Calculator
Investment Calculator
Project your portfolio's future value using monthly contributions, expected return assumptions, inflation and your chosen time horizon.
Assumptions explained in methodologyFigures are illustrative, not guarantees
Enter your figures
Common target amounts
Try a monthly amount
These percentages are examples, not forecasts. Real returns can be higher, lower or negative. Fees, inflation, tax and market conditions can all change the outcome.
The calculator assumes monthly contributions and monthly compounding. Changing the currency only changes how the figures are shown; it does not convert between currencies.
Your results
Result story
Use the headline number as a planning estimate, then compare it with total contributions and the target monthly amount. The useful question is not just “what could I have?” but “which inputs matter most?”
Growth Over Time
Save or share this scenario
The page URL captures everything you've entered above. Copy it to come back later or share these inputs with someone.
Milestone roadmap
A simple view of when you could reach each milestone, based on the numbers entered above.
Year-by-year projection
This table shows the estimate at the end of each year, along with the amount paid in and the estimated growth.
| Year | Age | Total contributed | Projected value | Estimated growth |
|---|
Tax and official tools
The projected value is shown before tax and platform fees. Inside an ISA the growth is sheltered, but in a General Investment Account gains and dividends can be taxed, so treat the figure as a ballpark rather than a net result.
Sources used for this calculator:
Where to open one
An investment calculator shows the growth; to act on it you would open a stocks and shares ISA or general investment account through an investment platform. The Knowledge Hub covers how to start investing and choosing an ISA platform.
Assumptions and review date
Last reviewed: 19 May 2026. This calculator compounds your chosen starting amount, monthly contribution, growth rate and time period.
It does not predict returns, adjust for inflation, model fees, tax, platform charges or investment volatility. Use conservative assumptions and treat the output as a planning estimate, not a forecast.
Understanding the investment calculator inputs
The calculator above is designed to be simple, but the assumptions you enter can make a big difference to the result.
Starting amount
The amount you already have invested or plan to invest at the beginning.
Monthly investment
The amount you plan to invest each month. Regular investing can make a big difference over long periods.
Target age
The age you want to model towards. A longer timeframe gives compound growth more time to work.
Expected growth
The annual return assumption you want to test. Compare several scenarios rather than relying on one forecast.
Investment growth examples
These examples show how regular monthly investing could build over 20 years using a 7% annual growth assumption.
| Monthly investment | Estimated value after 20 years | Total paid in | Estimated growth |
|---|---|---|---|
| £100/month | £52,093 | £24,000 | £28,093 |
| £250/month | £130,232 | £60,000 | £70,232 |
| £500/month | £260,463 | £120,000 | £140,463 |
| £1,000/month | £520,927 | £240,000 | £280,927 |
These figures are examples only. They are not predictions and do not include fees, tax, inflation or changes in market performance.
What annual return should I use?
No calculator can predict future returns, so it is usually better to compare a range of outcomes.
Cautious assumption
3%–4%A lower-growth example that may be useful when testing more conservative outcomes.
Moderate assumption
5%–7%A middle-ground example often used for long-term investment modelling.
Higher-growth assumption
8%–10%A more optimistic scenario. Higher assumptions should be treated with extra caution.
Why time matters when investing
Compound growth means your investment returns can start generating their own returns.
Over short periods, the effect of compounding may not look dramatic. Over 10, 20 or 30 years, the difference can become larger because both your original contributions and earlier investment growth have time to grow further.
This is why long-term investing is often about patience, consistency and staying invested through market ups and downs. The earlier money is invested, the longer it has to potentially benefit from compound growth.
Common investment calculator mistakes
Investment calculators are useful planning tools, but the result is only as helpful as the assumptions behind it.
Assuming returns are guaranteed
Investment calculators use assumptions. They cannot predict the future.
Ignoring inflation
A future portfolio value may look large, but inflation can reduce what that money is worth in real terms.
Forgetting fees
Platform fees, fund charges and advice fees can reduce returns over time.
Starting too late
Time is one of the biggest advantages an investor can have. Delaying can make a meaningful difference.
Using one scenario only
Test cautious, moderate and higher-growth assumptions so you understand a range of possible outcomes.
Investing without a cash buffer
Before investing, many people benefit from having accessible savings for unexpected costs.
Not sure how much cash to keep aside? Try the Emergency Fund Calculator.
Investment guides
Build your understanding with simple guides linked to long-term wealth planning.
Investment Calculator FAQs
Clear answers to common questions about using this calculator.
Is this investment calculator financial advice?
No. This calculator is for educational purposes only. It can help you explore possible outcomes, but it does not recommend any specific investment or financial product.
What return should I enter?
You can test different annual return assumptions, such as 4%, 6% or 8%. Since future returns are uncertain, it is usually better to compare several scenarios.
Does the calculator include inflation?
The current calculator focuses on nominal projected values. A future inflation-adjusted result can help show what a future portfolio could be worth in today’s money.
Can I use this for ISA investing?
Yes. You can use the calculator to estimate potential long-term investment growth inside a Stocks and Shares ISA, although it does not calculate tax or platform-specific rules.
Are investment returns guaranteed?
No. Investments can rise and fall in value, and you may get back less than you invest.