Zaden Park

M.S. Engineering · Exercise Science PhD (ABD).
Personal trainer by day, indie hacker by night 🏋️💻
6 products built, 1 with paying users. From Korea 🇰🇷

[Zaden Park @zadenpark]

How a personal trainer ended up indie hacking:

It started with one client, mid-session.

He's a developer at Krafton, the company behind PUBG. I mentioned I'd dabbled in coding back in grad school but never kept it up.

He said: "You should try vibe coding. It's gotten insanely good."

I was skeptical. A few years ago I tried AI-written code, and it was a mess. Errors everywhere, nothing actually ran.

But I gave it a shot.

He was right. It was a completely different world.

So I started building between PT sessions and in every spare minute.

Fast forward: 6 apps shipped.

Next up: the first app I built, and why it failed. Follow along.

[Zaden Park @zadenpark]

Thank you, that means a lot! Just checked out https://t.co/3ncAJzh65p and it feels like everything a small team or founder needs, packed into one place. Super interesting!

And you're a professor! In Korea, professors are basically gods to grad students lol. As a former grad student myself, huge respect.

[Zaden Park @zadenpark]

I'm a personal trainer by day, indie hacker by night.

Anyone else building on the side while working a completely different job?

Tell me your day job and what you're building 👇 I'll check out every single one. Let's connect!

[Zaden Park @zadenpark]

@addy_devx Let's connect! Just checked out https://t.co/NboCRqJtak and matching WASSCE scores to programs, scholarships, and mentors all in one place is such a socially valuable and rewarding problem to solve.

[Zaden Park @zadenpark]

@pablonasarre__ ¡Hola! Let's connect! Just checked out https://t.co/e3xSiAnpIZ and the "see your site first, pay only if you like it" model is so cool. You can really feel the confidence in your work. Respect!

[Zaden Park @zadenpark]

@Atulo1o Great to meet you! Just checked out https://t.co/95w88NUHxv and as a working personal trainer, I found it really interesting. Have you ever worked as a trainer yourself, too?

[Zaden Park @zadenpark]

@MorozVladyslav Let's connect! Just checked out your website, https://t.co/TxCnztzjfs, and wow, for a backend dev, your UI design is super cool. Did you design it yourself?

[Zaden Park @zadenpark]

Thank you! I'm not an engineer anymore, though.

I got my M.S. in Materials Science and Engineering, then went on to an Exercise Science PhD (ABD), and now I work as a personal trainer.

But training has a built-in limit: my income only grows with my hours. So I wanted to build something that makes money even while I'm coaching.

[Zaden Park @zadenpark]

@alokdataeng Hi Alok! Oh really? I've heard great things about Cursor. I'll give it a try, thanks! And a web-based game sounds fun. What kind of game is it?

[Zaden Park @zadenpark]

One of the hardest parts of app dev for me: responsive design.

Making it work on every phone, foldable, and tablet is brutal. I'll get a screen looking great on one size, and then it completely breaks on a different aspect ratio.

How do you all handle this? Any tips or tools that actually help?

[Zaden Park @zadenpark]

@alokdataeng He's one of my personal training clients, so we were in the middle of a training session when he said it. Looking back, that one conversation changed a lot for me.

[Zaden Park @zadenpark]

How a personal trainer ended up indie hacking:

It started with one client, mid-session.

He's a developer at Krafton, the company behind PUBG. I mentioned I'd dabbled in coding back in grad school but never kept it up.

He said: "You should try vibe coding. It's gotten insanely good."

I was skeptical. A few years ago I tried AI-written code, and it was a mess. Errors everywhere, nothing actually ran.

But I gave it a shot.

He was right. It was a completely different world.

So I started building between PT sessions and in every spare minute.

Fast forward: 6 apps shipped.

Next up: the first app I built, and why it failed. Follow along.

[Zaden Park @zadenpark]

@alokdataeng Agreed. If I were building an astrology service, I'd go with GPT too, not Claude. How's your astrology service going these days?

[Zaden Park @zadenpark]

I've used RAG once, in an app I built (shelved now). Snap a photo of a storefront sign, and it estimates that type of business's average revenue and operating profit. Snap a regular product, and it estimates the margin.

It all started from curiosity: how much would I make running that shop? How much do they keep from selling one of these?

[Zaden Park @zadenpark]

@michaeldeayaw Totally agree, the constraint keeps me focused. Right now it's a web project launching this week, plus an app I'm aiming to ship by the end of the year.

Hours-wise, about 30hr a week, mostly late nights and the gaps between PT sessions. What about you? What are you building?

[Zaden Park @zadenpark]

Honestly, just one: "Review this critically."

I knew LLMs lean toward telling you what you want to hear, but I didn't realize how much. Critical thinking was the most valuable thing I learned in grad school, so I tried asking AI to critically review my own ideas.

It opened up a whole new world. Way more objective, way more accurate feedback. My product quality jumped from that point on.

[Zaden Park @zadenpark]

Here's a habit of mine. Whenever I walk past a shop, I wonder: what's their MRR? When I see a product: how much margin do they make on each one?

Getting the picture?

Yep. Just like Cal AI does calories, I built an app where you snap a photo of a storefront sign or a product. It estimates the cost like zerob does, or uses the industry's average operating margin, and estimates the shop's monthly revenue.

Working name: BizScan (Video below: its onboarding flow)

Under the hood: OCR reads the sign or product, then an LLM pulls from Korea's national small business database using RAG.

But I shelved this one too. Why? More next time.

[Zaden Park @zadenpark]

How a personal trainer ended up indie hacking · Ep. 5

Quick recap:
Cal AI: snap a photo of your meal, get the calories.
zerob: buys the ingredients of store-bought food and calculates its real cost.

So what idea did these give me?👇

[Zaden Park @zadenpark]

From my own experience (6 products built with AI, 1 paying): yes, up to a point.

Getting to an MVP without a real coder is very doable now, as long as you have solid product sense.

But once real users show up, it's worth bringing in an expert at least a few times for security, performance, and UI/UX. That's the part that keeps a product alive as it grows.

So you can start without one. Just don't plan to scale without one.

[Zaden Park @zadenpark]

@SuiBuilds Same. What helped me: run /insights to see where your tokens actually go, then keep the default effort low and only drop "ultrathink" on the turns that really need it. Lasts way longer that way.

[Zaden Park @zadenpark]

@Dwriteway Exactly. People don't buy what you want to sell. They buy the life they want after buying it.