How to Use the Compound Interest Calculator
The Compound Interest Calculator shows you exactly how your savings or investments grow over time when interest is earned on both your principal and previously accumulated interest. It's one of the most powerful financial planning tools available — often called the 'eighth wonder of the world' by those who understand its effect.
Enter your starting balance, regular monthly contribution, annual interest rate, compounding frequency (daily, monthly or annual), and time horizon. The calculator plots your growth year by year, breaking down how much is from your contributions and how much is pure interest.
The key nuance is compounding frequency: daily compounding yields slightly more than monthly, which yields more than annual. On a $10,000 investment at 5% for 10 years, daily compounding produces $16,487 vs $16,470 for annual — a small difference at low rates, but significant with larger sums or higher rates.
📊 Worked Example
$5,000 starting balance, $200/month contributions, 7% annual return (monthly compounding), 30 years:
- Total contributions: $77,000
- Interest earned: $166,284
- Final balance: $243,284
- Interest is 68% of the final pot — more than contributions
Common Use Cases
- ✅ Projecting how a pension or retirement account grows over decades
- ✅ Comparing savings accounts with different interest rates
- ✅ Understanding the long-term cost of starting to save late
- ✅ Calculating how a lump-sum investment grows alongside monthly contributions
- ✅ Modelling ISA or 401(k) growth for retirement planning
- ✅ Showing children or students the power of starting early
- ✅ Comparing daily vs monthly vs annual compounding for the same rate
Frequently Asked Questions
What is compound interest?
Compound interest means you earn interest on your interest. If you have £1,000 at 5% and earn £50 in year one, in year two you earn 5% on £1,050 — not just the original £1,000. This snowball effect becomes enormous over long periods.
What is the Rule of 72?
The Rule of 72 is a quick mental maths shortcut: divide 72 by the interest rate to find roughly how many years it takes to double your money. At 6%, your money doubles in about 12 years (72 ÷ 6). At 9%, it doubles in about 8 years.
How does compounding frequency affect returns?
More frequent compounding means interest is calculated and added to the balance more often, so you earn interest on interest sooner. The effective annual rate (EAR) is always slightly higher than the stated rate for sub-annual compounding. At 6%, monthly compounding gives an EAR of 6.168%.
What return rate should I use for stock market projections?
The S&P 500 has historically returned around 10% annually before inflation and roughly 7% after inflation. Financial planners often use 6–7% for long-term real return projections. Always note that past performance doesn't guarantee future returns.
How much does starting 10 years earlier change my outcome?
The difference is dramatic. $200/month at 7% for 40 years grows to $525,000. Starting 10 years later (30 years) yields only $243,000 — less than half — even though you contribute for the same amount of time per year. This is why starting early matters so much.