one-line definition
A viral loop is a self-reinforcing cycle where existing users bring in new users as a natural byproduct of using the product.
formula: Viral coefficient (K) = Invites per user × Invite conversion rate. K > 1 means exponential growth.
tl;dr
A viral loop only works if the sharing action is embedded in the core workflow — not bolted on as a "refer a friend" button. The best loops make sharing the easiest path to completing a task (e.g., sharing a doc to collaborate).
Simple definition
A viral loop is a growth mechanism where using the product naturally leads to inviting others, who then become users and repeat the cycle. Unlike paid acquisition, a working viral loop compounds — each new cohort brings in the next one at zero marginal cost. What separates it from generic "word of mouth": the sharing is baked into the product's core use case, not bolted on after launch.
How to calculate it
Use the viral coefficient: K = Invites per user x Invite conversion rate. If each user invites 5 people and 25% of those invitees sign up, K = 5 x 0.25 = 1.25. A K above 1 means each user generates more than one new user, creating exponential growth. Below 1 and your loop is still valuable — it just supplements other channels rather than replacing them. Track K weekly and break it into the two components so you know whether to optimize for more invites or better conversion of those invites.
Example
You build a design feedback tool. Users upload a mockup and share a link with stakeholders to collect comments. Each active user shares with an average of 3 stakeholders per week. Of those stakeholders, 20% sign up because they want to use the tool for their own projects. That gives you K = 3 x 0.20 = 0.6. Not exponential yet, but each 100 users organically bring in 60 more. To push K higher, you add a feature letting commenters create their own project from the review page — now conversion jumps to 35% and K = 1.05. Your user base starts growing without additional ad spend.
Related reading
Related terms
- Virality
- Product-Led Growth
- CAC
FAQ
What is the difference between a viral loop and a referral program?+
A referral program incentivizes sharing with rewards. A viral loop makes sharing a natural part of using the product — like sending a Calendly link to book a meeting.
How do I know if my product can support a viral loop?+
Ask yourself: does the product become more useful when shared with others? If collaboration, sharing output, or inviting teammates is part of the core workflow, you have a loop candidate.