Free calculator
K-factor calculator
Find out whether your product grows by itself. Enter how many invites each user sends and how many of them convert.
K-factor
0.45
Low virality: referrals are a bonus, not an engine. Focus on other loops.
Users after 6 cycles
1,811
Starting from 1,000 users
| Cycle | New users | Total users |
|---|---|---|
| 0 | 1,000 | 1,000 |
| 1 | 450 | 1,450 |
| 2 | 202 | 1,653 |
| 3 | 91 | 1,744 |
| 4 | 41 | 1,785 |
| 5 | 18 | 1,803 |
| 6 | 8 | 1,811 |
What is the k-factor?
The k-factor (or viral coefficient) measures how many new users each existing user brings in. The formula is simple: k = invites sent per user, multiplied by the conversion rate of each invite. If every user sends 5 invites and 20 percent of them sign up, your k-factor is 1.0.
The magic threshold is 1. Above it, every cohort of users recruits a bigger cohort, and growth compounds without spending a dollar on acquisition. That is how Dropbox, PayPal and Calendly built their early curves. Below 1, the viral loop still matters: a k-factor of 0.5 means every paid signup effectively brings half of another user for free, cutting your blended acquisition cost by a third.
In practice, very few B2B products sustain k above 1 for long. Treat the k-factor as an amplifier on top of your other channels rather than a standalone engine. The two levers are exactly the two inputs of this calculator: increase the number of invites each user sends (make sharing a natural part of the product, not an afterthought), and increase the conversion rate of each invite (a compelling landing page, a clear reason to accept, an incentive on both sides).
Also watch cycle time, which this simple model hides: a k of 0.9 with a two-day cycle can beat a k of 1.1 with a two-month cycle over a quarter. Measure invites and conversions per cohort, not all-time, so you can see whether product changes actually move the loop.
Related reading: k-factor in the glossary and viral coefficient.