How much will my savings grow?

Compound interest on a starting balance plus regular deposits. £5,000 plus £300 a month at 4% for ten years grows to about £51,700 in an ISA. In an ordinary account a higher-rate taxpayer pays tax on the interest above the £500 personal savings allowance, so ends with less: this calculator shows both.

Do I pay tax on savings interest?

Interest is taxable income above the personal savings allowance: £1,000 for basic-rate taxpayers, £500 for higher-rate, nothing for additional-rate. Above that it is taxed at your band: 20/40/45% now, and 22/42/47% from April 2027. Interest inside an ISA is never taxed, which is why the ISA allowance of £20,000 a year matters even for cash.

Is a cash ISA worth it?

If your interest stays inside your personal savings allowance, an ordinary account paying the same rate is equally good. Once your balance or your tax band pushes interest above the allowance, the ISA keeps what the taxable account loses: the calculator shows the difference on your own figures.

Savings calculator: regular saving, compound interest, and the tax on it

A starting balance and monthly deposits compounded at your rate, then the same money in an ISA and in an ordinary account side by side, with the tax on interest above your personal savings allowance actually charged. Or turn it round: how much a month reaches a target by a date. Your figures never leave your browser.

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Worked example

Computed when this page was built by the same engine that runs the calculator above, so the figures below and the live tool can never disagree. Change any input above to see your own.

Inputs
Starting balance	£5,000
Saving a month	£300
Years	10
Interest rate	4.0%
Other taxable income	£55,000 (higher-rate taxpayer)

Results
Paid in	£41,000
In an ISA after 10 years	£51,629
In an ordinary account	£49,009
Tax paid on interest	£2,397
The ISA keeps	£2,620 more

The next question

ISA or pension for the next pound? →ISA vs pension calculator

Or answer it with everything else: the full Hundred Summers plan connects your pay, pensions, savings, property, tax and spending year by year, and stress-tests them across 500 simulated market histories.

Read the guides

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The layers of a UK pension charge (provider, fund OCF, transaction costs, adviser) the 0.75% auto-enrolment cap and the 1.5%/1% stakeholder cap, what 0.8 percentage points does to a pot over 25 years, and the guarantees worth more than the saving before you move an expensive old pension.
Overpay the mortgage or invest? The honest arithmetic →
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Inheriting money: the order of operations →
A lump sum arrives: inheritance, redundancy, a house sale. The unhurried sequence: debt, buffer, wrappers, the tax on what the money earns next, and the £2,880 trick for a non-earning partner.
Premium Bonds, cash and tax: where the money you might need should sit →
The personal savings allowance and why higher-rate savers cross it at ordinary balances, the after-tax comparison that decides Premium Bonds against a savings account, cash ISAs, and what inflation does to money that sits still.
Inflation: the assumption everything rests on →
What 2.5% a year does to money over a retirement, what UK prices actually did since 1988 on the ONS index, why every figure in a Hundred Summers plan is in today's money, and how the plan is stress-tested against inflation that does not behave.

The tax the savings calculators forget

Most savings calculators stop at compound interest. But interest outside an ISA is income: above the personal savings allowance (£1,000 for basic-rate taxpayers, £500 for higher-rate, nothing at the additional rate) it is taxed at your band, and from April 2027 the savings rates rise to 22/42/47%. At today’s cash rates a higher-rate taxpayer crosses the allowance with a balance of little over £10,000, so the “ISA or not” question is live for far more people than it was when rates were near zero.

Reading the result

The two columns use identical deposits and the identical rate; the only difference is tax, charged each year on that year’s interest and taken from the balance. If they match, your interest sits inside the allowance and the wrapper does not matter yet. The inflation calculator shows the real return on the same rate; the ISA vs pension calculator asks the bigger question about where long-term money should go; and the Hundred Summers app tracks every account with its wrapper, so the allowance and the £20,000 ISA limit are applied in every projected year.

Education, not advice. One assumed rate throughout; savings rates move with the Bank Rate. The starting rate for savings is applied: with other taxable income under £17,570, up to £5,000 of interest can be tax-free before the personal savings allowance is used. From 6 April 2027 an under-65 can put at most £12,000 a year into cash ISAs (the £20,000 overall allowance is unchanged). For a personal recommendation speak to an FCA-regulated financial adviser; free impartial guidance is available from MoneyHelper.

Sources

HMRC, Tax on savings interest (personal savings allowance and starting rate).
HMRC, Individual Savings Accounts (the £20,000 allowance).
Savings rates from April 2027: HM Treasury, Autumn Budget 2025.