One metric for
growth efficiency.
Speed alone lies. Cost alone lies. GPI captures both — how fast you grow and how efficiently you spend — in a single number.
The problem with single metrics
CPA alone is not enough
Low cost per subscriber sounds great — but what if you're only getting 2 subscribers a day? CPA ignores speed entirely. You could have the cheapest subs in the world and still barely grow.
Velocity alone is not enough
Fast subscriber growth feels great — but what if each subscriber costs $5? Velocity ignores cost. You could be growing fast while burning through your entire budget in days.
The formula
Expanded form
How GPI responds to change
| Scenario | Velocity | CPA | GPI | Happy? |
|---|---|---|---|---|
| More subs, same price | ↑ | = | ↑ | Yes |
| Same speed, cheaper subs | = | ↓ | ↑ | Yes |
| Faster AND cheaper | ↑ | ↓ | ↑↑ | Very much |
| Slower, but much cheaper | ↓ | ↓↓ | ↑ | Yes (efficient) |
| Faster, but much more expensive | ↑ | ↑↑ | ↓ | No (overspend) |
See it in action
Four users, same platform — GPI reveals who's actually winning.
1,000
1,000
2,500
50
A and B have the same GPI (1,000). B gets half as many subs — but at half the cost. GPI says they're equally efficient.
C is the winner (2,500). Fast growth AND low cost — the best of both worlds.
D is struggling (50). Slow growth AND expensive subs — needs optimization.
Period calculation
Daily GPI
GPIdaily = subs_today² / spend_today
A snapshot of today's growth efficiency. Useful for spotting day-to-day changes in campaign performance.
Rolling 7-day GPI
GPI7d = subs_7d² / (spend_7d × 7)
Smoothed over a week to reduce noise. The go-to number for tracking whether your growth is trending up or down.
Platform-level GPI
The sum of all active users' GPI scores. This number grows when new users join (more terms in the sum) and when existing users become more efficient (each term increases). A single metric for the health of the entire platform.