Viral Loop Calculator
“Make it go viral” is the least examined instruction in app marketing, because almost nobody puts a number on it. Two numbers you can measure decide the whole thing — and they usually say something you did not expect.
Your loop
These come from your own analytics. This site cannot see them — invites sent and invites converted exist inside your app and your attribution, nowhere public.
k-factor
0.24
new users each user brings
Multiplier over 90 days
1.29x
with 20% decay a cycle, this is most of what it gets
Users from the loop
294
on top of the 1,000 you brought in
Blended cost per user
$1.93
from $2.50 paid
Every 100 users you acquire become about 129
Real but not self-sustaining. The loop pays for itself as a discount on acquisition cost rather than as a growth engine, and it dies the moment you stop feeding it.
If your k is off by 10%
A k of 0.22 settles at 1.28x; a k of 0.26 settles at 1.36x. Comfortably below the range where a small measurement error swings the answer, so the multiplier above is worth acting on.
Cycle by cycle
One row per cycle of 7 days. The total is a sum of a shrinking series, not a trend line — most loops have delivered most of what they will ever deliver within the first few rows.
| Cycle | Day | Effective k | New users | Cumulative |
|---|---|---|---|---|
| seed | 0 | — | 1,000 | 1,000 |
| 1 | 7 | 0.240 | 240 | 1,240 |
| 2 | 14 | 0.192 | 46 | 1,286 |
| 3 | 21 | 0.154 | 7 | 1,293 |
| 4 | 28 | 0.123 | 1 | 1,294 |
7 later cycles hidden — each adds under one user. They are still in the total above.
At this k the loop never doubles the 1,000 you brought in — no amount of time gets there.
What would it take?
There is no published benchmark for a typical k in your category, and inventing one would be worse than useless. So this runs the question backwards instead — pick the multiplier you want and it says exactly what the loop has to do. That answer is exact and needs no benchmark.
k you would need
0.50
you are at 0.24
Invites per user
6.3
at your 8% conversion
…or conversion
16.7%
at your 3 invites per user
Note which side of that is cheaper to move. Invites sent is a product problem — how many users reach a moment worth sharing. Conversion is a product page problem, because that is the page every invite lands on, and it is the half that also improves every other channel you run.
What Apple lets an invite mechanic do
Quoted from the App Review Guidelines. These are the clauses an invite or referral feature runs into; they are not the whole document, and rejection is decided by review, not by this page.
3.2.2(x)
Rewarding a share is allowed. Gating the app behind one is not, and neither is any reward tied to leaving a review.
Apps must not force users to rate the app, review the app, download other apps, or other store-related actions in order to access functionality, content, or use of the app. Apps may otherwise incentivize users to take specific actions within apps (e.g. completing a level, watching an ad).
5.1.2(v)
This is the rule that kills the classic contact-list invite blast, and it is the one most invite flows are built to break.
Do not contact people using information collected via a user’s Contacts or Photos, except at the explicit initiative of that user on an individualized basis; do not include a Select All option or default the selection of all contacts. You must provide the user with a clear description of how the message will appear to the recipient before sending it (e.g. What will the message say? Who will appear to be the sender?).
5.1.2(iv)
Keeping the contacts you were given, to invite later or to match users against, is a separate violation from sending the first message.
Do not use information from Contacts, Photos, or other APIs that access user data to build a contact database for your own use or for sale/distribution to third parties, and don’t collect information about which other apps are installed on a user’s device for the purposes of analytics or advertising/marketing.
5.3.1
A prize draw for sharing is a sweepstake, and it cannot be run on a partner’s behalf.
Sweepstakes and contests must be sponsored by the developer of the app.
5.3.2
The rules have to be inside the app, and they have to say Apple is not involved.
Official rules for sweepstakes, contests, and raffles must be presented in the app and make clear that Apple is not a sponsor or involved in the activity in any manner.
These are your numbers, not our estimates. Nothing on this page is fetched or inferred — invites sent and invites converted are visible only inside your app and its attribution. If you have not instrumented both, the honest output of this page is that you do not have a k yet.
Everything cited here is Apple’s. The multiplier maths is standard branching-process arithmetic, checked against a simulation before it shipped. Apple publishes no ranking factor for sharing, so nothing here claims a loop moves your position in search — what it moves is how many users each acquired user brings.
The model assumes every cycle behaves like the last, apart from the decay you set. It cannot see a finite audience, a seasonal spike, or a loop that only works for one kind of user — all of which are real, and all of which pull the true number below this one.
Want the shared link to be measurable? Campaign link builder tags it so App Analytics can attribute what the loop brought. Working on the page the invite lands on? Screenshot teardown and the A/B test calculator work on the conversion half.