Every few weeks a thread blows up on X with a list of "app ideas that will make you rich." Most are noise. This one is worth a closer look — because it comes from someone with the receipts.
Jake Castillo (@jakecastilloooo) scaled Cal AI, a calorie-tracking app, from nothing to a reported $30M ARR in 18 months. In a recent thread he laid out the seven consumer app niches he would build in if he were starting over today — ranked. Below we summarize his list faithfully, then add our own take on how a solo builder or small team could realistically get started in each.
The filter: 4 things that make a niche worth building in
Before the list, Castillo shares the lens he uses to judge a niche. He looks for four signals:
- An expanding market — the total addressable market is growing, not shrinking.
- Cult-like audiences — people who are obsessed, not just mildly interested.
- Cheap creators — an existing army of content creators you can partner with for distribution without burning cash.
- Existing demand — people are already searching for and paying for solutions, so you do not have to educate the market from scratch.
"Pick a niche with all 4, nail your distribution, and you're well on your way to a profitable app." — Jake Castillo
That line about distribution is the part most builders underrate. A mediocre app with a creator pipeline beats a brilliant app nobody can find. Keep that in mind as you read the rankings.
#1 — Productivity
His take: The biggest and most durable niche of them all. Endless obsessed audiences, more creators than you can count, and a total addressable market that only grows as more of our work moves digital. You are, in his words, "immediately stepping into a market that's already begging for better tools."
Our take: Productivity is crowded — but that is a feature, not a bug. It proves people pay. The winning move is not "another Notion." It is solving one sharp, specific workflow better than anyone: a focus timer that genuinely blocks distractions, a meeting-notes tool built for a single profession, a keyboard-first task capture. Pick one job, do it ruthlessly well, and lean on creators inside that workflow for distribution.
#2 — Personal Finance Micro-Tools
His take: Consumers want control over their money without needing a finance degree. The audience is massive, the content is everywhere, and small, focused tools win. "Just solve one specific money headache and you can make bank."
Our take: The key word is micro. Do not try to build the next Mint. Build a subscription-cancellation tracker, a "can I actually afford this?" calculator, a bill-splitter, a debt-payoff visualizer. Single-purpose finance tools are easy to explain in one sentence — which is exactly what makes them shareable. (We are biased toward small, single-job tools; it is the same philosophy behind our own free utilities.)
#3 — Health & Fitness
His take: The cult-like following here is unmatched. Entire ecosystems of creators pump out content daily, and audiences care about their health more than ever. It is "one of the strongest existing niches on the internet" — and, notably, the exact space Cal AI itself plays in.
Our take: This is Castillo's home turf, so weight his confidence accordingly. For newcomers, the opening is in the underserved corners: a hydration tracker, a mobility app for desk workers, a streak tracker for one specific routine. Health audiences are loyal and pay monthly — but trust is everything, so accuracy and a calm, clean UX beat flashy features.
#4 — Photo, Video & Creative Utility
His take: Everyone is a creator now, and they all want to look better doing it. Huge audiences, a steady stream of new tools, and plenty of creators to partner with at fair prices. "Solve just one annoying creative problem and people pay happily."
Our take: "One annoying creative problem" is the entire strategy. Background remover, podcast-clip generator, thumbnail A/B tester, caption formatter. AI has made these cheaper to build than ever. The distribution is almost automatic: the creators you serve are the same people who will show your tool to their audience.
#5 — Education & Skill Coaching
His take: People are obsessed with self-improvement, and creators in every skill vertical already have engaged audiences. That means built-in demand and cheap distribution — you are "plugging into an existing movement instead of trying to start one from scratch."
Our take: The best opportunities here wrap around existing creators rather than competing with them. A spaced-repetition app for a specific certification exam, a practice tool for a language, an AI coach for a single skill like public speaking or chess openings. Partner with the creator who already teaches it; you provide the practice layer they cannot easily build themselves.
#6 — AI Agentic Apps
His take: Build apps with agents that automate the boring parts of day-to-day life. Demand is exploding, "the content writes itself," and there is massive upside for anyone who builds something that genuinely saves people time.
Our take: This is the highest-ceiling, highest-risk pick on the list. "Agent" is hype-bait, so the only bar that matters is: does it actually remove a chore? Inbox triage, appointment booking, form filling, research summarizing. Start narrow — one task done reliably end to end beats a do-everything agent that fails one time in five. If you are technical, this is where the frontier is, but remember that reliability, not novelty, is what gets people to pay and stay.
#7 — Female Health (FemTech)
His take: One of the fastest-growing markets out there, expanding rapidly with a loyal and vocal audience. Best of all, you "won't have to educate people on the problem, because they're already searching for solutions with their credit cards handy."
Our take: FemTech has been historically underbuilt relative to its market size, which is exactly why it earns a spot despite ranking seventh. Cycle tracking, fertility, menopause, pregnancy — each is a deep vertical in its own right. The "credit cards handy" point is the signal to watch: clear, urgent, recurring needs. Privacy and sensitivity are non-negotiable here; get them right and retention follows.
What to actually do with this list
The ranking is interesting, but the framework is the real takeaway. Notice that every niche scores on all four of Castillo's factors — expanding market, obsessed audience, cheap creators, existing demand. None require you to invent a category or convince people they have a problem.
If you are a builder, the practical sequence looks like this:
- Pick one niche from the list that you actually understand or use yourself.
- Narrow it to a single painful job — not the whole category, one specific headache.
- Map the creators already serving that audience, because they are your distribution channel.
- Ship the smallest useful version and let real usage tell you what to build next.
You do not need a novel idea. You need an obsessed audience, a clear problem, and a cheap way to get in front of them. As Castillo's own run with Cal AI shows, "boring but in-demand" beats "clever but unproven" almost every time.
Source: Jake Castillo's thread on X. The rankings and quoted lines are his; the commentary under each "Our take" is ours.