Viral coefficient (k)

The average number of new users each existing user brings in.

Example

k = 0.4 means every 10 users bring 4 more.

Important

k above 1 is genuinely rare and usually temporary. A k of 0.4 is still extremely valuable — it cuts your effective CAC by 40%.

Related terms

  • Growth loop — A cycle where the output of using the product feeds back into acquiring more users.
  • Referral — A new customer arriving because an existing one recommended you.
  • CAC (Customer Acquisition Cost) — The total sales and marketing spend to acquire one paying customer.

Lessons that explain it

  • Retention, referral and growth loops — A funnel needs constant refilling. A loop reinvests its own output. The difference decides whether growth compounds. (12 min)

All 176 terms in the Founder glossary