Grow It Investing
Compound interest: why time matters more than timing
Compound interest is the closest thing ordinary money has to magic: your growth earns its own growth, until the curve stops crawling and starts to soar. The catch is that it is painfully slow to start and startlingly fast to finish - which is why the people who win are simply the ones who started early and left it alone. Time, not timing, is the secret.
By Chris Willman · Last reviewed · About a 9 minute read
The short version
- Compounding means your returns earn returns. Given enough time, that snowball becomes most of your wealth.
- It is slow to start and fast to finish, which is exactly where most people give up - just before it accelerates.
- Time beats timing: starting earlier usually beats starting bigger or trying to pick the perfect moment.
- It runs faster inside tax wrappers (ISA, pension, LISA) and with an employer pension match. Your investments can still fall as well as rise.
What does compound interest actually mean?
In short: In short: your growth earns its own growth - and given enough time, that becomes most of what you have.
Simple interest earns on the original amount only. Put money in, earn a return on that amount, receive the same return next year on the same original amount. The growth is linear and predictable.
Compound interest earns on the original amount plus every return it has already generated. Each year the base grows - so the return, even at the same rate, is applied to a larger number. Over time, the base grows faster and faster, because there is more to compound on.
The curve this produces is not linear. It is exponential - which is why compounding feels almost imperceptible at the start and then suddenly dramatic. The early years are slow because the base is small. The later years are fast because the base has grown substantially, and the returns on it have grown too.
Most people abandon long-term investing during the slow early phase - the years where contributions dominate the portfolio and growth feels modest. This is precisely the phase that sets up everything that follows. Leaving during the slow phase means missing the fast phase entirely.
Why does starting earlier beat starting with more?
In short: In short: the early starter who did less ends up ahead of the late starter who did more.
The counterintuitive scenario: someone who starts investing in their mid-twenties and stops contributing after ten years ends up with more than someone who starts in their mid-thirties and contributes consistently for thirty years. This is purely a function of time, not amounts.
The early years are the most valuable, not because the amounts are larger, but because they have the longest runway to compound. Money invested in your mid-twenties has four decades to grow before retirement. Money invested in your mid-fifties has one decade. The same pound, invested thirty years earlier, does far more work.
This does not mean late starters should give up - starting now is always better than not starting. But it does make the case for not waiting: the cost of delay is real, and it compounds in reverse.
When does compound growth overtake your own contributions?
In short: In short: there is a point where your growth earns more each year than you put in - and then compounding takes over.
There is a specific point in any long-term investment journey - the acceleration point - where the annual returns generated by the portfolio exceed the annual contributions being made. At this point, the investor's own behaviour matters less, because the portfolio is doing most of the work.
Before this point, contributions drive growth. After it, compounding drives growth. The investor's role shifts from builder to caretaker: stay invested, keep costs low, do not interfere.
Reaching this point faster requires three things: starting earlier (the single biggest factor), contributing consistently without gaps, and keeping investment costs low so returns are not eroded before they compound. All three are within the investor's control.
Ongoing fund charges (expressed as OCF - ongoing charges figure) reduce the effective return rate. A difference of even a fraction of a percent in annual charges has a compounding effect of its own, in reverse. This is why low-cost index funds are the default recommendation for long-term investing.
How do UK tax wrappers make compounding run faster?
In short: In short: inside a tax wrapper, your returns compound without a tax bill reducing them each year.
Outside a tax wrapper, investment returns are potentially subject to capital gains tax on growth and income tax on dividends, depending on the amounts involved. Inside a Stocks and Shares ISA or a pension, those taxes do not apply. The full return compounds, not the after-tax return.
This matters more than it might initially seem. A tax drag applied each year reduces the effective rate at which returns compound. Over decades, the difference between compounding on the full return and compounding on an after-tax return becomes significant. Tax wrappers remove that drag entirely.
A Lifetime ISA (LISA) also carries a government bonus on contributions, up to the annual limit. This is a form of immediate return before compounding begins - though the rules around when you can access the money are specific, so it is worth understanding them fully.
Frequently asked questions
What is compound interest in simple terms?
It is when your growth earns growth of its own. Each year you earn returns on your original money and on all the returns it has already made, so given enough time the total accelerates sharply upward.
Why does starting early matter more than timing the market?
Because compounding rewards time in the market far more than perfect timing. An early start gives your money more years to snowball, which usually beats starting later with more, or waiting for the 'perfect' moment that never reliably arrives.
See compound growth in your numbers
Written by Chris Willman, founder of Money Matrix Unplugged. This is financial education, not personal advice.