This page works the arithmetic of money left in cash: the quiet cost inflation charges it, the tax line that interest crosses sooner than most people expect, and the carve-out that genuinely belongs in cash whatever the numbers say. It exists because "safe" has two meanings, and cash only delivers one of them.
A cash balance is safe in the first sense: the number never goes down, and no market day can touch it. The second sense of safe is about what the money can buy, and there cash is quietly exposed, because prices rise while the number stands still.
Neither sense is the "right" one. Money needed next month should care only about the first. Money not needed for a decade is mostly exposed to the second. The mistake this page exists to make visible is applying the first meaning to money that lives on the second timescale, which is what "idle cash" is: money holding a full-time defensive position against a risk it no longer faces.
At the 2.5% long-run inflation assumption the Sonuswealth engine uses, cash earning nothing loses about a fortieth of its buying power every year. Slow enough to be invisible in any month, large enough to matter over any decade.
Interest softens this but rarely cures it: a rate below inflation is a slower version of the same drag, and interest above the tax lines below is taxed on the way through. The comparison that matters is not cash against a bad year in markets, which cash always wins, but cash against a decade of invested growth, which, at the engine's long-run assumptions, cash has never been positioned to win. Both statements are arithmetic, not prediction.
Information and guidance only. Not personal advice. Inflation and growth assumptions are the engine's long-run modelling defaults, not forecasts. Investment values can fall as well as rise and you may get back less than you put in; cash balances cannot fall in nominal terms. Verify decisions with a qualified FCA-authorised adviser before acting.
The personal savings allowance shelters the first slice of interest each year: £1,000 for basic-rate taxpayers, £500 at higher rate, nothing at additional rate. At an illustrative 4.5% savings rate, the line arrives sooner than intuition suggests.
| Taxpayer band | Allowance (2026/27) | Cash balance where 4.5% interest crosses it |
|---|---|---|
| Basic rate | £1,000 of interest | about £22,200 |
| Higher rate | £500 of interest | about £11,100 |
| Additional rate | £0 | first pound of interest |
Above the line, interest is taxed as income at the saver's marginal rate, which for a higher-rate taxpayer takes a 4.5% headline rate to 2.7% after tax, below the inflation assumption. Cash ISAs shelter interest from this entirely, within the overall £20,000 ISA allowance, with the announced £12,000 cash ISA cap for under-65s arriving from April 2027. A low earner's starting rate for savings can shelter up to £5,000 more. The line moves with rates and bands, but the shape is constant: the larger an idle balance grows, the more of its already-modest return the tax system takes back.
None of the arithmetic above applies to the emergency fund. Its job is to be there on a bad Tuesday, and for that job, the inflation drag is simply the premium on an insurance policy that pays out in liquidity.
The planning convention sizes that fund at roughly 3 to 6 months of essential spending, held somewhere instant-access, and treats it as spent from the investable total, exactly as the payday order does. Known near-term costs, the house deposit, the September tax bill, sit in cash too, because their date is fixed and markets do not respect dates.
"Idle" is everything beyond those jobs: the balance that grew past the buffer years ago and stayed, out of inertia or an understandable memory of 2008 or 2022. The honest description of that money is not "safe". It is insured against the wrong risk, paying the inflation premium above for protection it no longer needs. What to do about it is a decision with real trade-offs, timing, access, risk tolerance among them, and that decision belongs to the household making it, with the numbers in front of them.
Methodology. Buying power: £20,000 ÷ 1.02510 = £15,624 and £20,000 ÷ 1.02520 = £12,205. Allowance lines: £1,000 ÷ 0.045 = £22,222 and £500 ÷ 0.045 = £11,111, shown rounded. After-tax rate at higher rate: 4.5% × (1 − 0.40) = 2.7%. All arithmetic machine-checked before publication. Published 7 August 2026 · figures last verified 7 August 2026.
Sonuswealth is pre-launch. The app separates the cash doing its job from the cash that is idle, and shows what the idle part is quietly costing.