How I Built It: $400K/Month Mobile App

Introduction

Last month, we made over $440K.

Meet Julian, a developer from Argentina who built an app that makes over $400,000 a month. But it didn't start that way. Before the success, Julian was a dude at the gym who discovered a different app that he thought was pretty cool. However, instead of just using it himself, he thought, "What if I built something better?" And that's exactly what he did. He built an MVP, posted about it on Reddit, and the rest is history.

About Julian & Gravel

Julian's Introduction:

Co-founder and developer of Gravel, an AI fitness app that provides smart workouts for the gym
Launched around 2 years ago
Over 70,000 subscribers
Last month: made over $440K

Background Story

Julian's Journey

Early Life:

Grew up in Argentina
Father owned a fitness center
Spent time outside of school playing sports and at the gym

Career Path:

Got into software engineering
Moved to Australia
Worked at:
- Small startups
- Big tech companies like Atlassian
- TV channels
- Investment fund

Previous Business:

Started with best friend as partner
Built an influencer marketing platform for mobile games
Eventually became a marketing agency
Learned a lot about user acquisition and marketing
Saw the revenue numbers and realized apps (especially fitness apps) make a lot of money

Building the App

The Turning Point

Initial Development:

Started with a workout tracker app (like Heavy, Strong)
Realized he wasn't adding any value - just copying existing apps

Discovery of Fitbot:

A friend showed him Fitbot (one of the biggest workout apps)
Initial reaction: "This is amazing" - it provided workouts on the spot
After using it: noticed the workouts were weird and sometimes dangerous
Realization: "We need to build FitBot UI/UX with an actual proper workout engine"
That's when they went all in

Development Timeline

MVP Development: 2-3 months

Two phases:

Pre-Fitbot phase: More of a tracker
Post-Fitbot phase: The tricky part

Challenges:

A lot of business logic around building custom workouts
Many different settings and combinations to consider:
- Equipment
- Weekly goal
- Gym frequency
- Consistency
- Gender, weight, age
- Many other factors

Launch Strategy

Reddit Strategy

The Launch:

Posted a thread on how he built Gravel (called "Games AI" back then)
Shared technical specs
Results within first couple hours:
- Over a couple hundred likes
- Over 300,000 impressions
- First couple thousand users

Target Audience Discovery:

Found that developers liked it because:
- They like tech
- Some like going to the gym but find it intimidating
- They were actually using the app

Validation:

Started getting feedback about bugs and feature requests
Realized: "We're onto something"
At this point, the app was free
Decision: "Now we need to build a serious business and start running ads"

Growth & Distribution

Paid Ads Strategy

Initial Success:

Started running ads as soon as they added subscription model
Got a subscription within the first 10 minutes of activating ads
"That was the biggest high probably I've ever had"

Expansion:

Translated app to Spanish
Started running ads in South America
Spent less than $50/day on ads initially
This strategy worked from the start until today

Top Tips for Building Apps in 2025

Tip #1: Validate Before Spending

"Validate before spending money on ads."

Not just that the product works
But also that people are willing to pay for it

Tip #2: Start with Cheaper Markets

They got lucky being Spanish speakers
Translated to Spanish and ran ads in South America
US market is expensive with lots of competition
Start where ads are cheaper

Tip #3: Start Small

Pay influencers or content creators as low as $50 for content
Use AI tools to create videos
Use CapCut to edit your own videos
Creating an ad isn't as hard as you think
The hard thing is creating the volume

Tip #4: UGC is King

Most ads that work are UGC content
AI videos are becoming popular
Cost of producing them is cheap
Easy to test

Tip #5: Copy, Copy, Copy

Meta Ads Library:

It's all public
You can see your competition's dashboard
See which ads are working for them
See where they're putting more money
No secret sauce - just copy what works
Start there

Product Demo Highlights

Gravel App Features

Onboarding Process:

Landing screen gets your name
Series of questions:
- Why you want to use a fitness app
- Your experience level (beginner/advanced)
- One rep max for certain exercises (for advanced users)
- Goals
- Training frequency
- Working split
- Excluded muscles and focus muscles
- Where you train (adapts to your gym)

Key Strategy:

Show users it's an AI app throughout onboarding
"That's sales"
Hard paywall before sign-in for the first time
"How many users will pay before seeing their product? The answer is a lot."

Main Features:

Workout card (main thing - 90% of user flow)
List of exercises
Works like a workout tracker
Proper content for every video
Description and how-to for each exercise

Smart AI Features:

If you sort exercises, weights are adjusted based on order
Recovery rate: tells you how tired your muscles are
Includes external workouts from Apple Health, Strava, runs, etc.
Learns and adapts workouts accordingly

Support:

24/7 support chat inside the app
Articles and messages
Real person replies (not AI)
Users value this a lot

Tech Stack

Frontend:

React Native and Expo

Backend:

.NET for most core functionality
Next and React for internal admin dashboards

AI:

Uses Cursor a lot
Very specific about which files to touch
Doesn't let AI "go rogue"

Business Financials

Expenses Breakdown

#1 Expense: Paid Ads

Meta and TikTok (main)
Some Google
Some Apple Search
About 1/3 of revenue
(This doesn't include making the ads, just running them)

#2 Expense: Salaries

First year: just the three of them
- Julian: development
- Matias and Aaron: growth and ads
Current team: 13-14 people (including part-times and contractors)
Estimate: $50K-$80K

Other Expenses:

Apple's 15% revenue cut
1% clip from revenue
MMPs: ~$1K/month
Infrastructure (servers, AI bills, tools): ~$1K/month

Advice to Founders

Be Proud of What You Build

"The motivation that comes from building something I'm excited about can't be matched by anything else."

It's always easier to grind and work for zero dollars when you love what you're doing

Keep Pushing, Don't Give Up Too Soon

It's a long road
Building it is not even half of it
Be ready to do all the hustling (Reddit, ads that don't work, etc.)
You're going to get some punches
You need to keep going

Know When to Give Up

"Knowing when to give up is also very important."

Their previous startup: they dragged it for an extra year
Should have called it days way earlier

Conclusion

Julian's app is crushing it at over $400,000 a month, and it's something he's super passionate about. Julian clearly loves fitness and working out, and he gets to build an app that makes hundreds of thousands of people's lives better while doing something he loves.

Key Takeaway: Build something you're passionate about, validate early, start small with distribution, and don't be afraid to copy what works.