What this calculator does
Takes current ISA balance, annual contribution amount (up to the ISA allowance), expected annual return, and a target value. Returns the projected time to reach £1,000,000 (or a custom target) and a year-by-year balance projection.
The formula
Future value with annual contributions (end-of-year contributions): V(n) = P × (1 + r)^n + C × [(1 + r)^n − 1] / r Where: P = current balance C = annual contribution r = annual return rate (decimal) n = years ISA allowance (2025/26): £20,000/year Target reached when V(n) ≥ target (default £1,000,000)
Assumptions
- Annual lump-sum contribution is made at the end of each tax year.
- Constant return rate throughout the projection.
- No tax on growth or income within the ISA wrapper.
- No platform or fund fees are modelled.
- The ISA allowance is modelled at the current year's figure and is not assumed to increase in future years.
Data sources
| Figure | Value used | Source | Last checked |
|---|---|---|---|
| ISA annual allowance | £20,000 | HMRC — Individual Savings Accounts | May 2026 |
Limitations
- Does not model ISA allowance increases in future years.
- Does not account for platform or fund fees, which reduce net returns over time.
- Uses a constant return rate — actual market returns vary year to year.
- Does not model withdrawals or flexible ISA recontributions.
Worked example
Inputs: £10,000 current balance, £15,000/year contributions, 7% annual return.
Year 1: £10,000 × 1.07 + £15,000 = £25,700 Year 5: ≈ £103,000 Year 10: ≈ £232,000 Year 15: ≈ £438,000 Year 20: ≈ £750,000 Year 23: ≈ £1,007,000 → target reached in approximately 23 years
Changelog
| Date | Change |
|---|---|
| May 2026 | Initial publication |