Viral coefficient
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The number of new users each existing user generates, measuring how effectively a product spreads through word of mouth and referrals.
The viral coefficient (K-factor) measures how many new users each existing user brings. If each user invites 10 people and 20% of those people sign up, your viral coefficient is 2.0. If it is above 1.0, every user generates more than one new user, and the product grows exponentially.
A viral coefficient above 1.0 is rare and usually temporary. Even products with strong viral mechanics (Dropbox, Slack, WhatsApp) had periods of viral growth followed by stabilization. Most SaaS products have a viral coefficient well below 1.0, which means they need paid or organic acquisition to grow.
But even a sub-1.0 viral coefficient is valuable. If your viral coefficient is 0.3, every 100 users you acquire through other channels generate 30 additional users for free. That reduces your effective customer acquisition cost by 30%. It is a key metric for product-led growth companies.
Examples
A collaboration tool calculates its viral coefficient.
Each user invites an average of 5 people. 30% of invited people sign up. The viral coefficient is 1.5 (5 x 0.30). Each user generates 1.5 new users on average. The product is growing virally.
A product increases its viral coefficient through design.
The team adds a 'share this dashboard' feature that makes it easy to send public links. Shared dashboards include a 'powered by' badge with a signup link. The viral coefficient increases from 0.2 to 0.4.
A company uses the viral coefficient to model growth.
With a viral coefficient of 0.5, the growth team knows that every 100 paid acquisitions will generate 50 additional users organically. They factor this into their CAC calculations and budget accordingly.
Frequently asked questions
What is a good viral coefficient?
Above 1.0 means truly viral growth (rare and usually temporary). Between 0.3 and 0.7 is strong for B2B SaaS. Even 0.1-0.2 adds meaningful free growth on top of paid acquisition. Any positive viral coefficient reduces effective CAC.
How do you increase the viral coefficient?
Two levers: increase the number of invitations per user (make sharing easy and natural) and increase the conversion rate of invited users (make the signup and first experience frictionless). Product design, not marketing, drives the viral coefficient.
Related terms
A closed cycle where the output of one step becomes the input of the next, creating compounding growth without linear investment.
A dynamic where a product becomes more valuable as more people use it, creating a self-reinforcing growth advantage.
A self-reinforcing business model where each component accelerates the others, creating compounding growth over time.
A go-to-market strategy where the product itself drives acquisition, conversion, and expansion through self-serve usage.
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