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Compound Interest Calculator UK
Estimate how a lump sum and regular monthly contributions could grow over time using compound interest or investment growth assumptions.
Savings and investment growth
Monthly contribution support
Growth breakdown
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Calculate compound growth
Enter your starting amount, monthly contribution, annual growth rate and time period. The calculator updates the projection automatically.
Your initial savings or investment balance.
Amount added each month.
Use a realistic estimate. Investments can fall.
Projection period in years.
Monthly is a useful default for planning. Daily and yearly are shown for comparison.
How to use this calculator
- Enter your starting amount.
- Add the amount you expect to save or invest each month.
- Choose an annual rate and projection period.
- Review the final balance, total contributions and estimated growth.
What your compound interest result means
The final balance is the estimated value after your starting amount, monthly contributions and compound growth have been applied over the selected period.
The growth earned figure shows how much of the final balance came from interest or investment growth rather than from your own contributions.
Projection warning: this calculator is an estimate. Savings rates can change and investment returns can be negative.
Want to use compounding in a tax wrapper?
Compare how compound growth could work inside an ISA, pension or general investment account.
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How compound interest works
Compound interest is growth on growth. Instead of only earning interest on your original amount, future interest is calculated on a balance that already includes earlier interest or investment growth.
This is why time matters. The longer money stays saved or invested, the more opportunity there is for previous growth to become part of the next calculation.
With monthly contributions, compounding works alongside regular saving. Each new contribution adds to the balance, and future growth can then apply to both the original money and the money added along the way.
The same principle can apply to savings accounts, ISAs, pensions and investments, although the risk is different. A savings account may offer more predictable interest, while investments can rise and fall.
What affects your compound interest result
- Starting amount: a larger starting balance gives growth more money to work on from day one.
- Monthly contribution: regular saving can become a major part of the final balance.
- Annual rate: a higher assumed rate increases the projection, but may also be less realistic.
- Time period: longer periods usually make compounding more powerful.
- Compounding frequency: more frequent compounding can slightly increase the result.
- Inflation: inflation can reduce the real-world value of the final amount.
Compound interest calculator formula
The calculator uses the compound growth formula for the starting amount and simulates monthly contributions so the contribution timing is easy to understand.
period_rate = annual_rate / compounding_periods
periods = years × compounding_periods
future_start =
starting_amount × (1 + period_rate)^periods
For monthly contributions:
balance = balance × (1 + monthly_rate) + monthly_contribution
total_contributed =
starting_amount + monthly_contribution × months
growth_earned =
final_balance - total_contributed
Compound growth example
This example shows how time changes the result when the starting balance and monthly contribution stay the same.
| Example assumption | Shorter period | Longer period |
|---|---|---|
| Starting amount | £1,000 | £1,000 |
| Monthly contribution | £100 | £100 |
| Annual rate | 5% | 5% |
| Time | 5 years | 20 years |
| What changes | Most of the balance is from contributions. | Growth becomes a much larger part of the result. |
Compound interest calculator FAQs
What is compound interest?
Compound interest means growth is added to the balance, then future growth is calculated on that larger balance.
How often should interest compound?
It depends on the account or investment. Monthly compounding is a useful planning default, but some accounts compound daily or annually.
Can I use this for investments?
Yes, you can use it as a growth projection for investments, but investment returns are not guaranteed and can be negative.
Why does time make such a big difference?
Time allows earlier growth to become part of the balance that earns future growth. This is why compounding often becomes more noticeable over longer periods.
Is the result guaranteed?
No. The result depends on the assumptions you enter. Real savings rates and investment returns can change.
Key terms used in this calculator
These glossary pages explain the main terms used when projecting savings and investment growth.