Promoter-to-referral rate

• Metric

  Viral coefficient (K)

  new users each user brings

  Invites (or shares) per active user multiplied by the share of invites that become activated users. K near zero while users love the product means fans exist but have no path to share.

• Metric

  Organic share of new activated users

  unpaid share of new activated users

  Share of new activated users who arrived without paid acquisition. A rising organic share corroborates product-market fit; a falling one is a warning even when total sign-ups grow.

• Metric

  Referral share of activated sign-ups

  new activated users who came through other users

  Share of new activated users who came through another user (an invite, a referral link, a shared result). The primary Spread metric; referred users should retain at least as well as organic ones.

• Metric

  Spam complaint rate

  emails marked as spam

  Share of delivered emails that recipients mark as spam, as reported by mailbox providers. Keep it under 0.1% and never reach 0.3%, the hard limit in Gmail and Yahoo bulk-sender rules since February 2024.

• Metric

  User quick ratio

  growth in versus churn out

  New plus resurrected users divided by churned users, over a rolling 28 days. Above 1 the user base grows; below 1 for two to three months marks a stalled or declining product.

• Metric

  Viral cycle time

  time for a user to bring in the next one

  Median time from a user's sign-up to the sign-up of someone they invited. Shorter cycles compound growth; products used daily give more chances to involve others.

Sources

Book content is paraphrased, never quoted at length. Last verified October 24, 2026.