Viral Coefficient(K-factor)

Also called: K-factor, 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 to 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.

Pitch deck pillar pages

Long-form deep dives on the slides Viral Coefficient most often shows up on.

Pitch Deck Traction Slide

Build a traction slide that proves the business is working: stage-appropriate metrics, the chart shape investors expect, and three failure patterns to avoid.

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