Methodology
No black boxes. Here is how every Casvia calculator works, the formula it uses, and the official source for each UK figure.
Core growth model
The ISA and Savings calculators share the same growth model. Interest or returns are compounded monthly, and regular contributions are assumed to be paid at the end of each month. The future value is:
FV = P(1 + r/12)12t + M × [ (1 + r/12)12t − 1 ] / (r/12)
- P = initial amount (already in the ISA, or starting balance)
- M = monthly contribution
- r = assumed annual return (as a decimal, e.g. 0.05 for 5%)
- t = number of years
Each result is split into contributions (money you put in) and growth (everything earned on top), so you can see how much of the pot came from your own saving.
Lifetime ISA bonus (ISA Calculator)
For a Lifetime ISA, each year's contribution is capped at £4,000 and the government adds 25% (max £1,000 per year). The bonus is modelled as credited at the end of each year before the next year compounds. The £4,000 counts within your overall £20,000 ISA allowance. GOV.UK · 2026/27
Inflation adjustment (Savings Calculator)
When enabled, the nominal future value is discounted back to today's purchasing power: Real value = FV / (1 + i)t, where i is the annual inflation rate. Each year's contributions are discounted separately to today's pounds.
Key assumptions
- A constant annual return with monthly compounding — real returns vary year to year and are not guaranteed.
- No fees, charges or platform costs.
- No withdrawals during the period.
- ISA growth is modelled tax-free (correct for ISAs in the UK).
Official data sources & years
Every UK figure on this site comes from an official source and is labelled with the tax year it applies to. Current figures (2026/27 tax year, 6 April 2026 – 5 April 2027):
| Figure | Value | Tax year | Source |
|---|---|---|---|
| Adult ISA annual allowance | £20,000 | 2026/27 | GOV.UK — ISAs |
| Lifetime ISA annual limit | £4,000 (25% bonus) | 2026/27 | GOV.UK — Lifetime ISA |
| Junior ISA annual allowance | £9,000 | 2026/27 | HMRC — Junior ISAs |
| Personal Savings Allowance (interest) | £1,000 / £500 / £0 | 2026/27 | GOV.UK — Tax on savings interest |
| Cash ISA limit (under 65) — future | £12,000 | 2027/28 (from 6 Apr 2027) | GOV.UK — Autumn Budget 2025 |
Rules and thresholds can change. Figures above were checked against official sources on the date of publication. Always verify against the latest official guidance before making financial decisions.