Savings & investing
Savings goal & compound interest calculator
Links marked * are commercial or go to our partner sites, which share our operator. Third-party companies may pay us commission. How we make money.
Plan how much to save each month to hit a target, or see how a lump sum and regular contributions grow over time with compound interest.
£
£
% AER
years
The Bank of England targets 2%. The real-terms figure below adjusts for this.% a year
- Future value
- £39,367
- In today's money
- £30,753
- Interest earned
- £8,367
| Starting balance | £1,000.00 |
|---|---|
| Total contributions£250.00 × 120 months | £31,000 |
| Interest / growth | £8,367 |
| Balance after 10 years | £39,367 |
Compound interest, monthly compounding. For a Cash ISA the headline rate is usually AER; for a Stocks & Shares ISA the figure you choose is a long-run assumption, not a guarantee — investments can go down as well as up.
Official rates & thresholds used
United Kingdom · tax year 2026/27 · last verified . These figures match the parameters embedded in this calculator.
| Parameter | Value | Notes |
|---|---|---|
| ISA allowance | £20,000 | Combined annual allowance across Cash, Stocks & Shares, Innovative Finance and Lifetime ISAs, fixed at £20,000 until April 2031. From 6 April 2027 no more than £12,000 of it can go into cash ISAs if you are under 65. |
| Junior ISA allowance | £9,000 | Per child each tax year. |
| Lifetime ISA allowance | £4,000 | Counts towards the £20,000 ISA cap. 25% government bonus on contributions. |
| Personal Savings Allowance | £1,000 / £500 / £0 | £1,000 basic rate, £500 higher rate, £0 additional rate. |
Primary sources: HMRC; HMRC; HMRC; HMRC. Always confirm live figures on gov.uk before acting.
How it works
- We compound your balance monthly. The starting balance grows as P × (1 + r)^n where r is the monthly rate and n is the number of months.
- Monthly contributions grow as a future-value annuity: PMT × ((1 + r)^n − 1) ÷ r. The two are added to give the total balance.
- The 'today's money' figure adjusts the future balance for your assumed inflation rate so you can see the real purchasing power, not just a nominal number.
Common questions
- What rate should I use?
- For cash, use the actual quoted AER. For long-term stock-market investing, many planners use 4–6% real (above inflation) as a long-run assumption — but past returns are not a guarantee.
- How do I work out how much to save each month?
- Try different monthly contribution figures until the future value matches your target. We will add a dedicated 'reverse' mode in a future update.
- Is the interest taxable?
- Inside an ISA, no. Outside, only the part above your Personal Savings Allowance is taxed — and only on cash interest, not capital growth or dividends.