Industry
FinTech
Case Study
ETF Investment Calculator
Long-term planning is difficult to understand from one final number. I built a local-first mobile toolkit that makes assumptions visible, models how money changes month by month, and explains the result in Serbian, German, and English.
Project Overview
The project started as an ETF calculator and grew into a broader personal-finance planning toolkit. Its current routes cover ETF investing, FIRE, pensions, inflation, fees, loans, lump-sum comparisons, real estate, and educational material.
The ETF module remains the deepest part of the product, with projection, goal, and withdrawal modes backed by an isolated calculation engine.
A multilingual mobile financial-planning toolkit for investment, goal, withdrawal, FIRE, pension, inflation, fee, loan, lump-sum, and real-estate scenarios.
Services
- UX/UI Design
- Web Development
- Product Strategy
- Mobile Development
Key Deliverables
- Responsive Design
- Custom Development
- User Experience Improvements
Platform Type
Mobile Application
Tech Stack
- TypeScript
- React Native
- Expo
- Expo Router
The Challenge
Most calculators return a large future value without exposing how contribution timing, fees, dividends, inflation, taxes, or withdrawals produced it. That makes scenarios hard to compare and easy to misread as predictions.
The interface also has to support advanced assumptions without turning every calculation into a wall of inputs.
Approach
I kept pure financial models separate from React Native screens. The engine advances month by month, while the interface progressively reveals assumptions and explains outputs in Serbian, German, and English.
The app stores preferences locally with SecureStore and does not require an account, analytics service, or backend to calculate a plan.
The Solution
ETF projections model recurring and one-time contributions, contribution timing, annual contribution increases, return, TER drag, reinvested dividends, inflation, and real purchasing value. The German tax model includes allowances, Teilfreistellung, church-tax effects, and Vorabpauschale logic.
Inverse solvers calculate the monthly contribution, return, or time required to reach a target. Withdrawal tools model depletion, sustainable withdrawals, and perpetual monthly income. Presets also cover indexes, gold, oil, bonds, real estate, technology, healthcare, and crypto.
Key Challenges
-
implementing month-by-month accumulation and decumulation without rounding drift
-
making German tax mechanics explicit and testable
-
solving inverse goals while handling impossible or unstable scenarios
-
presenting advanced assumptions without overwhelming a mobile screen
The Outcome
The result is no longer only a final-value calculator. It is a set of explainable planning tools that lets users compare how assumptions affect accumulation, goals, retirement income, and other long-term decisions.
Context
The product still carries its original name, but the architecture now supports a wider personal-finance toolkit. The case study reflects that current scope without pretending the projections are market forecasts.
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