Calculate exactly how much interest you'll pay on a loan or earn on savings. Compare simple vs compound interest side by side.
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Understanding how interest works is crucial for making smart financial decisions. Whether you're taking out a loan or building savings, this calculator helps you see the real cost or return.
Simple interest is calculated only on the original principal amount. The formula is straightforward: Interest = Principal × Rate × Time. For example, if you invest $10,000 at 5% simple interest for 3 years, you earn $10,000 × 0.05 × 3 = $1,500 in total interest. Simple interest is rarely used for loans or savings today, but it's useful for understanding the basics and calculating short-term interest.
Compound interest is calculated on the principal plus any previously earned interest. This is how most real-world loans and savings accounts work. The frequency of compounding matters: daily compounding produces more interest than monthly, which produces more than annual. The formula is A = P(1 + r/n)^(nt), where n is the number of times interest compounds per year. Albert Einstein allegedly called compound interest "the eighth wonder of the world" because of its power to grow wealth over time.
APY (Annual Percentage Yield) is the effective annual rate that accounts for compounding. If a bank advertises 5% interest compounded monthly, the APY is actually 5.12% because of compounding. Always compare APY, not just the nominal rate, when shopping for savings accounts or CDs. For loans, compare APR (Annual Percentage Rate), which includes fees.
For Loans: Use the results to understand the true cost of borrowing. A lower monthly payment with a longer term often means paying much more in total interest. Consider making extra payments to reduce total interest.
For Savings: See how your money grows over time. Even small increases in interest rate or regular contributions can significantly boost your final balance. Start early to maximize compound interest.
Pro Tip: The Rule of 72 helps estimate doubling time: divide 72 by your interest rate. At 6% interest, your money doubles in about 12 years (72 ÷ 6 = 12). At 8%, it doubles in 9 years.
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Simple interest is calculated using the formula: Interest = Principal × Rate × Time. For example, if you borrow $10,000 at 5% annual interest for 3 years, the total interest would be $10,000 × 0.05 × 3 = $1,500. Simple interest is calculated only on the original principal, never on accumulated interest.
Compound interest is calculated using the formula: A = P(1 + r/n)^(nt), where A is the final amount, P is principal, r is annual interest rate (as a decimal), n is the number of times interest compounds per year, and t is time in years. The interest earned is A - P. Compound interest grows faster because you earn interest on previously earned interest.
Simple interest is calculated only on the original principal amount, while compound interest is calculated on the principal plus any accumulated interest. Over time, compound interest results in significantly more interest earned (for savings) or paid (for loans) than simple interest. The difference becomes more pronounced with longer time periods and higher interest rates.
The more frequently interest compounds, the more interest accumulates. Daily compounding produces more interest than monthly, which produces more than quarterly, which produces more than annual. For example, $10,000 at 5% for 10 years compounds to $16,288.95 annually, but $16,470.09 daily—a difference of $181.14 from compounding frequency alone.
This calculator provides accurate estimates based on standard compound interest formulas used by most financial institutions. However, actual loan terms may vary due to origination fees, variable interest rates, payment schedules, prepayment penalties, or lender-specific calculation methods. Use this calculator for planning and comparison purposes, but always consult your official loan documents for exact figures.