What is viral coefficient (k-factor)?
The number of new users each existing user generates. If K > 1, you have exponential growth. Calculated by multiplying invites sent per user by the conversion rate of those invites.
Formula
K = (Invites Sent per User) × (Invite Conversion Rate)
Why it matters
A K-factor above 1 means viral growth - each user brings more than one new user. Dropbox achieved this with their two-sided referral program. Even K < 1 significantly reduces CAC.
How to improve
Make sharing natural (product-led growth). Give both sides incentives (two-sided referrals). Make the invite experience great for recipients. Time sharing prompts to moments of delight.
Related playbooks
Give both sides a reward
People hate asking friends for favors. Give both the sharer and the friend something, and sharing feels like a gift instead.
Make your product visible to non-users
Every time someone uses your product, someone who doesn't have it sees it working. That's the best ad you'll ever run.
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