Compound Investment Calculator

📖 Tool Introduction

Compound Investment Calculator is a free online projection tool built for fund dollar-cost averaging, retirement saving and long-term wealth planning. Most people have heard the phrase that compound interest is the eighth wonder of the world, yet when it comes to making an actual plan, very few can say with confidence what two thousand a month, kept up for twenty years, actually turns into. This tool breaks that question down into four inputs: an initial lump sum, a monthly contribution amount, an assumed annualized return, and a target number of years. It then projects the future value with the end-of-month annuity formula for the regular contributions and the classic compound growth formula for the lump sum, and after you click Calculate, three cards at the top show the final balance, the total amount you put in, and the total gain, with the gain highlighted in green so the reward from compounding is immediately visible. Below the cards, a pure CSS bar chart draws the growth curve year by year, and the way the bars climb from short to tall is a picture of what patience over time actually delivers to a disciplined saver. Everything runs locally inside your browser: no network request, no account, no data collection. The interface speaks Simplified Chinese, Traditional Chinese and English, adapts to narrow phone screens, and needs no registration at all. It is important to read the result as a long-run assumption rather than a promised return, because markets move and products fluctuate, so the tool is best used for rough order-of-magnitude planning and for comparing two saving strategies side by side, rather than as a forecast of any specific fund's future performance. Used that way, it becomes a quiet but powerful companion for anyone trying to translate a vague retirement dream into numbers they can actually act on every month.

✨ Key Features

• Monthly contributions grow with the exact end-of-month annuity formula M × ((1+r)^n − 1) / r
• The lump sum compounds at P × (1+r)^n and is added to the annuity future value
• Three result cards: final balance, total invested and total gain on one screen
• A year-by-year bar chart that visualises how the balance climbs over time
• Zero-return edge case handled gracefully, degrading to plain principal accumulation
• One-click copy and download, so the key figures can be saved or pasted anywhere

📝 How to Use

1. Type the one-time initial principal, or 0 if you are starting from zero
2. Enter the fixed amount you plan to invest every month
3. Enter an assumed annualized return as a percentage, e.g. 8 means 8%
4. Enter the number of years you intend to keep contributing, then click Calculate
5. Read the three cards and the bar chart, then tweak the numbers and recalculate

⚠️ Notes

• Parameter range: principal and monthly contribution cannot be negative, and years must be greater than zero
• Return assumption: the annualized figure is a long-run average assumption, not a guaranteed return, and real results will fluctuate
• Contribution timing: this tool uses end-of-month contributions, so beginning-of-month investing shows slightly different numbers
• Local only: every projection runs inside the browser with no network request at all
• Risk warning: results are for planning reference only; investing carries risk, so decide carefully