Viral Coefficient (K-factor)

The K-factor is a reality check more than a growth strategy. A true figure above 1 is rare and rarely lasts, because the easy invites are used up quickly. Below 1, virality still acts as a multiplier on other channels. At K = 0.4, each acquired user effectively becomes 1.4 users, which lowers the real cost of acquiring each one even though it will not sustain growth by itself. Tracked honestly, it shows whether to keep investing in the referral mechanic or treat it as a minor bonus.

• Count invites per user and the conversion rate separately, then multiply. The two halves point to different fixes.
• Track the figure over time, not as a snapshot. A mechanic decays once the easy invites are gone.
• Pair it with cycle time, the days between a user joining and their invitees joining. A K just above 1 that turns over in months compounds far more slowly than a smaller K that turns over in days.
• Break it down by acquisition source, since users from different channels often refer at different rates.
• Be suspicious of any K above 1 until it holds across more than one cohort.
• Use it to size the discount referrals give on acquisition cost, not as the headline growth plan.

K = invites sent per user × conversion rate of those invites. If each user invites 5 people and 1 in 5 of them signs up, K is 1.0. Above 1, every user brings in more than one new user, so the product grows without paid spend. Below 1, referrals still help but cannot carry growth alone.