Starting balance
This is the amount already saved or invested at the beginning.
One-off lump sums (optional)
Add extra contributions in specific years without changing the regular monthly amount.
No additional lump sums added.
Timing assumption: each lump sum is added at the start of the selected year. For example, Year 3 is added after 24 months and then compounds for the rest of the term.
Estimated final value—
Enter your details to calculate.
Compound interest calculator guide
Compound growth means returns can build on both your original balance and previous returns. Add regular monthly contributions and optional one-off lump sums to model a more realistic saving or investment plan.
Regular contributions
Adding money each month can have a major effect because each contribution gets more time to grow.
One-off lump sums
Add one or more extra contributions in a chosen year. Each is assumed to be invested at the start of that year.
Annual return
The rate is an estimate, not a guarantee. Small differences in rate can become more noticeable over long periods.
Projection assumptions & sensitivity
This is a mathematical projection, not a forecast. It assumes the same annual return every month for the whole period, regular contributions continue unchanged, selected lump sums are added at the start of their chosen year, and ignores investment fees, tax, inflation and market volatility.
The result now shows scenarios at the entered annual return and two percentage points lower/higher. This is a sensitivity check, not an expected range of outcomes.
Why this matters: long-term compound results are very sensitive to the return assumption. A small change in annual return can create a large difference over many years.
Consumer guidance: MoneyHelper investing guidance
Frequently asked questions
What is compound interest?
It is growth earned on both the original amount and previously accumulated growth.
Why does time matter so much?
Compounding has more opportunities to build on itself over longer periods.
Are investment returns guaranteed?
No. Investment returns can rise or fall, so projections are estimates only.
Built for quick, transparent estimates. See how our calculators work and check page-specific assumptions before relying on a result.