Why I Built Splync – Easy Budget Tracker for Couples

Hello! I'm Kohei Koyanagi, a full-stack engineer based in Shonan, Japan’s laid-back surf area. I built Splync and launched it on the App Store as my first app. This article is about why I decided to do so. Honestly, I had never studied computer science before. I majored in mathematical science at Kyoto University and later earned an MBA from the University of the People, a non-profit, tuition-free, peer-learning, American-accredited online university. I only began learning programming quite recently, and I never imagined that I would one day release an iOS app on my own.

In the summer of 2023, I joined a Piscine—the month-long entrance exam of 42, a non-profit, tuition-free programming school originally founded in Paris and based on peer learning. Piscine means “swimming pool” in French. The four-week code camp was exactly that: a place where dozens of beginners were thrown into deep water with no teachers to guide us. We spent day and night solving increasingly difficult problems. Some quit. Others struggled to continue. A few managed to succeed.

I was fortunate enough to be among those who passed and enrolled at 42. It wasn’t because I was more intelligent than others—quite the opposite. Most of the time, many candidates around me were far smarter. I just succeeded by learning from them, struggling together, and encouraging each other. If you ask me whether you can become a programmer by joining Piscine, my answer is “I don’t know.” It depends on you—what you give to others and what you take from others. Overall, I deeply appreciate the great opportunity. It taught me that what seems impossible can become possible.

Months later, I left 42 because I needed cash. I started working as a full-time employee at a consulting company. My new job role had nothing to do with coding. However, I continued programming before and after work. With C, the only programming language I had learned, I built small text-based games like Tic-Tac-Toe, Reversi, Wordle, and Scrabble. It was fun to turn game logic into something visible and playful. Slowly, a thought grew in me: Why stop at games? Why not build something useful? Why not create something more tangible, like an iOS app? But what should I make?

I couldn’t find a good app to share expenses with my partner. We had been together for six years before getting engaged in June 2025. From the very beginning of our relationship, I realized our financial habits were very different. I felt stressed when money was spent in ways I didn’t value. It was even painful to see those differences cause conflict in our life. One day, I bought a paper notebook with the illustaration of a sweet panda illustration and suggested we use it for bookkeeping. Soon, the pages were filled with pasted receipts. At the end of each month, we manually calculated our expenses to see who owed whom, and how much.

In the second year, we switched to a digital approach. A shared Excel spreadsheet on Google Drive felt like the natural solution. We could access it from our laptops or smartphones. Creating our own format was fun, and choosing colors for sections and cells even increased our motivation. The spreadsheet automatically calculated the result of expense sharing with simple functions. It was not so bad, but entering expenses from a smartphone was a bit of a tedious task. We preferred to do it on a laptop, which meant we had to keep our receipts in our wallets until we got back home.

We tried many mobile apps in search of an easier solution. Some could split expenses well, but they didn’t include budget settings. For us, it wasn’t only about splitting fairly. It was also about controlling our budgets to reach long-term goals. Most of the apps had limitations in their free plans, while the paid versions seemed expensive. Weren’t there any apps that were easier, more affordable, better for budget control, more customizable, and visually appealing? Every app had both good and bad points. And as we kept trying them, the years passed.

I decided to create a new expense-sharing app that would work best for us. I searched with Google and ChatGPT and learned that iOS apps are written in a language called Swift. I installed Xcode, made an account on AWS, rented a server, and registered a domain. As I had done at the Piscine, I cleared new challenges one after another—new terms, new logic, new tools. I started writing Python, MariaDB, JSON, and more. Almost everything was new to me, but little by little the app began to come to life. I was deeply passionate about building this app. What I wanted might also be what you want.

On July 1, 2025, after Apple’s review, the app was released on the App Store. The app’s name Splync is coined from "Split + Link + Sync" and is pronounced /splɪŋk/. It is designed to help share living expenses by linking and syncing data in real-time. It's my approach to managing shared expenses in an easy and simple way. I am both the developer and a daily user of Splync. As a user-focused app, Splync keeps growing to meet your needs. It is also great for friends and individuals who want to manage their finances. I hope Splync helps make your life better.