Use this free compound interest calculator to project how savings or an investment grow over time. Enter your starting amount, interest rate and time period, and see the future value with interest earned on interest. No sign-up and no data stored.
What Compound Interest Is
Simple interest is paid only on the original amount. Compound interest is paid on the original amount plus the interest already earned. Over short periods the difference is small. Over long periods it becomes large, because each period’s interest starts earning interest of its own.
The Compound Interest Formula
A = P x (1 + r / n) ^ (n x t)
- A: the future value
- P: the starting amount (principal)
- r: the annual interest rate as a decimal
- n: how many times interest is compounded per year
- t: the number of years
Worked example: 10,000 invested at 5 percent for 10 years.
- Compounded annually: 10,000 x 1.05^10 = about 16,288.95
- Compounded monthly: 10,000 x (1 + 0.05/12)^120 = about 16,470.09
The same money earns an extra 181 simply because interest is added more often. With simple interest the result would be 15,000, so compounding added more than 1,288 in the annual case.
For another reference, the US Securities and Exchange Commission’s investor site offers its own compound interest calculator.
How to Use the Compound Interest Calculator
- Enter your starting amount.
- Enter the annual interest rate.
- Set the time period in years.
- Choose how often interest compounds, such as annually, monthly or daily.
- Add regular contributions if the tool offers that field.
- Read the future value and the interest earned.
The Rule of 72
A quick mental shortcut: divide 72 by the annual rate to estimate how many years it takes money to double. At 6 percent, money doubles in roughly 12 years. At 9 percent, it doubles in roughly 8 years. The rule is an approximation, but it helps you sense-check any result from the compound interest calculator.
What Changes the Result Most
- Time. Starting earlier matters more than almost anything else, because growth is exponential.
- Rate. A difference of one or two percentage points compounds into a large gap over decades.
- Contributions. Regular deposits add to the base that earns interest.
- Compounding frequency. More frequent compounding gives a slightly higher result.
- Fees and taxes. Management fees and taxes reduce your real return. Subtract them from the rate if you want a realistic projection.
- Inflation. A 5 percent return with 3 percent inflation grows your buying power by roughly 2 percent.
When to Use a Compound Interest Calculator
- Savings goals. Work out how long it takes to reach a target balance.
- Retirement planning. See how regular contributions grow over decades.
- Comparing accounts. Test two savings products with different rates and compounding schedules.
- Understanding debt. Credit card balances compound against you, so the same math shows how quickly debt grows.
- Family and diaspora savings. Project the growth of money set aside for a future move, land purchase or business.
Limits of the Projection
This tool assumes a constant rate. Real investments rise and fall, and savings rates change. Treat the output as an estimate for planning, not a guarantee. Nothing here is financial advice. For decisions about investments, speak with a licensed financial advisor.
FAQ SECTION
What is compound interest and how is it different from simple interest?
Simple interest is calculated only on your original principal. If you invest $1,000 at 10% simple interest for 3 years, you earn $100 per year — $300 total. Compound interest is calculated on your principal plus all the interest already earned. In year 1 you earn $100, but in year 2 you earn interest on $1,100 — and so on. Over long time periods, this compounding effect creates exponential growth rather than linear growth, which is why Albert Einstein is (perhaps apocryphally) credited with calling compound interest the eighth wonder of the world.
What does compounding frequency mean and does it make a big difference?
Compounding frequency is how often your interest is calculated and added to your balance. Common options are daily, monthly, quarterly, and annually. The more frequently interest compounds, the slightly higher your effective annual return — daily compounding yields marginally more than monthly, which yields more than annual. For most practical purposes the difference between daily and monthly compounding is small, but annual versus monthly compounding can make a meaningful difference over decades. A savings account that compounds daily at 5% AER will slightly outperform one that compounds annually at the same stated rate. Use this calculator’s frequency selector to compare the difference directly.
How much do regular monthly contributions change the final result?
Significantly. Regular contributions have a compounding effect of their own — each contribution earns interest for the remainder of the investment period. A $200 monthly contribution added to a $5,000 initial investment at 8% over 20 years adds more than $60,000 to the final balance compared to investing the lump sum alone. The earlier and the more consistently you contribute, the more powerful the effect. This is the mathematical basis for the advice to start saving early and contribute regularly, even in small amounts.
Can I use this calculator for a savings account or only for investments?
You can use it for any scenario where interest or returns compound over time — savings accounts, cash ISAs, fixed-term deposits, pension funds, index fund investments, or business reinvestment projections. Simply enter the interest or return rate that applies to your specific account or product. For savings accounts, use the AER (Annual Equivalent Rate) stated by your bank. For investment projections, use a realistic long-term average return for your asset class and remember that investment returns fluctuate and are not guaranteed.
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