Compound Interest Calculator

What Is a Compound Interest Calculator?

There's a reason people call compound interest the eighth wonder of the world — and a reason credit card companies love customers who only make minimum payments. The same mechanism that quietly builds your ISA into a meaningful sum is the one that turns a £500 credit card debt into £1,500 if you let it run.

The numbers: £10,000 at 5% simple interest earns exactly £5,000 over 10 years. The same amount at 5% compounded monthly earns £6,470 — nearly 30% more. Add a modest £200 monthly contribution and after 10 years you have £44,677. The gap between simple and compound interest widens dramatically with time, which is why starting a pension or ISA in your twenties produces results that feel almost unfair compared to starting in your forties.

Enter your starting amount, annual interest rate, compounding frequency, and the time period. Add monthly contributions to see how regular saving accelerates growth. The output shows your projected balance, total contributions, and total interest earned separately — so you can see exactly how hard your money is working versus how much you actually put in.

How Do You Use This Compound Interest Calculator?

Enter your initial deposit, annual interest rate, compounding frequency, and time period. Optionally add regular monthly contributions. Click Calculate to see your projected balance.

1. Enter your initial deposit or investment amount in pounds.
2. Set the annual interest rate as a percentage.
3. Choose the compounding frequency (daily, monthly, quarterly, or yearly).
4. Enter the investment period in years.
5. Add any regular monthly contributions if applicable.
6. Click Calculate to view the projected balance and interest earned.
7. Compare results by adjusting the compounding frequency or rate.

How Does the Compound Interest Calculator Formula Work?

The formula used: A = P(1 + r/n)^(nt) where P = principal, r = annual rate, n = compounds per year, t = years

What Are Some Example Calculations?

£10,000 at 5% compounded monthly for 10 years with £200/month contributions: Final balance = £44,677. Total contributions = £34,000. Total interest earned = £10,677.

When Should You Use a Compound Interest Calculator?

Use it to model your Cash ISA, Stocks and Shares ISA, or savings account. Enter the current balance, your provider's AER, and how many years until you need the money. Then test what adding £50 or £100 a month does to the final figure — the difference over a 15-year period is almost always larger than people expect.

Run it in reverse before taking on high-interest debt. Credit cards often charge 25–40% APR, and that compounds against you fast. A £2,000 balance at 29.9% with no repayments becomes over £4,000 in under three years. Seeing that number written out tends to focus the mind considerably.

What Do These Terms Mean?

How Do the Options Compare?

What Are the Best Tips to Know?

• Start contributing as early as possible — time in the market matters more than timing the market.
• Increase monthly contributions by even £25 per month to see a significant long-term difference.
• Compare accounts by entering each provider's rate and compounding frequency side by side.
• Use the Rule of 72 for quick estimates: divide 72 by the interest rate to find doubling time.
• Reinvest dividends and interest rather than withdrawing to maximise compounding.

What Mistakes Should You Avoid?

• Using the gross interest rate instead of the AER when comparing savings accounts.
• Forgetting that inflation erodes the real value of future returns.
• Assuming past investment returns will continue at the same rate in the future.
• Overlooking fees and charges that reduce the effective rate of return.
• Confusing compounding frequency with contribution frequency.