---
title: K-Factor — definition | RGM® Glossary
url: https://realgrowthmatters.com/glossary/k-factor/
updated: 2026-06-10
source_html: https://realgrowthmatters.com/glossary/k-factor/
---

# K-Factor

k-fac·tor/keɪ ˈfæktəɹ/noun

Borrowed from epidemiology with the same threshold — above 1 it spreads, below 1 it fizzles.

Term
:   K-Factor

Synonym
:   Viral coefficient

Formula
:   K = invites per user × invite conversion

Threshold
:   K > 1 = self-sustaining

## Forms & parts of speech

K · noun (the bare letter)

The viral coefficient itself.

"The loop's **K** is 0.4 — every hundred users recruit forty. Useful, not viral."

## Definition in plain terms

K-factor measures how many new users each existing user generates. K = i × c, where i is the average number of invitations (or shares, exposures) a user sends and c is the rate at which those convert into users. K = 2 means each user brings two more — exponential growth. K = 0.4 means each hundred users recruit forty, who recruit sixteen — a boost on top of other channels, decaying geometrically rather than compounding.

## The mechanics

The threshold is 1, exactly as in the epidemiology it borrows from: above it, spread self-sustains; below, it dies out without external acquisition. Cycle time matters as much as K — a K of 1.2 with a two-day invite cycle beats a K of 1.5 that takes two months. Measure K per loop (referral program, content sharing, collaboration invites each have their own), instrument both i and c separately (they're different levers), and expect decay: early-adopter enthusiasm overstates steady-state K every time.

## When it matters

K matters for products with a genuine sharing mechanic — collaboration tools (the invite IS usage), referral-incentive programs, content platforms. Sustained K above 1 is vanishingly rare (the famous cases — early Hotmail, Dropbox referrals, viral-era Facebook games — are famous because they're rare); the practical use is engineering K upward as a CAC subsidy: a K of 0.5 halves your effective acquisition cost, since every paid user brings half a free one.

**Worked example.** A file-sharing product's growth team measures the loop honestly: users send 1.8 invites on average (i), 22% convert (c) — K = 0.4, well below self-sustaining. Instead of chasing 'virality,' they treat K as a CAC subsidy and work both levers: the invite moment moves to the post-value moment (file delivered), lifting i to 2.6; the landing page gets the inviter's file as context, lifting c to 31%. K reaches 0.8 — still not 'viral,' but effective CAC nearly halves, which funds the paid growth that actually scales.

**Failure modes to watch.** Chasing K>1 in a product with no natural sharing moment; measuring K once (it decays); ignoring cycle time; and spamming invites to lift i while c collapses along with sender reputation.

## Synonyms & antonyms

### Synonyms

K-factorviral coefficientK

### Antonyms

paid-only acquisitionK below 1 (sub-viral)

## Origin & history

Borrowed from epidemiology, where the k/R factor describes how many people each infected person infects — the >1 spread threshold transfers intact. Adopted into tech vocabulary by 2000s social-gaming and viral-marketing practitioners (Facebook-platform era), with the growth literature (Chen, Skok, Ellis) standardizing the i × c formulation.

Etymology: [source](https://en.wikipedia.org/wiki/Viral_coefficient).

## Usage trends

Search interest for this term over the last five years:

[View interest-over-time on Google Trends →](https://trends.google.com/trends/explore?q=viral%20coefficient&date=today%205-y)

## Common questions

What is K-factor?
:   The viral coefficient — invites per user times invite conversion rate. K above 1 means self-sustaining growth.

Where does the term come from?
:   Epidemiology and medicine — the infection-spread factor — adopted by social-gaming and startup growth teams in the 2000s.

Is K above 1 realistic?
:   Rarely, and rarely sustained. The practical goal is raising K as a CAC subsidy — K of 0.5 halves effective acquisition cost.

## Related tools & calculators

- tool[Funnel drop-off analyzer](/tools/funnel-drop-off-analyzer/)

## Resources & people to follow

- book*Hacking Growth* — Ellis & Brown (viral loops chapter)
- referenceAndrew Chen — viral-loop essays
- referenceDavid Skok — the viral-growth math (forEntrepreneurs)

Curated, non-competitor resources verified per term.

## Related training

- module[Marketing analytics](/training/marketing-analytics/)

## Disciplines

Areas of marketing where k-factor is a core concern:

[Measurement](/training/marketing-analytics/)[Growth strategy](/training/growth-marketing-foundations/)

## Read next

## Related terms

[Growth marketing](/glossary/growth-marketing/)[Referral program](/glossary/referral-program/)[CAC](/glossary/cac/)[Sean Ellis](/glossary/sean-ellis/)[Pirate metrics (AARRR)](/glossary/pirate-metrics-aarrr/)

## Sources

1. trends[Google Trends — "viral coefficient"](https://trends.google.com/trends/explore?q=viral%20coefficient&date=today%205-y)