Compound Interest Calculator
Calculate how your savings or investments grow over time with the power of compounding. Enter your principal, interest rate, time horizon, and compounding frequency to see your final amount and total interest earned instantly.
Calculate Your Investment Growth
What is Compound Interest?
Compound interest is interest calculated not just on your original principal, but also on all the interest you have previously earned. This self-reinforcing cycle — where interest earns interest — is what separates compound interest from simple interest, where interest is only ever calculated on the original principal. Over long time periods, this difference becomes enormous, which is why Albert Einstein reportedly called compound interest "the eighth wonder of the world".
Consider a simple example: you invest $10,000 at an annual interest rate of 5%. With simple interest, you earn $500 every year — exactly $5,000 over 10 years. With compound interest (monthly compounding), your investment grows to $16,470.09 — earning $6,470.09 in total interest, or nearly 30% more than simple interest over the same period. After 30 years, the gap becomes dramatic: simple interest gives you $25,000, while monthly compounding delivers $44,812.12.
Compound interest works in your favour when you are saving or investing — your wealth accelerates over time. It works against you when you are borrowing — credit card debt, personal loans, and mortgages all use compounding, which is why outstanding balances can grow rapidly if left unpaid.
The Compound Interest Formula
The standard compound interest formula, as used in this calculator, is:
A = P × (1 + r/n)^(n×t)
Final Amount
The total value of your investment at the end of the period, including principal and all compound interest earned.
Principal
Your initial investment or deposit — the original sum of money you put in before any interest is added.
Annual Interest Rate
The nominal annual interest rate expressed as a decimal. For example, 5% annual rate = 0.05 in the formula.
Compounding Frequency
How many times per year interest is added to your balance. Monthly = 12, quarterly = 4, daily = 365.
Time (years)
The investment duration in years. You can use decimals — for example, 18 months = 1.5 years.
Effective Annual Rate (EAR): When interest compounds more frequently than once per year, the effective annual rate is always higher than the nominal rate. For example, a 5% nominal rate compounding monthly produces an EAR of 5.1162%. The formula is: EAR = (1 + r/n)^n − 1. Our calculator shows you this figure automatically in the results.
How Compounding Frequency Affects Your Returns
The more frequently interest compounds, the more you earn — but the differences between frequencies narrow at higher compounding rates. Here is what $10,000 invested at 5% for 10 years produces at different compounding frequencies:
| Compounding Frequency | Final Amount | Interest Earned |
|---|---|---|
| Annually (n=1) | $16,288.95 | $6,288.95 |
| Semi-annually (n=2) | $16,386.16 | $6,386.16 |
| Quarterly (n=4) | $16,436.19 | $6,436.19 |
| Monthly (n=12) | $16,470.09 | $6,470.09 |
| Weekly (n=52) | $16,483.25 | $6,483.25 |
| Daily (n=365) | $16,486.65 | $6,486.65 |
As you can see, switching from annual to monthly compounding adds over $100 in extra interest on a $10,000 investment over 10 years. Going from monthly to daily adds only a few dollars more — the gains from ultra-frequent compounding diminish rapidly. For most savings accounts, mortgages, and investment products, monthly compounding is the standard and provides an excellent balance of frequency and simplicity.
Compound Interest vs Simple Interest
Understanding the difference between compound and simple interest is fundamental to making informed financial decisions — whether you are choosing a savings account, taking out a loan, or evaluating an investment.
| Feature | Simple Interest | Compound Interest |
|---|---|---|
| Calculated on | Principal only | Principal + accumulated interest |
| Formula | A = P × (1 + r × t) | A = P × (1 + r/n)^(nt) |
| Growth pattern | Linear — grows at a steady rate | Exponential — accelerates over time |
| Better for | Short-term loans, easy comparison | Long-term investments and savings |
| Common uses | Car loans, personal loans, bonds | Savings accounts, mortgages, ETFs |
| Risk | Predictable, easy to calculate | Can work against you with debt |
Real-World Compound Interest Examples
The table below shows what different starting amounts grow to under various realistic investment scenarios using monthly compounding — the standard for most savings accounts and investment funds:
| Scenario | Final Amount | Interest Earned |
|---|---|---|
| $10,000 at 5% for 10 years (monthly) | $16,470.09 | $6,470.09 |
| $10,000 at 5% for 10 years (yearly) | $16,288.95 | $6,288.95 |
| $10,000 at 7% for 20 years (monthly) | $40,387.39 | $30,387.39 |
| $50,000 at 4% for 15 years (monthly) | $91,015.08 | $41,015.08 |
| $10,000 at 10% for 30 years (monthly) | $198,373.99 | $188,373.99 |
The last row illustrates the extraordinary power of long time horizons. A single $10,000 investment at 10% for 30 years grows to nearly $200,000 — roughly 19× the original amount — entirely through compounding. This is why starting to invest early, even in small amounts, is one of the most impactful financial decisions a person can make.
