── ── Startups
K-Factor and Viral Coefficient
K = i × c, where i = average invites per existing user, c = conversion rate into new activated users. K > 1 → exponential growth; K < 1 → finite ceiling; K = 1 → linear. Most "viral" products have K in the 0.1-0.6 range — social transmission, not a growth engine.
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How it works
Step 1 — Define units: New user = activated (not signed up). Identify invite event and conversion event. Set cohort window (typically 7-30 days).
Step 2 — Calculate: i = total invites / N users. c = conversions / total invites. K = i × c.
Step 3 — Growth math: K < 1 → ceiling = N₀/(1-K). K = 1 → linear. K > 1 → exponential. (1000 users: K=0.9 → ~10K ceiling; K=1.5 → ~3.3M after 20 cycles.)
When to use it
- user says 'viral coefficient,' 'K-factor,' 'going viral,' 'our product is viral,' 'referral program,' 'invite mechanic,' 'built in sharing,' growth plateauing despite viral elements, or a growth forecast is being justified by virality without a K calculation
When not to use it
When the decision is routine and reversible, applying a formal method costs more than it returns.
Worked example
Hotmail, 1996-1998
The case that defined viral marketing as an operational discipline — and the case Steve Jurvetson cited when he coined the term — is Hotmail, the free web-based email service founded by Sabeer Bhatia and Jack Smith in 1996.
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FAQ
How do you calculate the K-factor?
K = i × c: the average number of invites each existing user sends, times the rate at which invites convert into new activated users. If 100 users send 200 invites (i = 2) and 15% convert (c = 0.15), K = 0.3 — each user generates 0.3 new users.
What does K greater than 1 actually mean?
Each user brings in more than one new user, so growth compounds without paid acquisition — true virality. K below 1 means every viral loop shrinks: the product still benefits (each paid user brings a fraction more), but virality is an amplifier on other channels, not an engine.
Why do so few products achieve K above 1?
Sustained K > 1 requires the product to be inherently collaborative or communicative — the invite must be part of using the product, not a growth team's afterthought. Most 'viral' products run K between 0.1 and 0.6: valuable word-of-mouth, but a finite multiplier that must be paired with a real acquisition channel.
Related mental models
Mr.
Every market — capital, talent, customers, ideas — prices the consensus view into its current state.
The North Star Metric (NSM) is the single metric that most directly measures value delivered to customers and predicts revenue over time.
Organic growth builds value from within (product, operations, retention, unit economics).