Viral coefficient measures how many new users each existing user brings through referrals/invites. Formula: (Number of invites sent per user) × (Conversion rate of invites). Above 1.0 = exponential growth.
Viral coefficient (K-factor) determines if your product can grow organically without paid marketing. Calculate: K = (Average invites sent per user) × (% of invites that convert). Example: Each user invites 5 friends, 20% sign up → K = 5 × 0.2 = 1.0. Magic thresholds: K < 1.0 = linear growth (need paid marketing). K = 1.0 = each user brings exactly 1 more user (sustainable). K > 1.0 = exponential viral growth (rare, holy grail). K = 1.5 = every 100 users bring 150, then those 150 bring 225, etc. (hockey stick). Real viral loops: Dropbox ("Get 500MB per friend referred"), WhatsApp (need friends on platform to get value), Zoom (host shares meeting link), Hotmail (signature "Get your free email at Hotmail.com"). Viral time matters too: Viral cycle time—how long invite-to-signup takes. Daily cycle (social apps) vs weekly (B2B apps) makes huge difference in growth speed. Increase K through: Incentivized referrals, forced invites to use product (Clubhouse invite-only), network effects (more users = more value), social sharing built into product.
K = (Invites sent per user) × (Invite conversion rate) | K > 1.0 = Viral growthK = 1.0+ through referral program. 35% of daily signups came from referrals. Saved millions on marketing, grew to 500M users.
K = 1.5-2.0. Natural viral loop—can't use WhatsApp unless friends are on it. SMS invites had 60%+ conversion. Grew to 1B users with zero marketing spend.
K = 0.1-0.5 (each user brings 0.1-0.5 users). Not viral. Must rely on paid marketing or content/SEO for growth.
K > 1.0 means free exponential growth. Companies with K > 1.0 can outspend competitors on CAC (because existing users bring new users for free). K < 0.5 means growth is entirely dependent on paid marketing budget.
Track: (1) How many invites each user sends on average, (2) What % of invites result in signups. Multiply them. Example: 10 invites sent, 5% convert → K = 0.5.
K = 0.5-0.7 is decent (some organic growth). K = 0.7-1.0 is good (significant organic growth). K > 1.0 is exceptional (viral/exponential growth). Most products have K = 0.1-0.3.
Very hard. B2B viral loops are slower (long sales cycles). Examples that worked: Slack (teams invite teammates), Zoom (hosts invite attendees). Need natural reason to invite others.
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