Viral Coefficient (K-Factor) Calculator
The viral coefficient of a product or referral scheme from invitations sent and the share that convert, with the users it brings in over a number of cycles.
Results
The dashed line marks the value you entered.
What this tool does
K is the number of new users each existing user brings in: invitations sent times the share that accept. If K is above 1, every cohort is larger than the last and growth feeds itself; below 1, each cohort is smaller, the effect fades and the total settles at N₀ ÷ (1 − K) — a viral boost that multiplies what you pay to acquire, not a substitute for it. Most products have a K well under 1, and what matters as much is how long a cycle takes, which this model leaves out.
Formula
K = invitations per user × conversion rate · users after n cycles = N₀ (1 − Kⁿ⁺¹) ÷ (1 − K)
Variables
| Symbol | Meaning | Unit |
|---|---|---|
n0 | Users you start with | — |
iv | Invitations each user sends | — |
cv | Invitations that turn into users | % |
cy | Viral cycles to simulate | — |
K | Viral coefficient (K) | — |
ST | Growth by word of mouth | — |
T | Users after those cycles | — |
L | Users added in the last cycle | — |
MX | Most users it can ever reach (when K < 1) | — |
Worked example
- Users you start with1000
- Invitations each user sends5
- Invitations that turn into users15 %
- Viral cycles to simulate6
- Viral coefficient (K)0.75
- Growth by word of mouthFades out unless you keep adding users
- Users after those cycles3466
- Users added in the last cycle178
- Most users it can ever reach (when K < 1)4000
Limitations
- The result is an estimate based only on the values you type. Real situations often include factors this calculator does not know about.
- The formula assumes ideal conditions: no friction losses, no air resistance and no efficiency losses unless you enter them.