How does compound interest work in simple terms?

Compound interest means you earn interest on your interest, not just on your original amount. If you invest £1,000 at 7% and earn £70 in year one, in year two you earn 7% on £1,070 — not just £1,000. Each year the interest base grows, so growth accelerates over time. After 30 years at 7%, that £1,000 becomes £7,612 without adding another penny.

What is the Rule of 72?

The Rule of 72 is a shortcut for estimating how long it takes your money to double. Divide 72 by the annual interest rate. At 7%, your money doubles roughly every 10 years (72 ÷ 7 = 10.3). At 4%, it doubles every 18 years. It's a useful mental check — not an exact formula, but accurate enough for planning purposes.

Is compound interest good or bad?

Compound interest is good when it's working for you (savings, investments) and bad when it's working against you (debt). A credit card at 20% APR compounds just as relentlessly as a stocks and shares ISA at 7% — it just compounds in the wrong direction. The maths is identical. The difference is who benefits.

What is a realistic compound interest rate for UK savers?

It depends on what you're investing in. UK cash ISAs currently offer around 4–5% AER (this tracks the Bank of England base rate and fluctuates). UK stocks, over long periods, have historically returned 6–8% per year including dividends. The Barclays Equity Gilt Study puts the long-run real return on UK equities at around 7.3% annually over 20+ year periods. Past performance doesn't guarantee future returns, but it gives you a planning baseline.

Should I pay off debt or invest for compound interest?

If your debt interest rate is higher than what you'd earn investing, pay the debt first. A credit card at 20% APR is mathematically guaranteed to cost you more than a stocks and shares ISA is likely to earn. Once high-interest debt is clear, investing for compound interest makes sense. Low-rate debt (like a student loan or a mortgage below 4%) is a closer call and depends on your personal situation.

Finance

Compound Interest: The One Number That Changes How You Think About Money

One number explains why starting early matters more than earning more. Here's how compound interest actually works, the Rule of 72, and why ignoring it in both directions — savings and debt — is the most expensive mistake most people make.

7 April 2026

By Krishna Chaitanya, Software Engineer

At 18, someone puts £5,000 into a stocks and shares ISA and then, because life gets busy, largely forgets about it. No additional contributions. Just that £5,000 earning the UK equity average of around 7% a year. By 65, that forgotten investment is worth roughly £132,000.

Their university flatmate never invested a penny. Earned more at every stage of their career. But only started thinking seriously about retirement savings at 45. Put in £2,000 a month for 20 years. Total contributions: £480,000. Final pot: around £524,000.

The person who contributed nearly ten times more ends up with roughly four times the result. The difference is not discipline or income. It is time. Compound interest rewards the clock, not effort. Most people discover this far too late.

The Real Problem

There are two ways people get compound interest wrong, and both cost money.

The first is underestimating it. The standard objection to investing early is that small amounts "won't make a difference." A 22-year-old putting away £50 a month feels trivial, almost embarrassing compared to the savings rates people talk about online. But at 7% over 40 years, that £50 a month grows to around £131,000 from £24,000 in contributions. The gap between what you put in and what you end up with is entirely compound interest. Small amounts do matter. Dramatically.

The second mistake is ignoring debt. Compound interest does not care whether it is working for you or against you. A credit card at 20% APR compounds with exactly the same mathematical force as a well-performing investment portfolio. It just moves in the opposite direction. A £3,000 credit card balance left untouched for five years at 20% becomes over £7,400. Nobody frames it that way on the credit card statement, but that is what is happening.

The variable nobody talks about enough is time. Not rate, not principal. Time. Start five years earlier and the final number changes more dramatically than almost any other lever you can pull.

What You Need to Know

The Formula

The compound interest formula is:

A = P(1 + r/n)^(nt)

Where:

A = the final amount (what you end up with)
P = the principal (your starting amount)
r = the annual interest rate as a decimal (7% = 0.07)
n = how many times interest compounds per year (monthly = 12, annually = 1)
t = time in years

Run that for £10,000 at 7% compounded annually for 30 years: A = 10,000 x (1.07)^30 = £76,123.

You put in £10,000. You get back £76,123. The £66,123 difference is compound interest: interest on interest on interest, stacked for three decades.

The Rule of 72

There is a simpler mental tool that belongs in everyone's head. Divide 72 by your annual interest rate and you get a rough estimate of how many years it takes your money to double.

At 4% (current cash ISA range): money doubles in roughly 18 years
At 7% (UK equity historical average): money doubles in roughly 10 years
At 10%: money doubles in roughly 7.2 years

This is a mathematical identity, not a rule of thumb someone invented. It works because of how exponential growth approximates at typical interest rates. Use it as a quick sanity check before running the full calculation.

