Viral Coefficient

Some start-ups grow mainly by advertising. Others grow because their own users bring in the next users — a friend shares a link, a colleague gets an invite. That second kind of growth is called viral, and the viral coefficient is how an AIF manager puts a number on it.

The idea, per the workbook (section 14.8.1): it "represents the degree of exponential growth which a company experiences, by analyzing the number of referrals sent per user, the conversion rate of those referrals, and the total number of current users." It matters most in businesses "wherein word-of-mouth marketing strategy can be efficiently deployed" — a social app, a fintech referral scheme, a marketplace.

The workbook gives the formula through its own worked figures (section 14.8.1):

"a start-up determines that on average, each user sends out three referrals for new users to join and the conversion rate of those referrals is 33%. This indicates that three existing users can get one more paid user, on average."

Viral coefficient = Referrals sent per user × Conversion rate of referrals
                  = 3 × 33%
                  ≈ 0.99, about 1 new user per existing user

Note the workbook's own inconsistency. Its sentence says "three existing users can get one more paid user" — that would be a coefficient of about 0.33. Its own figures (3 referrals × 33%) give about 1 new user per existing user (one new user for every three referrals, not every three users). Work from the formula.

A coefficient above 1 means each existing user brings in, on average, more than one new user — growth compounds without extra marketing spend. A coefficient below 1 means the loop still needs some paid acquisition to sustain growth; it adds users but does not, by itself, multiply the base. The workbook groups it under "Revenue Growth and Customer Retention" metrics alongside Net Promoter Score, churn rate and DAU/MAU — together they tell an investor whether growth is organic and sticky, or bought.

Illustrative figures, using the workbook's own 3-referrals / 33%-conversion example as the base case.

Kisan Setu, an agri-marketplace app backed by a Category I AIF, has 40,000 active users. Each user sends out an average of 3 referrals a month, and 33% convert into new sign-ups — the workbook's own figures.

Viral coefficient        = 3 × 0.33 = 0.99
New users this month     = 40,000 × 0.99 ≈ 39,600

A rival app in the manager's watchlist reports referrals of 5 per user at the same 33% conversion:

Viral coefficient = 5 × 0.33 = 1.65

At a coefficient above 1, that rival's existing base of users alone regenerates a growing base each cycle — every 40,000 users become roughly 66,000 the next cycle, then over 1,08,000 the one after, purely from the referral loop, before any fresh paid acquisition. The fund manager weighs this against Kisan Setu's Customer Acquisition Cost — the workbook's companion metric — because a low coefficient combined with a high CAC signals a growth model that depends on continued marketing spend rather than the product marketing itself.

Chapter 14, section 14.8.1 (Valuation Metrics), lists the viral coefficient among the metrics used for start-up valuation and performance assessment, immediately after Net Promoter Score. The workbook's own numeric example — 3 referrals, 33% conversion, "one more paid user" per three referrals — is the exact number the question is built from. Expect either a direct recall of the formula (referrals per user × conversion rate) or a plug-in question using different figures to the same shape.

• The formula multiplies two rates; it does not add them. Referrals per user is a count, conversion rate is a percentage — multiply, don't sum.
• A coefficient below 1 does not mean zero growth. The workbook's own example (0.99) still adds users; it only means the loop alone does not multiply the existing base.
• Viral coefficient measures referral-driven growth, not overall growth. A company can grow fast through paid acquisition with a low viral coefficient — the two are different engines.
• Do not confuse it with Net Promoter Score, which measures willingness to recommend, not the referral-and-conversion arithmetic that actually produces new users.