ISA Calculator UK — Grow Your Money Tax-Free

ISA Calculator UK

Understanding Your Investment Returns

This calculator projects your returns using compound interest, where your earnings generate their own earnings over time. The power of compounding means that even small regular investments can grow into substantial wealth over long periods. For example, investing just Rs 5,000 per month at 12% expected returns for 25 years can grow to over Rs 1 crore — of which only Rs 15 lakh is your own money and Rs 85 lakh is compounding returns. The key factors that determine your final corpus are: the amount invested, the rate of return, the duration of investment, and the frequency of compounding.

Important Considerations

Past returns do not guarantee future performance, especially for market-linked instruments like mutual funds and equities. The returns shown are estimates based on the rate you enter. Equity investments carry market risk but have historically delivered 12-15% CAGR over 15+ year periods in India. Fixed income options like PPF (7.1%) and FD (6-7.5%) offer lower but more predictable returns. Diversifying across asset classes — equity, debt, gold, and real estate — reduces overall portfolio risk while optimizing returns for your risk tolerance.

ISA Allowance 2026/27 — What Changed

The overall ISA allowance for the 2026/27 tax year is £20,000, per the most recent Budget — a figure frozen since 2017/18, when it rose from £15,240. That overall £20,000 stands for 2026/27, but from 6 April 2027 the amount you can put into a cash ISA each year is capped at £12,000 for savers under 65 — a confirmed change announced at Autumn Budget 2025. The full £20,000 can still be used across stocks and shares and Innovative Finance ISAs, and over-65s keep the £20,000 cash limit. Always confirm against HMRC guidance before subscribing late in the year. Within that £20,000, the Lifetime ISA carries a £4,000 sub-limit: pay £4,000 into a LISA and only £16,000 remains for cash, stocks and shares, or Innovative Finance ISAs. The Junior ISA runs on a separate £9,000 allowance per child and does not touch your adult £20,000. Two structural rules matter more than the headline number. First, since 6 April 2024 you can subscribe to multiple ISAs of the same type in a single tax year — for example, two cash ISAs with different banks to chase rates — something previously banned (LISAs and JISAs are excluded from this easing). Second, flexible ISAs let you withdraw and replace money within the same tax year without burning allowance: pull £5,000 from a flexible cash ISA in June, repay it by 5 April, and your subscription total is unchanged. Flexibility is provider-optional and many stocks and shares ISAs still are not flexible, so check the terms before you rely on it.

Using the Full £20,000 — Split Strategies

How you split £20,000 should follow time horizon, not product marketing. Money needed within roughly five years belongs in cash; money untouched for five-plus years has historically been better rewarded in equities, though values can fall. A worked three-way split for a 30-year-old saving toward a first home and retirement: £4,000 into a Lifetime ISA (the government adds a 25% bonus — £1,000 — making £5,000 invested); £6,000 into a fixed-rate cash ISA at around 4% AER as a near-term goal pot; £10,000 into a stocks and shares ISA in a global index fund assuming 6% nominal annual growth. The maths: the £6,000 cash portion reaches about £6,749 after a three-year fix (£6,000 × 1.04³); the £10,000 equity portion at 6% compounds to roughly £17,908 over ten years (£10,000 × 1.06¹⁰); and the £5,000 LISA pot grows to about £8,954 on the same assumption — with the LISA locked until age 60 or a first-home purchase up to £450,000. Fees compound too: on that £10,000 over ten years, a 0.25% platform beats a 0.45% one by roughly £330 in end value. All rates shown are illustrative — cash ISA rates reset with the Bank of England base rate, so re-shop at every fixed term's maturity rather than rolling over by default.