Free app growth calculator

Viral Coefficient Calculator (K-Factor)

A viral coefficient calculator measures your app's K-factor: how many new users each existing user brings in through invites and referrals. Enter your sharing rate, invites per user, and invite conversion to see whether your referral loop is self-sustaining, how many users it adds cycle by cycle, and exactly what it would take to reach K = 1.

Everything runs in your browser. Nothing you enter is sent to a server or saved.

Enter your referral loop

Use numbers from the same cohort or period. Results update as you type.

Launch cohort, beta list, or the new users you acquire this period.

Share of users who send at least one invite or referral link.

Average invites, shares, or referral links sent by each sharing user.

Share of invites that become a new signed-up user.

Days from a user joining to their invitee joining.

How many viral cycles to project, up to 52.

Projections are planning estimates based on a simplified viral loop model, not a forecast. Measure your real K-factor from cohort data once you have it.

Your viral growth

Your viral coefficient is K = 0.40, so referrals amplify growth but do not sustain it on their own.

Sub-viral (K below 1)

Viral coefficient (K)
0.40
2 invites per user x 20% conversion
Total users after 6 cycles
1,664
About 84 days from 1,000 starting users
Users from referrals
664
4 new users in the final cycle
Long-run amplification
1.67x
Levels off near 1,667 total users
Conversion needed for K = 1
50%
At your current sharing rate and invites
Invites per sharer for K = 1
10
At your current sharing and conversion rates
  • At K = 0.40, referrals amplify your acquisition by 1.67x: every 100 users you acquire directly turn into about 167 over time. That still lowers effective acquisition cost, but growth stops when direct acquisition stops.
  • To reach K = 1 with everything else unchanged, raise invite conversion to 50%, or get each sharing user to send 10 invites.

Growth by viral cycle

CycleDayNew usersTotal users
Start01,0001,000
1144001,400
2281601,560
342641,624
456261,650
570101,660
68441,664

Formulas used

Invites per user
Users who send invites % x Invites per sharing user
Viral coefficient (K)
Invites per user x Invite conversion rate %
New users in cycle n
Starting users x K^n
Total users after N cycles
Starting users x (1 + K + K^2 + ... + K^N)
Long-run amplification (K < 1)
1 / (1 - K)
Conversion needed for K = 1
1 / Invites per user

Assumptions

  • Every new user behaves like the starting cohort: same sharing rate, invites, and conversion.
  • Each user invites once, in the cycle after they join, and invites never convert later.
  • No churn, duplicate invites, or audience saturation. Real K-factors usually decline as you grow.
  • Growth comes only from the viral loop. Paid, organic, and content acquisition are not included.

Treat the projection as a model of your loop, not a forecast. Recalculate with real cohort data as soon as you have it.

How to calculate your viral coefficient

  1. 1Enter how many users you are starting with, such as a launch cohort or this month's new signups.
  2. 2Enter the percentage of users who send at least one invite and how many invites each sharing user sends.
  3. 3Enter the invite conversion rate: the percentage of invites that turn into a new signed-up user.
  4. 4Enter your viral cycle time in days and how many cycles to project.
  5. 5Review your K-factor, total users, referred users, and the cycle-by-cycle growth table, then use the K = 1 levers to plan improvements.

Viral coefficient FAQ

What is a viral coefficient?

A viral coefficient, also called the K-factor, is the average number of new users each existing user brings in through invites or referrals. It equals invites sent per user multiplied by the share of invites that convert into new users.

How do you calculate the viral coefficient?

Multiply the share of users who send invites by the average invites each of them sends, then multiply by the invite conversion rate. For example, 50% of users sending 4 invites each with a 20% conversion rate gives K = 0.5 x 4 x 0.2 = 0.4.

What is a good viral coefficient for an app?

A K-factor of 1 or higher means each user replaces themselves with at least one new user, so growth compounds on its own. Most successful consumer apps run well below 1, often 0.15 to 0.5, and use referrals to amplify paid and organic acquisition rather than replace it.

What is viral cycle time and why does it matter?

Viral cycle time is how long it takes a new user to sign up, experience the product, and invite someone who then signs up. Two apps with the same K-factor grow at very different speeds if one loop takes 2 days and the other takes 30.

What does a K-factor below 1 still do for growth?

A K-factor below 1 multiplies every user you acquire by 1 / (1 - K) over time. At K = 0.4, every 100 users you acquire directly become about 167, which lowers your effective customer acquisition cost by 40%.

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