The viral coefficient, or k-factor, multiplies the number of invitations an average user sends by the rate at which invitations convert. A value above one would mean each cohort of users brings in a larger one, though true exponential growth is rare and fades over time.
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Calculating it
Multiply the average invites sent per user by the percentage of invites that become users. Measure by cohort over a fixed window so the numbers can be compared.
Realistic use
Most products have a coefficient below one, and still benefit. Even a modest figure lowers acquisition cost by adding referred users on top of paid and organic ones.
Common mistakes
Counting invites that never convert
Only signed-up, active users count toward the coefficient.
Frequently Asked Questions
Collaborative and network-based products, where inviting others is part of using it.
Keep reading
- Growth Loops: Self-reinforcing systems where the output of one cycle, such as new users or content, becomes the input for the next, so growth compounds instead of resetting.
- Word of Mouth: Customers recommending a product to others through conversation, communities, social posts and reviews. A strong, hard-to-measure SaaS acquisition source.
- Product-Led Growth: A go-to-market model where the product itself drives acquisition, activation and expansion, usually through a free plan or trial that buyers start alone.
- Cohort analysis: Cohort analysis groups users by a shared starting point, such as sign-up week, and tracks how each group behaves over time.
- Account-Based Marketing: A B2B strategy that targets a defined list of high-value companies with coordinated, personalized marketing and sales, rather than casting a wide net.
- Alternatives Page: A page that answers “[competitor] alternatives” by listing several real options, written for buyers who are leaving or ruling out a product.
- Buying Committee: The group of people inside a company who influence, evaluate and approve a B2B software purchase, from end users to finance, IT and legal.
- CAC Payback Period: How many months of gross profit from a new customer it takes to recover what it cost to acquire them. A measure of how much cash growth ties up.
