What is Compound Interest?
Compound interest is the process of earning interest not just on your original principal, but also on every dollar of interest you have previously accumulated. Unlike simple interest — which applies only to the starting balance — compound interest causes your money to grow at an accelerating rate over time. The longer you stay invested, the more dramatic the effect becomes.
Consider $10,000 invested at 7% per year. With simple interest you would earn $700 every year, adding up to $17,000 after ten years. With annual compounding, your balance grows to roughly $19,672 over the same period — nearly $2,700 more, without a single extra dollar invested. At 30 years, that same $10,000 reaches over $76,000, illustrating why time is the most powerful variable in wealth building.
Adding regular monthly contributions amplifies the effect even further. Small, consistent deposits — even $100 or $200 a month — can dramatically increase the final balance because each new contribution immediately starts earning compound returns of its own. Use the compound interest calculator to model your own numbers and see the exact growth curve.
How Loan Interest Works
Most personal loans, auto loans, and mortgages use an amortizing repayment schedule. Each monthly payment is fixed, but the way it is split between interest and principal changes over the life of the loan. In the early months, the outstanding balance is at its highest, so the majority of each payment goes toward interest charges. Only a small portion reduces the principal you actually owe.
As you make payments and the balance falls, the interest portion of each payment shrinks and more of your fixed monthly amount chips away at the principal. By the final years of a 30-year mortgage, nearly the entire payment is eliminating debt rather than paying interest. This is why paying even a small amount extra each month early in a loan term can save thousands of dollars — those extra dollars skip the interest charge and directly reduce the balance on which future interest is calculated.
The three levers that control your total interest cost are the loan amount, the interest rate, and the loan term. Shortening the term increases monthly payments but slashes total interest. Use the loan calculator to compare different term and rate combinations side by side before you sign.
Why Inflation Matters
Inflation is the gradual increase in the price of goods and services over time — which is the same as saying the purchasing power of a dollar quietly erodes every year. At a modest 3% annual inflation rate, something that costs $100 today will cost roughly $134 in ten years and $181 in twenty years. The rule of 72 offers a quick way to visualize this: divide 72 by the inflation rate and you get the number of years it takes for prices to double.
For savers, this has a critical implication: cash sitting in a low-yield account loses real value every single year. If your savings account pays 0.5% interest and inflation runs at 3%, you are effectively losing about 2.5% of your purchasing power annually. This is why financial planners emphasize investing in assets — like stocks and real estate — that have historically produced returns above the rate of inflation.
Retirement planning must account for inflation carefully. A nest egg of $1 million may sound comfortable today, but 25 years from now its real value at 3% inflation is closer to $478,000 in today's dollars. Building a retirement plan that targets enough savings to maintain your desired lifestyle in inflation-adjusted dollars is essential. Use the inflation calculator to see exactly how inflation will affect any amount over your chosen time frame.