How long to £100,000?
Set what you have, what you can add and what you expect to earn on it. The answer comes in two forms: the number the account will show, and what that money will be worth by the time you get there.
Why there are two answers
£100,000 in twenty years is not £100,000. At 3% inflation it buys what about £55,000 buys today. A calculator that shows only the cash figure is not wrong, exactly — the account really will say £100,000 — but it answers a question nobody is asking. People want to know what the money will do, so both numbers are on the page and the honest one is the one we lead the sentence with.
One more thing, quietly: the annual growth rate is converted to its true monthly equivalent rather than divided by twelve. Dividing by twelve turns a 12% assumption into 12.68% by the end of the first year, and compounds that error every year after. It makes every projection look better. It is also wrong.
Questions
Two answers, and which one to believe
Why are there two answers, and which one is real?
Because “£100,000” in fifteen years is not £100,000. The first answer is when the balance on the screen reaches the target. The second is when it is worth the target in today's money, after inflation.
The second one is the real answer, and it is the one almost nobody publishes. At £200 a month the gap between them is about twenty-one years — the cash figure arrives in 22 years 10 months, and the figure that actually buys what you had in mind arrives in 43 years 8 months. Quoting only the first is the most flattering thing a calculator can do, so we show both and treat the second as the answer.
What growth rate should I assume?
Nobody knows, and any site that tells you otherwise is guessing with more confidence than the evidence supports. The calculator starts at 5 per cent a year because it is a defensible middle, not because it is a forecast.
What is worth doing is moving it and watching what happens. The durations are far more sensitive to what you pay in than to what you assume about returns, which is the opposite of where most people put their attention. Two points on the growth rate change the answer by a couple of years; doubling the contribution halves it.
One technical note, because it changes the numbers more than people expect: we convert the annual rate to its true monthly equivalent rather than dividing by twelve. Dividing by twelve and compounding monthly quietly turns a 5 per cent assumption into slightly more than 5 per cent, and over decades that is not a rounding difference.
Where should the money actually sit while it grows?
For a target of this size, usually inside an ISA, and the tax question mostly answers itself. £20,000 can go into ISAs in the 2026 to 2027 tax year, which is more than £1,600 a month — more than any of the contribution levels on this page. Growth and withdrawals inside one are untaxed.
If you are saving towards a first home, a Lifetime ISA is worth knowing about: £4,000 a year, with a 25% bonus up to £1,000 — and it counts towards the £20,000 ISA limit. On a £200-a-month contribution that bonus is worth more than any plausible difference in investment return. It is also restrictive — pay in before 50, open before 40. withdrawing for anything other than a first home or age 60 carries a charge.
This is information, not a recommendation. Which wrapper suits you depends on things this page knows nothing about.
Does the calculator account for tax, fees or pay rises?
No, and it says so rather than hiding it. It models one pot, one contribution and one growth rate. It does not model platform fees, fund charges, tax on anything held outside a wrapper, or the fact that most people pay in more as they earn more.
Fees and unwrapped tax push the answer out; rising contributions pull it in. We would rather be clear about what is missing than produce a more complicated number that is wrong in ways you cannot see.
The Swealth letter
One letter a week, on Thursday
Plain writing about British money, and the UK Wealth Blueprint to start with.