The Finance Substack conversion index
An imperfect yet intriguing proxy.
I wrote about the highest-converting finance Substacks a couple of weeks ago, flagging an impressive anomaly from author James Bulltard who has 361k fewer free subs than Doomberg (now 15k vs 376k) yet sits only one bestseller rank behind (#4 vs #3).
This new Conversion Index formalises that measure into one ranked number across the current 100 bestsellers, rewarding those with a low free-audience-to-paid-rank ratio. It’s really important to say that this is a mere proxy for conversion and by no means a purified measure of publication value: authors often import loyal audiences from other platforms and many use Substack as a payment rail for off-platform services, like the MarketStack Terminal, which is going paid very soon. So it’s rarely apples to apples. Nonetheless I thought it was an interesting ratio to rank and analyse.
Conversion Index = 1 / ( bestseller rank × free subscribers )
Higher = more paid standing per free reader.
Here’s the top 15. I couldn’t believe an author ranked above James Bulltard. J-Griff is ranked 54 in Bestsellers with only 1k free subs, looks like he has only just arrived on Substack too.
I was really pleased to discover that 11 of the top 15 converting authors have appeared in Author Signals since it started on 2 May 2026. Author Signals is a live board in the Terminal that surfaces authors showing statistically unusual activity on the Rising list. Each signal is scored across five cross-referenced dimensions, with an author’s MarketStack Index rank acting as a credibility weight — a moderate spike from a consistently tracked voice outscores a lone burst from an unknown.
Accidental proof that the MarketStack Terminal is working
What I didn't expect was for this to double as a stress-test of my own tooling.
That’s a genuinely strong overlap, nearly three-quarters of the most efficient converters have also tripped the unusual-activity signal at some point.
And now taking a meta-view of the conversion index…
The conversion frontier
This plots free subs on a log scale vs paid bestseller rank. The story reads at a glance: J-Griff and James Bulltard big and bright.
And here’s the next 16-30
I’m working on setting up the complete list in the MarketStack Terminal, alongside the Recommendation Index, refreshed daily of course. If you’d like to see today's complete list ASAP, feel free to reach me at hi@marketstack.pro. And by the way it’s 98 not 100 because 2 bestselling authors don’t display their free sub count.
None of this is a verdict but I think it's worth watching. The writers who convert thin air into loyalty are usually doing something the subscriber numbers alone won't show you, and I'd rather find them early than read about them later.
p.s. The whale I had never heard of
I accidentally discovered an author with more free subs than Compounding Quality, who ranked top of my original free sub rank (>500k). The Capitalist has 787k subs but didn’t appear because they aren’t in bestsellers. This is how they describe themselves…
They do not rank in the MarketStack Index 100.
See also: PoliticStack, the MarketStack Terminal and MarketStack The Edit
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