ISA Calculator
Project a Stocks & Shares or Cash ISA for 2026/27 and beyond: monthly or yearly contributions within the £20,000 allowance (and the £12,000 cash limit from April 2027), fees, inflation, and the tax you avoid compared with a taxable account.
How it works
Each month: balance × (1 + (return − fees) ÷ 12) + monthly contribution · yearly contribution ≤ £20,000 − other ISAs (cash ISA ≤ £12,000 from 2027/28 if under 65) · taxable account: dividends above £500 taxed at 10.75% / 35.75% / 39.35%, gains above £3,000 at 18% / 24%, interest above the Personal Savings Allowance at 20/40/45% (22/42/47% from 2027/28)
An Individual Savings Account shelters interest, dividends and capital gains from UK tax, and withdrawals are tax-free. In the 2026/27 tax year (6 April 2026 to 5 April 2027) you can pay in up to £20,000 across Cash, Stocks & Shares, Lifetime and Innovative Finance ISAs; unused allowance does not roll over, and transfers of old ISA money do not use it. From 6 April 2027 the overall limit stays at £20,000, but people under 65 can put at most £12,000 of it into cash ISAs. The calculator grows your balance month by month at the expected return minus fees, adds contributions (monthly, or a lump sum each April), caps them at the allowance left after any other ISAs, and can raise them each year. It then runs the same money through a taxable account: for investments, dividends above the £500 allowance are taxed each year at the 2026/27 dividend rates (10.75%, 35.75% or 39.35%) and the remaining gain above the £3,000 exemption is taxed at 18% or 24% when sold; for cash, interest above your Personal Savings Allowance is taxed at your savings rate, which rises by 2 points from April 2027. The difference is what the ISA saves. Figures in today’s money divide by the inflation rate you choose.
Worked example
£15,000 already invested plus £500 a month at 6% with 0.5% fees for 20 years: the net monthly rate is 5.5% ÷ 12 = 0.4583%, so the ISA grows to about £262,763, of which £127,763 is growth (about £160,357 in today’s money at 2.5% inflation). The same money in a taxable account, with a 2% dividend yield, pays about £12,485 of dividend tax at the higher rate over the 20 years and £18,090 of capital gains tax on sale, ending near £227,588, so the ISA is about £35,175 ahead. Fees cost about £17,910 over the period.
Sources: GOV.UK — Individual Savings Accounts (2026 to 2027 allowance) · GOV.UK — Cash ISA limit reduction from 6 April 2027 · GOV.UK — Change to tax rates for property, savings and dividend income (technical note)
Frequently asked questions
Can I have more than one ISA?
Yes. Since April 2024 you can pay into several ISAs of the same type in one tax year, as long as the total stays within £20,000. Enter what you pay into the others under “Paid into other ISAs each year” and the calculator caps this one at the room left.
What changes for cash ISAs in April 2027?
From 6 April 2027 the yearly limit for cash ISAs falls to £12,000 for anyone under 65. The overall £20,000 allowance stays, so the other £8,000 can go into a Stocks & Shares ISA. People aged 65 or over keep the full £20,000 cash limit. Choose Cash ISA and enter your age to see the effect.
What happens if I withdraw?
With a flexible ISA you can take money out and put it back in the same tax year without using more allowance. With other ISAs, money you withdraw has still used that year’s allowance.
How does the taxable-account comparison work?
It applies the 2026/27 dividend rates (10.75%, 35.75% or 39.35%) to dividends above £500 each year, and capital gains tax at 18% or 24% to the gain above £3,000 if you sell at the end. For cash it taxes interest above your Personal Savings Allowance. Turn off “allowances are free” if other savings already use them.
Is a monthly contribution or a yearly lump sum better?
If you have the money in April, investing it at the start of the tax year gives it longer to grow, so the lump-sum option usually ends slightly higher. Monthly payments spread the risk of investing just before a fall.
Related calculators
Uses 2026/27 UK rates and allowances (England, Wales and Northern Ireland income tax bands; dividend and savings rates apply UK-wide). Thresholds change every April — check GOV.UK for the current figures. Investments can fall as well as rise; projections are not guaranteed. Educational estimate, not tax or financial advice.