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K-factor, also called the viral coefficient, is the number growth teams use to quantify how much a product spreads on its own through invites and shares. It answers a specific question: for every existing user, how many new users does that person bring in. This calculator takes your existing user count, invites sent and new users gained from those invites, and returns the K-factor instantly along with the two numbers that make it up.
K-factor is invites per user multiplied by the conversion rate of those invites. First divide total invites sent by existing users to get invites per user. Then divide new users gained through invites by total invites sent to get the conversion rate. Multiply the two together for K-factor. With 5,000 users sending 9,000 invites that convert 630 new users, invites per user is 9,000 divided by 5,000, which is 1.8, and the conversion rate is 630 divided by 9,000, which is 7 percent. Multiply 1.8 by 0.07 and K-factor comes out to 0.126.
A K-factor of 1 or higher means each existing user is, on average, bringing in at least one more user through invites alone, the textbook condition for a self sustaining viral loop. It is a genuinely rare state to hold for long, since the pool of people worth inviting tends to get used up as a network matures, and most products that briefly cross 1 during a launch spike settle back down afterward. Do not treat sustained K-factor above 1 as a normal target.
Most products with any referral mechanism at all sit somewhere between 0.1 and 0.5, and that range is still valuable. A K-factor of 0.2 means one in five new users arrives essentially for free through referrals rather than paid channels, which lowers your blended cost per acquisition even without hitting viral growth on its own. Improving K-factor is usually a matter of getting more users to send invites in the first place, or improving how well those invites convert once sent, prompted at a moment when the value being shared is obvious to both sides.
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