── ── 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.
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.
Install this skill (free, MIT)
npx skills add deciqAI/knowledge-skills