Airgead is an interactive compound interest calculator designed to make the relationship between time, contributions, and investment growth visible and tangible. It accepts four inputs — starting balance, monthly deposit, annual interest rate, and investment period — and immediately shows how those variables interact across every year of the projection. The application runs entirely in the browser with no backend, no database, no npm packages, and no build step required.Documentation Index
Fetch the complete documentation index at: https://mintlify.com/apursley2012/airgead-investment-calculator/llms.txt
Use this file to discover all available pages before exploring further.
What is Airgead?
Airgead models two parallel investment scenarios side by side. The first scenario shows how an initial balance grows on its own through compound interest alone. The second scenario adds a recurring monthly deposit to that same starting balance, illustrating how consistent contributions change the outcome over time. Comparing these two paths is the core purpose of the application: the difference between them makes the effect of monthly deposits concrete rather than abstract. The calculation uses monthly compounding throughout. Each month, the monthly interest rate (annual rate ÷ 12) is applied to both balances, and the monthly deposit is added to the with-deposits scenario. At the end of each year, the application records the balance, total principal contributed, and interest earned for both scenarios. Currency values are rounded to whole dollars for display and export. The educational goal is to help users understand fiscal responsibility through interaction rather than definitions. Rather than presenting a single large final number, Airgead separates total principal from earned interest and shows how the balance evolves year by year, so the mechanism of compound growth remains visible throughout the entire projection.Application pages
Airgead is organized as a multi-page static site. Each page is a self-contained HTML file that shares a common header, navigation, and footer through a single shell component.Calculator
Collects the four investment inputs — starting balance, monthly deposit, annual rate, and years — and previews both projected balances as values change.
Results
Displays the full breakdown: final balance with deposits, total principal, interest earned, final balance without deposits, a growth chart, and a year-by-year data table.
Compare
Places two independently configured investment scenarios side by side, each with its own inputs, summary values, and a shared comparison chart.
Saved Plans
Lists calculations saved to the current browser’s localStorage, with options to reopen any plan on the Results page or delete it.
How It Works
Explains the monthly compounding model, the two-scenario approach, and the static demo behavior including the mock sign-in flow.
Articles
Central index for full-length articles covering how monthly deposits affect long-term growth and how the original C++ console app was rebuilt as a static web application.
Case Study
Documents the project problem, the architecture decisions made during the rebuild, and possible future enhancements.
Project origins
Airgead started as a C++ console application. The original program accepted an initial investment, monthly deposit, annual interest rate, and investment period, then printed yearly reports for both growth scenarios to the terminal. The project was later rebuilt as a responsive static website so the same calculation logic could be explored directly in a browser without compiling any source code. The web version preserves the original educational purpose while expanding the experience with live calculation updates, custom Canvas charts, scenario comparison, browser-based saved plans, and CSV export. Thesource/ folder in the repository keeps the original C++ files — airgeadBankingApp.cpp, investment.cpp, and investment.h — intact for technical reference. Both the browser demo and the C++ source represent the same core project concept expressed through two different interfaces.
Technologies
| Technology | Use |
|---|---|
| HTML5 | Semantic page structure, forms, tables, articles, and navigation |
| CSS3 | Layout, visual design, responsive behavior, typography, and interaction states |
| JavaScript ES Modules | Calculation logic, page behavior, storage, exports, and shared components |
| Canvas API | Custom year-by-year growth chart and scenario comparison chart |
| Web Storage API | Current calculation state, saved plans, and mock session data |
| Blob and Object URL APIs | Client-side CSV generation and download |
| C++ | Original console application and object-oriented calculation implementation |
| GitHub Pages | Static site hosting with no server-side configuration |
Key limitations
Airgead is an educational projection tool, not a financial planning service. Before relying on any projection, be aware of the following constraints:
- Fixed rates — the calculator uses the single annual rate you enter; it does not connect to any market data source or adjust for rate changes over time.
- Whole-dollar display — all currency values are rounded to the nearest dollar at the end of each year.
- No taxes, fees, or inflation — the model does not account for account fees, tax treatment, inflation adjustments, or withdrawal activity.
- Browser-only storage — saved plans are stored in
localStorageand remain only in the current browser. Clearing site data removes them. Plans are not synchronized between devices. - Mock sign-in — the Sign In page is a clearly labeled demonstration. It does not create a real account, provide authentication, or transmit any information to a server.