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Business & Tax
CAC, LTV, gross margin, and contribution margin decide whether each new customer makes you richer or poorer. The formulas, the ratios that matter, and a worked case.
By FreeCalculators Editorial · Published 2026-09-01 · Updated 2026-09-04 · 5 min read · 1,112 words
Unit economics is the profit or loss on a single unit of sale — one customer, one subscription, one transaction — measured after every cost that varies with that unit. If the unit loses money, growth multiplies the loss rather than curing it, which is why the question is settled before scaling and not after. Four numbers carry the whole analysis: customer acquisition cost, lifetime value, gross margin, and contribution margin.
CAC is total sales and marketing spend for a period divided by new customers acquired in that period — including salaries, ad spend, tooling, content, and events, not just media. LTV is average revenue per customer per period, multiplied by gross margin, multiplied by the expected number of periods retained. Gross margin is revenue less direct cost of delivery. Contribution margin goes one step further and removes every variable cost attributable to the customer: payment processing, support hours, onboarding, hosting.
| Metric | Formula | What it excludes |
|---|---|---|
| CAC | Sales + marketing spend ÷ new customers | Excludes cost to serve after the sale |
| Gross margin % | (Revenue − direct COGS) ÷ revenue | Excludes support, processing, overhead |
| Contribution margin | Revenue − COGS − variable cost to serve | Excludes fixed overhead and salaries |
| LTV | ARPU × gross margin % × periods retained | Excludes acquisition cost — that is CAC |
| LTV:CAC | LTV ÷ CAC | Excludes payback timing, which matters separately |
The 3:1 rule — three dollars of lifetime margin for every dollar of acquisition cost — is the most repeated benchmark in the field and also the least sourced. No agency publishes it; it originated as venture-capital shorthand for a business that can fund its own growth, and it is folk wisdom rather than data. It is still useful as a band, provided you know why each band sits where it does.
| LTV:CAC | What it usually means | The action it implies |
|---|---|---|
| Below 1:1 | Every new customer destroys value | Stop spending; fix pricing or retention first |
| 1:1 to 2:1 | Thin — no margin for a bad quarter | Raise price or cut CAC before adding spend |
| 3:1 to 5:1 | Healthy and self-funding | Scale acquisition while watching payback months |
| Above 5:1 | Likely under-investing in growth | Test higher acquisition spend deliberately |
A 4:1 ratio earned over a seven-year customer life is a very different business from a 4:1 ratio earned in nine months, because the first one needs years of financing to bridge the gap. CAC payback — months of contribution margin needed to repay acquisition cost — is the number that determines whether growth consumes cash. The Federal Reserve Small Business Credit Survey, published annually by the twelve Reserve Banks, repeatedly finds paying operating expenses among the most commonly cited financial challenges for small employer firms, which is what a long payback period looks like from the inside.
Worked example: a subscription business (2026)
Monthly price = $80 Direct COGS (hosting, licenses) = $16 -> gross margin 80% Variable cost to serve (support, fees) = $12 Contribution margin per month = $80 - $16 - $12 = $52 Monthly churn 3% -> average life = 1 / 0.03 = 33 months LTV (margin basis) = $52 x 33 = $1,716 Sales + marketing spend, quarter = $96,000 New customers in quarter = 120 CAC = $96,000 / 120 = $800 LTV:CAC = $1,716 / $800 = 2.1 : 1 CAC payback = $800 / $52 = 15.4 months
That business is viable but not yet fundable at scale: a 2.1:1 ratio with a 15-month payback means every 120 new customers tie up $96,000 for well over a year. Cutting churn from 3% to 2% extends average life to 50 months and lifts LTV to $2,600, moving the ratio to 3.3:1 without spending a dollar more on acquisition. Retention is almost always the cheapest lever in the model.
Comprehensive Guide
Read our business and tax guide for margins, payroll, and tax planning.
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How this guide was created
This guide was written and reviewed by FreeCalculators Editorial, drawing on published formulas, official government sources, and real calculator outputs from our 4 calculators in this category. Every claim is sourced; every formula is auditable. Read our review policy.