How Compound Interest Works in the Stock Market

The value of investments can fall as well as rise, and you may get back less than you put in. This article is general information, not personal advice.

Why Stocks Compound

Stocks compound through two channels working together: price appreciationas the underlying companies grow earnings, and reinvested dividends as those earnings get distributed to shareholders and then ploughed back into more shares. The interaction of these two forces is what makes equity returns so powerful over decades.

A single year of 10% growth is uninteresting. A century of 10% turns £1 into roughly £13,800. That is compounding.

The Math (Quick Recap)

The compound growth formula is:

FV = PV × (1 + r)^t

• FV = future value
• PV = present value (your initial investment)
• r = annualised return (as a decimal)
• t = number of years

For regular contributions, the future value of an ordinary annuity formula is more useful: FV = PMT × [((1 + r)^t − 1) / r] where PMT is the periodic contribution.

Example: $10,000 Growing for 35 Years

These two growth rates are illustrations, not a statement of what any index returned. The lower rate stands for growth from price alone and the higher rate for growth with dividends reinvested. Here is how a single $10,000 investment would grow at each rate over 35 years:

On these assumed rates, reinvested dividends turn a 14.8× return into a 35.1× return, roughly 2.4 times more wealth from the same starting capital, simply by ticking "reinvest" rather than "take as cash".

Dividend Reinvestment Plans (DRIP)

A DRIP automatically uses cash dividends to buy more shares of the same stock or fund, often commission-free and including fractional shares. Most major brokerages offer DRIPs. For long-term investors not yet drawing income, turning DRIP on is essentially free — the only thing it changes is whether the dividend appears as cash or as additional shares.

Inside an ISA, SIPP, 401(k), Roth IRA or TFSA, dividends are not taxed at all. Outside a wrapper, dividends are taxed in the year received even if reinvested — something to bear in mind for taxable accounts.

The Effect of Fees on Compounding

Fees compound just like returns, only against you. Consider £50,000 invested for 30 years at a 7% gross return:

Going from a 0.05% ETF to a 2.00% advisor-plus-fund stack costs you over 40% of your terminal wealth. Fees are the most controllable variable in long-term returns.

Test fee scenarios in our Compound Interest Calculator

Compounding Through Bear Markets

Bear markets are not a bug in the compounding machine — they are part of how it works for regular investors. When prices fall, your monthly contribution buys more shares, which then compound from a lower base. Investors who kept buying through 2008–2009 saw their average cost basis fall sharply, then rode the recovery up.

The danger is selling. If you crystallise a 35% paper loss, you have replaced a temporary drawdown with a permanent one and missed all the subsequent compounding.

Real vs Nominal Returns

Nominal returns are the headline number. Real returns subtract inflation, which is what actually matters for purchasing power. Real returns are lower than headline returns and vary by market and by period.

At 3% inflation, money in a 0% interest account loses half its purchasing power in 24 years. At an assumed 7% real return, wealth roughly doubles every 10 years (the rule of 72), but returns are not guaranteed.

Long-Term Examples Table

£200/month invested at different real (inflation-adjusted) rates:

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