Your K-factor is the number of new users each existing user generates through referrals. The formula is:

K = (invites sent per user) × (conversion rate per invite)

If 100 users each send 5 invites and 20% convert, your K-factor is 5 × 0.20 = 1.0 — every user brings exactly one new user.

What the number means

K > 1 — true viral growth: the user base grows on its own, exponentially. Rare and usually temporary.
K = 1 — each user replaces themselves through referral. Self-sustaining.
K < 1 — referral amplifies growth but doesn't sustain it alone. This is normal and still valuable.

Most real products live between 0.1 and 0.5. That's not failure — a K-factor of 0.4 means referral effectively reduces your acquisition cost by ~40%.

A second way to calculate it

When you model a loop as a series of steps, K-factor is the product of the conversion rates across the loop:

K = rate₁ × rate₂ × … × rateₙ

Example — a referral loop: use product (85%) → send invite (40%) → invite opened (70%) → sign up (45%) gives K = 0.85 × 0.40 × 0.70 × 0.45 ≈ 0.107. See live modeled examples in the GrowthPilot gallery.

How to improve your K-factor

Increase invites sent — prompt at the moment of peak intent (right after the "aha moment"), not at signup.
Increase invite conversion — a personal invite from a friend converts far better than an ad. Make the landing experience match the promise.
Make the reward improve the product — Dropbox gave storage (which made the product better), not cash (which attracts bounty hunters).
Shorten the cycle time — a loop with K = 0.5 that completes weekly beats K = 0.8 that completes quarterly.

Cycle time matters as much as K

Two products with the same K-factor grow at very different speeds if one loop completes in 3 days and the other in 30. Always pair K-factor with cycle time when you forecast growth.

FAQ

What is a good K-factor? Anything above 0 adds value. K > 1 means self-sustaining viral growth, but most successful products operate at 0.1–0.5 and rely on loops to lower acquisition cost.

How do you calculate K-factor? Multiply the average number of invites each user sends by the conversion rate of those invites: K = invites × conversion rate.

Can K-factor be greater than 1? Yes, briefly — that produces exponential growth — but it rarely stays above 1 for long as the addressable network saturates.

Why is my K-factor below 1? That's normal. It still reduces your effective customer acquisition cost and extends lifetime value alongside your other channels.

Simulate your K-factor and forecast growth at 30/60/90 days with the GrowthPilot loop simulator.