Calculate compound interest with initial investment and regular contributions. See growth over time with charts and accumulation schedule. Supports tax and inflation.
Ending balance
$53,153.79
Total principal
$45,000.00
Total contributions
$25,000.00
Total interest
$8,153.79
Interest of initial investment
$5,525.63
Interest of the contributions
$2,628.16
| Year | Balance at start | Contribution | Interest | Balance at end | |
|---|---|---|---|---|---|
| 2026 | $20,000.00 | $5,000.00 | $1,000.00 | $26,000.00 | |
| 2027 | $26,000.00 | $5,000.00 | $1,300.00 | $32,300.00 | |
| 2028 | $32,300.00 | $5,000.00 | $1,615.00 | $38,915.00 | |
| 2029 | $38,915.00 | $5,000.00 | $1,945.75 | $45,860.75 | |
| 2030 | $45,860.75 | $5,000.00 | $2,293.04 | $53,153.79 |
Compound interest is one of the most powerful concepts in finance. Unlike simple interest, which only earns on the original amount, compound interest earns on both the principal and accumulated interest. This "interest on interest" effect can significantly grow your wealth over time. Here's how to use this calculator:
Compound interest is the process where interest is added to the principal, and then future interest is calculated on this new, larger balance. Albert Einstein allegedly called it the "eighth wonder of the world" because of its remarkable ability to grow wealth exponentially over time. The key difference from simple interest is that your earnings generate their own earnings, creating a snowball effect that accelerates over longer time periods.
Understanding the difference between these two types of interest is crucial for making informed financial decisions.
Simple interest is calculated only on the original principal amount. The interest earned each period remains constant, regardless of how long you invest. Formula:
Example: £10,000 at 5% simple interest for 10 years earns £500/year, totaling £5,000 in interest.
Compound interest is calculated on the principal plus all accumulated interest. Each period, your interest earns interest, creating exponential growth. Formula:
Example: £10,000 at 5% compound interest (annual) for 10 years grows to £16,289 – that's £6,289 in interest!
Where: P = principal, r = annual rate (decimal), n = compounding periods per year, t = time in years, A = final amount.
The frequency at which interest is compounded can make a meaningful difference to your final balance. While the difference may seem small in percentage terms, it can add up to significant amounts over long investment periods.
Annually (1x/year): Interest is calculated and added once per year. This is the simplest form and serves as the baseline for comparison.
Semiannually (2x/year): Interest compounds twice per year. Common for bonds and some savings products.
Quarterly (4x/year): Interest compounds four times per year. Often used for dividend reinvestment plans.
Monthly (12x/year): Interest compounds every month. Most savings accounts and many investments use monthly compounding.
Daily (365x/year): Interest compounds every day. Offers the highest effective return but the difference from monthly is minimal.
The Rule of 72 is a simple mental math shortcut to estimate how long it takes for an investment to double at a given annual interest rate. Simply divide 72 by the annual percentage rate.
Quick Examples:
Understanding the mathematical formula helps you grasp how compound interest works and verify calculations.
Future value with regular contributions (end of period):
If you contribute at the start of each period, the contribution term is multiplied by (1+r), giving slightly higher returns.
Where:
Interest rates can be either fixed (constant over time) or floating (variable, changing based on market conditions). This calculator assumes a fixed rate for simplicity.
Fixed Rate: The interest rate remains constant throughout the investment period. This provides predictable returns and is ideal for planning. Common for fixed deposits, bonds, and some savings accounts.
Floating Rate: The rate changes periodically based on a benchmark (like the Bank of England base rate). While you might benefit from rate increases, your returns are less predictable. Common for premium bonds and some savings accounts.
Tip: For variable rate investments, recalculate periodically with updated rates to track your projected growth.
Compound interest is often called the investor's best friend. Understanding and harnessing its power can transform your financial future.
Exponential Growth: Unlike linear growth with simple interest, compound interest creates exponential growth that accelerates over time. The longer you invest, the more dramatic the effect.
Passive Wealth Building: Once invested, your money works for you 24/7. You earn returns on your returns without any additional effort on your part.
Time Advantage: Starting early gives you a massive advantage. Someone who starts investing at 25 can often accumulate more than someone who starts at 35, even with smaller contributions.
Inflation Hedge: Compound returns that outpace inflation help preserve and grow your purchasing power over time.
Small changes in your investment strategy can lead to significant differences in your final wealth.
Start Early: Time is the most powerful factor in compounding. Starting 10 years earlier can more than double your final balance.
Contribute Regularly: Consistent monthly or annual contributions dramatically boost your growth. Even small amounts add up significantly over time.
Reinvest All Returns: Always reinvest dividends and interest to maximise the compounding effect. Don't withdraw earnings prematurely.
Minimise Fees: High fees erode compound growth. A 1% difference in fees can cost tens of thousands over a lifetime.
More frequent compounding (monthly vs annually) increases your effective annual return slightly, as interest is added to the balance more often and then earns interest itself. However, the difference is typically small – around 0.1-0.3% annually between monthly and annual compounding.
Contributing at the start of each period gives slightly higher returns, because that contribution earns interest for the full period. Over a 30-year investment, this could mean several thousand pounds extra.
For stock market investments, 7-10% annually is historically reasonable (before inflation). For savings accounts, 3-5% is typical. For government bonds, 2-4%. Always consider fees and taxes which reduce your effective return.
Inflation reduces the purchasing power of your money over time. If you earn 6% but inflation is 3%, your real return is only about 3%. Use the inflation adjustment feature to see what your future balance will actually buy in today's terms.
You invest £20,000 in a diversified index fund and contribute £5,000 at the end of each year. With an average annual return of 7% compounded annually over 20 years:
Ending balance: £295,685 (contributions: £120,000, interest: £175,685)
Starting with £5,000 and saving £200 monthly at 4.5% interest compounded monthly for 10 years:
Ending balance: £37,250 (contributions: £29,000, interest: £8,250)
A 25-year-old invests £10,000 initially and adds £500 monthly at 8% compounded monthly until age 65 (40 years):
Ending balance: £1,745,000 – the power of starting early!