How to Maximise Compound Interest on Your Savings
Understanding compound interest is only useful if you apply it. Here are the key principles that determine how much compound interest you actually earn:
Start as early as possible
Time is the most powerful variable in the compound interest formula. An investor who starts at 25 with $10,000 at 7% will have nearly twice as much at 65 as someone who starts at 35 with the same amount — despite investing for only 10 extra years.
Reinvest all interest and dividends
Compound interest only works if the interest stays invested. Withdrawing interest payments reverts your investment to simple interest behaviour. Most ETFs and index funds automatically reinvest dividends (accumulating share classes), which is ideal for long-term compounding.
Maximise your compounding frequency
Choose savings accounts and investment products that compound monthly or daily rather than annually. Over a 10-year period, monthly compounding at 5% produces meaningfully more than annual compounding at the same rate, with no extra risk.
Minimise fees — they compound too
Annual management fees on investment funds reduce your effective return every year — and those reductions compound just like gains do. A 1% annual fee sounds small, but on a $100,000 portfolio at 7% over 30 years, it costs over $180,000 in lost compound growth.
Increase your principal regularly
Regular contributions — monthly deposits into a savings account or investment account — multiply the compounding effect significantly. Even small regular contributions ($100–$200/month) dramatically outperform a single lump-sum investment over long periods.
Where Compound Interest Applies in Real Life
Compound interest is not just a theoretical concept — it is embedded in the financial products most people use every day, on both sides of the ledger:
Works in your favour (saving / investing)
- High-yield savings accounts (HYSA)
- Cash ISAs and fixed-term deposit accounts
- Index funds and ETFs (accumulating class)
- Pension funds and retirement accounts (401k, ISA)
- Government and corporate bonds (reinvested coupons)
- Cryptocurrency staking rewards (reinvested)
✕ Works against you (borrowing)
- Credit card balances (often 20–30% APR, daily compounding)
- Payday loans and short-term borrowing
- Student loan interest (in some countries)
- Mortgage interest (especially early years)
- Buy Now Pay Later (BNPL) deferred interest
- Unpaid tax liabilities and penalties
Credit card warning: A $5,000 credit card balance at 25% APR with daily compounding grows to over $6,300 in just one year if no payments are made — and to nearly $46,000 over 10 years. Paying off high-interest compound debt is mathematically equivalent to earning a guaranteed 25%+ annual return on investment.
The Rule of 72 — How Long to Double Your Money?
The Rule of 72 is a quick mental shortcut for estimating how long it takes to double an investment under compound interest. Simply divide 72 by the annual interest rate to get the approximate number of years to double your money:
Years to double ≈ 72 ÷ rate (%)
| Annual Rate | Rule of 72 (approx.) | Exact years |
|---|---|---|
| 2% | 36.0 years | 35.00 years |
| 3% | 24.0 years | 23.45 years |
| 4% | 18.0 years | 17.67 years |
| 5% | 14.4 years | 14.21 years |
| 6% | 12.0 years | 11.90 years |
| 7% | 10.3 years | 10.24 years |
| 8% | 9.0 years | 9.01 years |
| 10% | 7.2 years | 7.27 years |
| 12% | 6.0 years | 6.12 years |
The Rule of 72 is remarkably accurate for interest rates between 2% and 12%. At a 6% annual return, your money doubles in approximately 12 years. At 10%, it doubles in just over 7 years. This rule is useful for quick mental calculations, comparing investment options, and understanding the impact of inflation on purchasing power.