A free K-factor calculator for SaaS. Enter your user base, invites per user, and conversion rate to calculate your viral coefficient, project growth over time, and benchmark against real SaaS averages.
| Cycle | New from loop | Total | Growth |
|---|
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Viral Loop Kit — the toolkit teams use to design, audit, and ship working viral loops. Coming soon.
Join the waitlist →The K-factor — also called the viral coefficient — measures how many new users each existing user brings in. It is the single most important number for understanding whether your product grows on its own or needs constant paid acquisition.
k = invites per user × conversion rate
If each user invites 4 people and 25% of them sign up, your k-factor is 4 × 0.25 = 1.0. A k of 1.0 means every user brings exactly one more — the threshold of self-sustaining viral growth. Most SaaS products sit between 0.1 and 0.5, and even there the effect is powerful: at k = 0.4, every 100 paid users generate about 67 extra users for free.
The calculator needs four inputs. Two minutes with your analytics dashboard is enough:
Your k-factor lands in one of five bands. Here is what each means for your growth:
| K-factor | What it means | Grade |
|---|---|---|
| k < 0.15 | Negligible virality — growth depends almost entirely on paid and organic channels | F |
| 0.15 – 0.4 | Meaningful assist — the loop noticeably lowers your blended acquisition cost | C |
| 0.4 – 0.7 | Strong loop — a large share of new users arrives for free | B |
| 0.7 – 1.0 | Excellent — close to self-sustaining growth | A |
| k ≥ 1.0 | Viral — exponential growth with no extra spend | A+ |
Cycle time matters as much as the coefficient. A product with k = 0.6 and a 3-day cycle outgrows one with k = 0.9 and a 30-day cycle. Speed compounds.
Say your product has 1,000 users. Each invites 3 people, 15% convert, and the cycle takes 10 days. Your k-factor is 3 × 0.15 = 0.45 — a strong loop (grade B).
That means every 100 users you acquire will generate about 82 more for free as the loop settles (1 / (1 − 0.45) ≈ 1.82×). To push toward k = 1.0, you would lift invites per user, improve the invite conversion rate, or both — the calculator lets you test each lever instantly.
Want the full method behind these numbers? Read how to calculate the viral coefficient, the 7 viral loop metrics that matter, and what a viral loop is.