Viral coefficient (K-factor): when a product grows by itself

Some products grow "by themselves" — users bring users. The viral coefficient (K-factor) measures this. Let's break down how to calculate it and what it shows.

What the K-factor is

K-factor is how many new users one existing user brings on average. The formula is simple:

K = (invites per user) × (conversion of an invite into a new user).

Example: each invites 5 people, 20% of whom register → K = 5 × 0.2 = 1.

What the numbers mean

• K > 1 — each user brings more than one new one → the product grows virally, exponentially, with no ads.
• K = 1 — self-sustaining (on the edge).
• K < 1 — there's virality, but it fades: it makes acquisition cheaper but doesn't fully replace it.

Most products have K < 1 — and that's normal. Even K = 0.5 seriously lowers acquisition cost.

How to raise the K-factor

1. More reasons to share — a referral mechanic, rewards for inviting (see the piece on the referral program).
2. Simplify sharing — an "invite" button, ready text, a referral link in one tap.
3. Raise the invite conversion — so the invited person easily grasps the value and stays.
4. Viral triggers in the product — something worth sharing in itself.

An important nuance — the cycle

What matters isn't only K but also speed: how fast a user brings new ones. A fast cycle at K just under 1 can be more valuable than a slow one at K just over.

Takeaway

K-factor shows how much a product grows by itself: K>1 is a viral explosion, K<1 is cheaper acquisition. It's raised by referral mechanics and ease of sharing. We help build acquisition, including viral and word-of-mouth mechanics.