Compound Interest Calculator — Investment Growth

Compound interest is the reason starting early beats saving more later. Your returns earn their own returns, and over decades that snowball does more work than your contributions do. This calculator shows the growth of a lump sum, regular monthly savings, or both — and splits the final pot into money you put in versus growth the money generated itself.

How it works

Enter an initial amount, a monthly contribution (leave it at zero for a pure lump sum), an expected annual return, and the number of years. You can change how often interest compounds — annually, monthly or daily — to see how much difference compounding frequency makes.

The output shows the final balance, your total contributions, and the total interest earned. The year-by-year view is worth a look: growth in the final five years of a long projection routinely exceeds everything earned in the first ten, which is the whole point of starting now rather than waiting for a “better” moment.

How the calculation works

For a lump sum, the formula is:

A = P × (1 + r ÷ n)ⁿᵗ

where P is the starting amount, r is the annual rate, n is how many times interest compounds per year, and t is the number of years.

Regular contributions are calculated as a series of smaller lump sums, each compounding for the time it is invested. This calculator assumes monthly contributions are made at the end of each month and compounded monthly for the projections shown — the convention most savings and investment illustrations use.

Two honest caveats. First, real investment returns are not smooth: markets deliver the average in lumpy, sometimes negative, instalments, so treat the projection as an illustration of a rate, not a prediction. Second, the figures are before tax, fees and inflation unless you adjust the rate. Many people model long-term stock market growth at 5–7% after inflation, cash savings at whatever their account pays, and then compare like with like.

Worked example: $10,000 lump sum plus $200 a month at 7% for 20 years

Step 1 — The lump sum. $10,000 × (1.07)²⁰ = $10,000 × 3.8697 = $38,697.

Step 2 — The monthly contributions. $200 a month for 240 months, growing at 7% a year compounded monthly, accumulates to about $104,185.

Step 3 — The total. $38,697 + $104,185 ≈ $142,882.

Step 4 — What you actually paid in. $10,000 + ($200 × 240) = $58,000 of contributions. The remaining ~$84,882 is pure growth — more than everything you deposited. Extend the same plan to 30 years and the final pot reaches roughly $364,000, with growth supplying over three-quarters of it.

Things to watch

• Fees compound too — against you. A 1% annual charge does not sound like much, but at 7% gross growth it cuts the 30-year outcome in the example above by roughly a fifth. When comparing platforms or funds, a lower fee is a guaranteed “return” you do not have to take risk to earn.
• Inflation quietly taxes cash. Money in an account paying 3% while inflation runs at 3% is standing still in real terms, and UK savers may pay tax on the interest as well. For long-term goals, check whether your assumed rate is a real (after-inflation) figure or a nominal one, and be consistent.
• Use tax shelters first where they exist. Growth inside a UK ISA, a US Roth IRA or a 401(k)/pension is not taxed each year, which means the compounding in this calculator actually happens. In a taxable account, annual tax on interest and dividends creates a drag the headline formula ignores.
• Doubling time is a handy mental shortcut. Divide 72 by your rate to estimate how long money takes to double: at 7%, about 10.3 years; at 4%, 18 years; at 10%, about 7.2 years. It explains why a debt at 24% APR doubles in roughly three years if unpaid.
• Sequence matters as much as averages near retirement. A crash in year 19 of 20 hurts far more than one in year 2, because the pot is largest at the end. Illustrations smooth this out; real plans near a goal often reduce risk gradually for this reason.

Frequently asked questions

What rate of return should I assume?
For a diversified stock portfolio, long-run historical averages are around 7–10% a year before inflation, but past performance guarantees nothing. Conservative planners model 5%. For cash savings, use your actual account rate. Running the calculator at two or three rates gives a more honest range than one precise-looking number.

How much difference does starting five years earlier make?
A large one, because the extra years are added at the end where growth is fastest. In the worked example, starting the same plan at 25 years instead of 20 lifts the final pot from about $142,900 to about $218,000 — the extra $12,000 of contributions generates roughly $75,000 of extra value.

Is monthly or annual compounding better?
Monthly compounding produces slightly more than annual at the same headline rate, and daily slightly more again, but the differences are small next to the rate itself and the time invested. In the UK, savings interest is usually calculated daily and paid monthly or annually; investments do not compound on a schedule at all — their value simply changes.

Does compound interest work the same way on debts?
Yes, and faster, because debt rates are higher. Credit card balances compound monthly at APRs above 20%, which is why minimum payments barely shrink a balance. The same maths that builds savings dismantles borrowers.

Should I pay off debt or invest?
Compare rates. If a debt costs more than your realistic after-tax investment return — credit cards almost always do — clearing the debt is the better guaranteed “return”. Low-rate mortgage debt versus long-term investing is a closer, more personal call involving risk tolerance, not just arithmetic.

This calculator is for guidance only and isn’t financial advice.