UK Context: What Rate Should You Use?

For cash ISAs and high-interest savings accounts, UK rates are currently sitting at 4-5% AER, tracking the Bank of England base rate. This fluctuates. Rates were near zero just a few years ago, so do not build a 30-year plan on the assumption they stay high.

For equities, the Barclays Equity Gilt Study (2024 edition) puts the long-run real return on UK equities at approximately 7.3% annually over 20-plus year periods. That is after inflation, which makes it a meaningful figure. Over the long run, UK stocks have significantly outpaced cash savings. The trade-off is volatility: equity returns are not smooth and you will see down years. That is the price of the higher average.

My view: for any time horizon over 10 years, an equity ISA is almost certainly the better vehicle over cash. The maths on this is hard to argue with once you look at the numbers side by side.

How the Compound Interest Calculator Solves This

Rather than running the formula by hand every time, the Compound Interest Calculator does the work in seconds. You input:

Principal -- your starting lump sum
Annual interest rate -- as a percentage
Time period -- in years
Monthly contribution -- optional, but often the most important variable

The calculator outputs three numbers: your final balance, total contributions you made, and total interest earned. That third number tends to change how people think about investing. When you see that £46,000 in contributions becomes £198,000, the £152,000 gap is not magic. It is maths that started running the moment you made the first contribution.

Try running your own numbers at the Compound Interest Calculator before reading the worked example below.

Worked Example

Compound Interest Calculator showing £10,000 growing to £76,123 over 30 years at 7%

Two scenarios, both at 7% annual return over 30 years:

Scenario A -- lump sum only You invest £10,000 today and add nothing more. After 30 years: £76,123. Your £10,000 grew to more than seven times its original value without any further contributions.

Scenario B -- monthly contributions only You invest nothing upfront but add £100 every month for 30 years. Total contributions: £36,000. After 30 years: £121,997. The market did £85,997 of the work.

Combined Invest the £10,000 lump sum and contribute £100 a month. After 30 years: approximately £198,000. Total money you put in: £46,000. Total interest earned: roughly £152,000.

The gap between £46,000 contributed and £198,000 received is not an accounting trick. That is compound interest: you earned interest on your returns, year after year, for three decades. No additional effort required after setting it up.

The monthly contribution scenario is the one most people overlook. You do not need a large lump sum to build serious wealth. A regular standing order into an ISA, starting in your twenties, does more work than a large investment made at 45.

What to Do With the Result

Start as early as possible, with whatever you have. Waiting until you have "enough" is, ironically, one of the most expensive decisions you can make with money. The cost of a five-year delay at 7% is larger than increasing your monthly contribution by 50% after that delay.

In the UK, the most tax-efficient way to capture compound growth is through a Stocks and Shares ISA. Your annual ISA allowance is £20,000. All gains within the ISA are free of capital gains tax and income tax, permanently, not just deferred. Over 30 years, the tax saving alone compounds into a meaningful sum.

Once you know your target figure, the next question is how much to contribute monthly to reach it. The Savings Goal Calculator works backwards from a target: input your goal, timeline, starting balance, and expected rate, and it tells you the monthly contribution required. Use both calculators together, one to understand what compound interest can do, the other to reverse-engineer a plan to make it happen.

Common Mistakes

Waiting until you have "enough" to invest. The threshold keeps moving. First it is £1,000, then it is after you pay off the car, then it is after you renovate the kitchen. The cost of waiting five years from 25 to 30, at 7%, is roughly 40% of your final portfolio, because you miss two doublings that would have happened in your thirties and forties. There is never a perfect time to start. Earlier always wins.

Ignoring fund fees. A 1% annual management fee sounds negligible. Over 30 years at 7%, it costs you roughly 25% of your final portfolio. Fees compound too: you pay 1% every year not just on your original investment but on your gains as well. Over three decades, a 1% fee versus a 0.2% fee (widely available with index funds) is tens of thousands of pounds on a mid-sized portfolio. Check the ongoing charges figure (OCF) before you invest.

Investing before clearing high-interest debt. Compound interest working for you at 7% cannot outrun compound interest working against you at 20% APR. If you have credit card debt, the guaranteed return from paying it off exceeds almost anything available in the market. Clear high-rate debt first, then redirect the payments into an ISA.

Start Now, Not Later

The most useful thing you can do with this information is run your own numbers, not to see impressive projections, but to understand the specific cost of waiting. Open the Compound Interest Calculator, put in your current savings or a starting amount, pick a realistic rate, and see what 10, 20, and 30 years looks like. Then move the start date back by five years and see what you lose.

That number has a name. It is the cost of waiting. And it is almost always larger than people expect.