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SavingEnter a starting balance, monthly contribution, expected annual rate and time horizon. This calculator projects month-by-month growth with monthly compounding and shows how much of the final total came from your money vs interest.
Runs entirely in your browser — your numbers are never sent to a server.
Compound interest is interest paid on both your original balance AND the interest already earned. Over decades this "interest on interest" becomes the majority of your final balance — often two or three times what you contributed.
Example: £200/month at 6% for 30 years:
We use the standard future value of a series with monthly compounding: FV = P(1+r/12)^(12n) + PMT × (((1+r/12)^(12n) − 1) / (r/12)) — where P is starting balance, PMT is monthly contribution, r is annual rate, n is years.
| Product type | Realistic long-run rate |
|---|---|
| UK easy-access savings | 3–5% (variable) |
| UK fixed-rate bonds | 4–5% |
| Global equity index tracker | 6–8% (real, long-run) |
| Balanced 60/40 portfolio | 4–6% |
| UK cash ISA | 3–5% (tax-free) |
£100/month at 6% for 40 years (starting age 25): ~£197,000
£100/month at 6% for 20 years (starting age 45): ~£46,000
Same £48,000 total contributed — different starting ages. Time in market matters more than amount contributed.
Projections use a fixed annual rate for illustration. Real markets don't return the same rate every year. Capital at risk for equity investments. General information only, not personalised advice.