Viral Coefficient (K-factor)

TL;DR

The average number of new users each existing user invites who themselves convert into active users.

The viral coefficient (K) measures whether a product grows on its own. K = (invites per user) × (conversion rate of invites). A K above 1 means each user produces more than one new user, and the product compounds without paid acquisition. A K of 0.4–0.7 still meaningfully reduces CAC even though the product is not 'viral' in the lay sense.

Virality interacts with cycle time: a low K with a fast invite cycle can outperform a high K with a slow one. Real-world K above 1 is rare and almost never sustainable.

Formula

K = i × c

• K — Viral coefficient
• i — Average invites sent per active user
• c — Conversion rate of invitees into active users

K > 1 implies organic compounding growth; K < 1 still reduces effective CAC.

Worked example

A referral program: each new user sends 4 invites (i = 4) and 12% accept (c = 0.12). K = 4 × 0.12 = 0.48. Since K < 1, growth from referrals decays — the program supports paid acquisition but doesn't sustain growth alone.

Common pitfalls

• Reporting share counts as virality without measuring conversion.
• Confusing referral incentives with true product virality.
• Optimizing K above the customer's actual social tolerance.

When this shows up in a pitch deck

The Growth slide cites K alongside cycle time when virality is part of the thesis; otherwise omit it.

See Viral Coefficient in context

Viral Coefficient shows up most often in these scoring rubrics and investor profiles — jump straight to who cares about it and how to pitch them.

In VC frameworks

• 500 Global — pitch deck framework

Related terms

• AARRR (Pirate Metrics) — Dave McClure's five-stage growth funnel: Acquisition, Activation, Retention, Referral, and Revenue.
• Acquisition Channel — A repeatable source of new users or customers, such as paid search, content SEO, partnerships, outbound sales, or virality.
• CAC — Customer Acquisition Cost — the total sales and marketing spend required to acquire one new paying customer over a given period.
• Land and Expand — A motion where a small initial deployment grows into a much larger account through additional seats, products, or use cases.
• Network Effects — A property where each additional user makes the product more valuable for existing users, creating compounding defensibility.

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