Visualize the exponential power of time and money By Hemon Vongprachith
Compound interest is interest calculated not only on your original principal, but on all accumulated interest from prior periods. Unlike simple interest — which grows linearly — compound interest bends the curve exponentially upward.
The gap between simple and compound growth starts small but widens dramatically over time. This is why starting early matters so much: a dollar invested at 25 does far more work than one invested at 45, because it benefits from additional compounding cycles every single year.
Imagine rolling a snowball down a long hill. At first it gains size slowly. But as it grows, it picks up more snow with each turn — more surface area contacts more snow. By the bottom it's enormous, not because it rolled faster, but because its size kept compounding. Your money works the same way: every dollar of interest earned becomes new principal that earns its own interest, accelerating without extra effort on your part.
The Rule of 72 is one of finance's most elegant mental shortcuts. It answers the question everyone asks: "How long will it take to double my money?" — and you can calculate it in your head in seconds.
Simply divide 72 by your annual interest rate as a whole number. The result is approximately how many years it takes for your investment to double under compound growth.
The rule works because 72 is divisible by many common rates (2, 3, 4, 6, 8, 9, 12) and the logarithmic math of compounding makes it remarkably accurate across a wide range. It slightly overestimates at low rates and underestimates at high ones — but as a quick sanity check, it's nearly perfect.
If inflation runs at 3%, your purchasing power halves in 24 years. If your savings account pays 1%, your money doubles in 72 years. But if your portfolio returns 9% annually, you double every 8 years — meaning four doublings in 32 years, or roughly 16× growth. The Rule of 72 turns abstract rates into visceral timelines.
| Year | Starting Balance | Interest Earned | Contributions | Ending Balance | Growth % |
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