affordable CAC = contribution margin per order. Waiting 90 days or 6 months means affordable CAC = margin x the orders a customer places in that window. Longer tolerance buys a higher CAC but risks more cash.
- Budget ceiling = affordable CAC x target new customers. Decide how many customers you want this month, then multiply by the affordable CAC. That's the most you can spend on acquisition without breaking your own payback rule. That's the budget, not a guess.
- Scale only while payback holds. Raise the budget only when actual CAC stays inside affordable CAC. The moment real payback drifts past tolerance, stop scaling; that's the channel saturating, not a reason to spend more.
Worked example (labeled): Contribution margin $33/order, first-order payback, target 300 new customers. Affordable CAC = $33; budget ceiling = 33 x 300 = $9,900.
Current blended CAC is $28 (under the line), so there's room to scale, but only while CAC stays under $33. For the matching ROAS floor, see break-even-roas under marketing: this note sets the spend ceiling, that one sets the return threshold.
Derive the budget from margin every month; never let "let's try $X" set it. Cross-check with cac-and-payback, and route the ROAS math to break-even-roas under marketing.