Frequently Asked Questions
Everything you need to know about compound interest, our calculators, and making your money work harder.
About Compound Interest
What is compound interest?
Compound interest is interest calculated on both the initial principal and accumulated interest from previous periods. Unlike simple interest, which only earns on the original amount, compound interest creates exponential growth over time.
How is compound interest calculated?
The formula is A = P(1 + r/n)^(nt), where A is the final amount, P is principal, r is the annual interest rate (decimal), n is compounding frequency per year, and t is years.
What is the difference between APR and APY?
APR (Annual Percentage Rate) is the nominal rate without compounding. APY (Annual Percentage Yield) includes the effect of compounding. A 5% APR compounded monthly gives a 5.12% APY.
How does compounding frequency affect returns?
More frequent compounding produces higher returns. Daily compounding earns more than monthly, which earns more than annual. However, the differences are relatively small — the rate and time period matter far more.
What is the Rule of 72?
Divide 72 by your annual interest rate to estimate years to double your money. At 8%, money doubles in roughly 9 years. It's a quick mental math shortcut for compound growth estimation.
Using Our Calculators
Are your calculators free?
Yes, all 25+ calculators are completely free with no sign-up, subscription, or hidden fees required.
How accurate are the calculations?
Our calculators use standard financial formulas and produce results that match professional financial software. All calculations run in your browser for instant results.
Do you save my financial data?
No. All calculations happen entirely in your browser. We never collect, store, or transmit the numbers you enter.
Which calculator should I use?
For general savings growth, use the Compound Interest Calculator. For retirement, use the Retirement Calculator. For loans, use the Loan or Mortgage Calculator. Each tool page explains its specific use case.
Can I use these on mobile devices?
Yes, all calculators are fully responsive and work on phones, tablets, and desktop computers.
Financial Planning
How much should I save for retirement?
A common guideline is 25× your annual expenses (the 4% rule). If you spend $50,000/year, target $1.25 million. Use our Retirement Calculator to build a personalized plan.
Should I pay off debt or invest?
Compare your debt interest rate to expected investment returns. If your debt rate exceeds investment returns, prioritize debt payoff. Always maintain an emergency fund regardless.
How does inflation affect my savings?
At 3% inflation, the cost of living doubles in 24 years. Your investments need to outpace inflation to maintain purchasing power. Use our Inflation Calculator to see the impact.
What is dollar-cost averaging?
Investing a fixed amount at regular intervals regardless of market conditions. This strategy reduces the risk of investing a lump sum at a market peak and is the basis of SIP investing.
Is compound interest guaranteed?
For savings accounts and CDs (up to FDIC limits), yes. For stock market investments, returns are not guaranteed but have historically been positive over long periods (20+ years).