KPIMarketing

Viral / Referral Coefficient (K-Factor)

Average number of new users each existing user generates through referral or word-of-mouth

What is Viral / Referral Coefficient (K-Factor)?

The viral coefficient, or K-factor, measures how many new users each existing user brings in. It combines how many invitations users send with the share of those invitations that convert.

A value above 1 means every user brings in more than one new user on average, so growth feeds itself. Even a value below 1 is useful, because it reduces the cost of acquiring each customer.

How this KPI is measured

Function: Marketing
Sub-function: Product-Led Growth
Industry: All Industries
Type: Leading
Impact: Revenue
Unit: x
Measurement frequency: Monthly / Quarterly
Formula: Average invitations sent per user × Invitation conversion rate

How to calculate it

Multiply the average number of invitations sent per user by the conversion rate of those invitations.

For example, if each user sends 8 invitations on average and 15% of them become new users, the coefficient is 8 x 0.15 = 1.2.

A value of 1.2 would mean the user base grows on its own, though in practice referral growth also depends on how quickly invitations are sent and on how long the effect lasts. Measure it by cohort and be realistic, since most products land well below 1.

Where it is used

Evaluating referral programmes
See whether an invitation feature or incentive is bringing in new users.
Estimating organic growth
Understand how much of growth comes from users rather than paid acquisition.
Prioritising product changes
Test changes that make sharing easier or more rewarding.

How to improve it

Make sharing part of the productPlace invitation and sharing options where users experience value, and keep them simple.
Give both sides a reasonOffer benefits to the person inviting and to the person invited.
Improve the invited user's first experienceMake the landing and sign-up path for referred users fast and relevant.
Measure sends and conversion separatelyFind whether the weak point is too few invitations or too few of them converting.
Test incentives carefullyCompare rewards and check they attract users who stay, not only users who claim the reward.

Tips and common mistakes

Do not expect a value above 1. Very few products reach it, and reducing acquisition cost is still a real gain.
Measure the time it takes for invitations to convert as well as the rate.
Check the quality of referred users. Some incentives bring in people who never engage.

More Marketing kpi library

← All kpi library

Build better plans, faster

Join the waitlist for early access to curated insights, flexible templates, and rapid iteration across strategy, planning, and project management.