Customer Acquisition Cost

What is customer acquisition cost?#

CAC = attributable acquisition cost / new customers acquired in the matched cohort

Both terms are policy choices. Similarweb, which reports CAC as an operating metric, defines it as "the portion of sales and marketing expenses allocated to acquire new customers," and defines a separate customer retention cost (CRC) as "the portion of sales and marketing expenses allocated to retain existing customers and to increase existing customers' subscriptions." That split is the single most important structural decision in the metric, and it is a decision — not a rule.

Three views are worth maintaining. They answer different questions and are not interchangeable:

Do not mix the numerator from one view with the label from another. The gap between them is routinely 3x.

The metric contract#

Before computing anything, write down and freeze six things:

Then reconcile back to the ledger, every period:

total sales & marketing expense
  = acquisition cost + retention/expansion cost + brand and market development + excluded items

The bridge is what stops a flattering operating metric from quietly excluding a growing share of spend. It is also the first thing a competent diligence process asks for.

Why does CAC matter to founders?#

It decides whether growth creates or consumes value. Revenue can accelerate while cash efficiency deteriorates. The test is not growth; it is growth measured against the gross profit that recovers the spend.

It exposes channel quality, not just channel cost. Two channels with identical lead cost can produce customers with different fit, sales effort, activation, retention, and expansion. Channel decisions belong to cohort economics, never to cost-per-lead.

It tests whether your price can support your sales motion. A field-sales motion is very hard to fund at a $6,000 contract value. When CAC looks impossible, the problem is usually the segment or the packaging, not the reps.

It makes fundraising claims auditable. Investors will probe whether CAC excludes founder selling time, harvests organic demand created by years of past spend, or benefits from a channel that is about to saturate. A reconciliation from the general ledger to the metric is worth more than a confident single number.

How do you calculate CAC?#

1. Freeze the cohort and match the lag#

If the median sales cycle is 90 days, dividing March spend by March wins measures timing noise. Use lead-created or opportunity-created cohorts, or a weighted spend-to-cohort schedule that you update as opportunities resolve.

2. Classify every dollar of sales and marketing spend#

3. Pick the denominator and hold it#

Signed accounts and activated accounts give different answers. Both are defensible; switching between them without restating is not.

4. Segment, then look at concentration#

Compute CAC by channel, segment, product, contract size, and geography — and always show volume alongside it, so a five-account channel does not set the company narrative.

5. Distinguish attributed CAC from incremental CAC#

A campaign gets attribution for customers who would have arrived anyway. Where the spend is material, use holdouts or geo tests. Where you cannot, say "attributed" and mean it.

paid-media CAC   = $120,000 / 30 = $4,000
variable CAC     = $240,000 / 30 = $8,000
fully loaded CAC = $360,000 / 30 = $12,000

fully loaded CAC per activated account = $360,000 / 25 = $14,400

Related concepts#

• CAC Payback Period — how long gross profit takes to recover the acquisition investment.
• Customer Lifetime Value — the survival-weighted gross profit CAC is spent against.
• LTV:CAC Ratio — compare matched value and acquisition cost.
• Gross Margin — the margin that actually recovers CAC.
• Churn Rate — how long the acquired customer stays to pay it back.
• Cohort Analysis — keep spend and outcomes in reproducible groups.
• Sales Funnel Metrics — connect stage conversion and cycle lag to acquisition.
• Distribution Channels — the mechanism that creates and converts demand.

Sources#

1. Similarweb Ltd., Q2 2025 Shareholder Letter (Form 6-K, Exhibit 99.2), August 12, 2025. The 45–50% land / 50–55% expand split of sales and marketing cost, the 21–22 month average CAC payback, and the definitions of CAC and customer retention cost.
2. Similarweb Ltd., First Quarter 2026 Results (Form 6-K, Exhibit 99.1), May 13, 2026. Current published definitions of CAC, CRC, and CAC payback period as supplemental operating metrics.
3. Gupta, S., Lehmann, D. R., & Stuart, J. A., Valuing Customers, Journal of Marketing Research 41(1), 7–18, 2004. Acquisition-cost elasticity of 0.02–0.32 versus margin elasticity of ~1 and retention elasticity of 3–7.
4. Skok, D., SaaS Metrics 2.0 — A Guide to Measuring and Improving What Matters, For Entrepreneurs. The HubSpot direct-versus-channel LTV:CAC comparison (1.5 versus 5.0) and the origin of the LTV:CAC and months-to-recover-CAC guidelines.
5. US Securities and Exchange Commission, Non-GAAP Financial Measures — Compliance and Disclosure Interpretations, Division of Corporation Finance. Staff position on defining, reconciling, and consistently applying supplemental measures.

Source-use note: The worked example is illustrative and constructed for this page; it is not drawn from any company's filings. Similarweb's figures are the company's own disclosures about its own business and are not a benchmark for any other company.

Note: This page is educational and does not constitute accounting, financial, or investment advice. CAC is an unaudited operating metric outside GAAP and IFRS, and companies exercise real discretion over its cost boundary, denominator, and cohort. Consult a qualified accountant before relying on it in investor reporting or a financing document.