Savings Growth Calculator UK
Compound interest is the most reliable force in personal finance — but it's almost impossible to visualise without seeing the numbers. This tool shows you not just the end balance, but when the real growth starts, how much is interest versus contributions, and what inflation does to the real value of what you're building.
Compound interest means you earn interest not just on your original deposit, but on all the interest you have already earned. This creates a snowball effect: in the early years, growth is slow because the balance is small. In later years, the same percentage rate is applied to a much larger balance, generating substantially more interest in absolute terms.
On a £10,000 deposit at 5% annual interest, you earn £500 in year one. By year 10, assuming monthly compounding and no additional contributions, the balance is £16,470 and the interest in that year alone is £783. By year 20 the balance is £27,126 and annual interest is £1,290. The rate hasn't changed — the base it applies to has. This is why starting early matters far more than starting with a large sum.
The frequency with which interest is calculated and added to your balance affects the final outcome, though for most savings accounts the difference is modest. Monthly compounding means interest is calculated on the current balance each month and added to it, so subsequent months earn interest on a slightly larger total. Annual compounding adds interest only once a year.
On £10,000 at 5% over 10 years: annual compounding produces £16,289; monthly compounding produces £16,470 — a difference of £181. Over longer periods and higher balances the difference grows, but it is rarely the deciding factor when choosing between accounts. The rate itself matters far more than the compounding frequency. Most UK savings accounts use monthly or annual compounding — check the account terms or AER (Annual Equivalent Rate), which standardises the effective rate regardless of compounding frequency.
UK savings accounts advertise two rates: the gross rate (the annual interest rate before tax) and the AER (Annual Equivalent Rate), which accounts for how often interest is compounded. When comparing accounts, always use the AER — it is the standardised figure that allows like-for-like comparison regardless of whether interest is paid monthly, quarterly, or annually.
Interest on savings outside an ISA is paid gross and counts as taxable income. Basic rate taxpayers have a £1,000 Personal Savings Allowance (PSA) — meaning the first £1,000 of savings interest each tax year is tax-free. Higher rate taxpayers have a £500 PSA. Additional rate taxpayers have no allowance. Above these thresholds, interest is added to your income and taxed at your marginal rate. At 4.5% AER, you would need around £22,000 saved to exceed the basic rate PSA — so for most savers, tax on savings interest is not a current concern.
The nominal balance shown in the calculator is what your account will show. The real value — shown adjusted for 3% inflation — is what that balance is worth in terms of today's spending power. If inflation averages 3% and your savings earn 4.5%, your real return is approximately 1.5% per year. If inflation runs at 4% and your account pays 4%, you are treading water in real terms despite your balance growing.
This is particularly relevant for long-term savings goals. A house deposit target of £30,000 today will need to be higher in five years if property prices keep pace with inflation. Building your savings target with inflation in mind — rather than a fixed nominal figure — gives a more accurate view of whether you are genuinely making progress.
The optimal savings account depends on how long you can leave the money untouched. Easy access accounts pay competitive rates and allow withdrawals at any time — currently the best pay around 4.5–4.75% AER. Fixed-rate bonds lock your money for a set term (typically 1–5 years) in exchange for a higher rate and rate certainty. At current rates, the premium for fixing for 1 year over easy access is small — around 0.1–0.2%. For 5-year fixes the premium has been slightly larger.
For short-term goals (under 2 years), easy access or a short-term fixed bond makes sense. For medium-term goals (2–5 years), a fixed bond or cash ISA with a competitive rate locks in your return. For goals beyond 5 years, a Stocks and Shares ISA invested in diversified index funds has historically returned 7–9% annually — significantly above any cash savings rate — though with short-term volatility. Use this calculator to see how much that rate difference compounds over your chosen timeframe.