Saving & Investing
See what your money could grow into.
Estimate how a starting balance and regular contributions could grow through compound interest. Your result is separated into the money you contributed and the growth earned over time.
Compound Interest Calculator
Model a starting balance, regular contributions and compound growth.
Your starting amount and regular contributions could grow over time through compound interest.
See how much comes from your own money and how much comes from estimated growth.
How your balance could grow
Illustrative year-by-year projection based on your inputs.
Try changing your contribution, interest rate or saving period to see which makes the greatest difference.
This is an illustrative estimate, not a guaranteed return. Actual savings or investment growth may differ because rates, charges, taxes and market performance can change.
What Your Compound Interest Result Means
Your projected final balance is made up of the money you contribute and the estimated growth produced by the assumptions entered into the calculator.
The projected total at the end of the selected period.
Your starting amount plus any regular contributions.
The difference created by the assumed rate and compounding over time.
The final balance is not all interest or investment growth. Part of it is the money you put in yourself, while the remainder represents the estimated growth produced over time. Looking at this split helps you understand how much of the result comes from contributions and how much comes from compounding.
The calculator result is illustrative. Savings rates and investment returns can change, and actual outcomes may differ.
How Compound Interest Builds Your Balance
Compound interest means that interest or growth already added to your balance can itself contribute to future growth. Instead of calculating every future return only from the money you originally deposited, the calculation gradually uses a larger balance.
The effect is often relatively modest during the early years because there has not yet been much time for previous growth to build upon itself. As the period becomes longer, the difference can become more noticeable because the process has had more opportunities to repeat.
If you want a complete explanation before exploring the numbers further, What Is Compound Interest? explains the principle from the beginning, while Why Compound Interest Is So Powerful looks more closely at why the effect can accelerate over longer periods.
How Compound Growth Builds Over Time
The calculation repeats the same basic process throughout the period you selected.
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Money is added
Your starting balance and any regular contributions create the amount available to grow.
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Interest or growth is applied
The calculator applies the annual rate you entered according to the selected compounding frequency.
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Growth joins the balance
The estimated return becomes part of the balance rather than remaining separate.
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The larger balance grows again
Future growth is then calculated using both your contributions and earlier growth.
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The process repeats
Over longer periods, repeated compounding can make an increasing share of the final balance come from growth.
Compound interest becomes powerful because earlier growth is given further opportunities to generate growth of its own. Time therefore influences more than the number of years in the calculation.
This is a simplified explanation. Real savings rates and investment returns may change over time.
Understanding Your Calculator Results
Estimated final balance
The estimated final balance is the projected value at the end of the period you selected. It combines your starting balance, all regular contributions and the estimated growth produced by the annual rate and compounding assumptions.
This figure should be treated as an illustration rather than a forecast. If the actual interest rate or investment return changes, the eventual balance will also change.
Total contributed
Total contributed represents the money you have supplied yourself. This normally includes the starting balance together with all regular contributions made throughout the calculation period.
Separating contributions from growth is useful because a large final balance does not necessarily mean that most of the money came from compound interest. Particularly during shorter periods, your own deposits may still account for most of the total.
Estimated growth
Estimated growth is the difference between the money contributed and the projected final balance. For savings accounts this may represent interest earned, while for investment illustrations it represents assumed growth rather than a fixed or guaranteed return.
As the investment or saving period becomes longer, this part of the balance may become increasingly significant because earlier growth has more time to participate in future growth.
Growth multiple
If the calculator shows a growth multiple, this compares the projected final balance with the total amount contributed. For example, a multiple of 1.50× means the projected balance is one and a half times the amount contributed.
It does not mean that the annual return was 50%, and it should not be interpreted as a guaranteed investment return. It is simply another way of showing the relationship between your deposits and the projected final value.
The Four Main Factors Behind Compound Growth
Each input affects the calculation differently. Changing one factor at a time makes it easier to understand what is driving the projected result.
Starting Amount
Immediate effectA larger opening balance gives compound growth more money to work on from the beginning.
Regular Contributions
Builds steadilyEach contribution increases the balance available to earn future interest or investment growth.
Annual Rate
Compounds over timeSmall differences in the assumed rate can create much wider differences when repeated over many years.
Time Period
More years to growA longer period gives contributions and previous growth more opportunities to compound.
The four factors work together. A larger balance provides more money to grow, regular contributions keep adding to it, the assumed rate determines the pace of growth and time allows the process to repeat.
The relative importance of each factor depends on the figures entered. Higher assumed returns may also involve greater uncertainty where the calculation is being used to illustrate investment growth.
What Does Compound Growth Look Like in Practice?
The easiest way to understand the interaction between these inputs is to keep most of them fixed and look at one realistic example. The figures below are not intended to represent an expected investment return; they simply demonstrate how a starting balance, regular contributions, an assumed rate and time combine within the calculation.
£10,000 With £250 Added Each Month
This illustration shows how regular contributions and compound growth can combine over a 20-year period.
£10,000 starting balance with £250 added every month
The example assumes a fixed 5% annual rate, monthly compounding and contributions made at the end of each month.
- Starting amount
- £10,000
- Monthly contribution
- £250
- Assumed annual rate
- 5%
- Time period
- 20 years
The £70,000 contributed remains an important part of the final balance, but a longer period also gives the starting amount, monthly contributions and previous growth repeated opportunities to compound.
Use the calculator above to change one assumption at a time and see how your own scenario responds.
Return to the Compound Interest CalculatorFigures are rounded for illustration. The example assumes a constant 5% annual rate throughout and does not account for fees, tax, inflation or changing returns.
What the Compound Interest Calculator Includes and Excludes
The calculator applies the figures you enter consistently across the selected period. Understanding what is and is not included helps you interpret the projected result more realistically.
Included in the calculation
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Starting balance
The amount entered is treated as being available from the beginning of the calculation.
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Regular contributions
The contribution amount and frequency you select are assumed to continue throughout the full calculation period.
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Annual rate
The same annual interest rate or assumed return is applied throughout the illustration.
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Compounding frequency
Interest or growth is added according to the daily, monthly, quarterly or yearly frequency selected.
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Contribution timing
The result reflects whether regular contributions are added at the beginning or end of each contribution period.
Not included in the calculation
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Changing rates or returns
Savings rates and investment returns can change over time and may be higher or lower than the figure entered.
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Fees and charges
Platform fees, fund charges, account fees and transaction costs are not deducted unless they are specifically modelled elsewhere.
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Tax
The calculation does not account for income tax, dividend tax, capital gains tax or your individual tax circumstances.
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Inflation
The projected balance is shown in future pounds and does not reflect how rising prices may reduce its spending power.
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Withdrawals or interrupted contributions
The calculation assumes the money remains in place and scheduled contributions continue without interruption.
The projected balance is an illustration based on consistent assumptions rather than a prediction of what will happen. Changing real-world conditions can produce a different outcome.
Investment values can rise or fall, and actual savings rates, investment returns, fees, tax and inflation may differ from the assumptions entered.
Why Calculator Assumptions Matter
A compound interest calculation can only work with the assumptions supplied to it. If you enter a 5% annual rate for 30 years, for example, the calculator applies that rate consistently throughout the period even though real savings rates and investment returns may change many times.
This does not make the calculation unhelpful. It means the result is best used for comparing scenarios rather than predicting an exact future balance. You might compare 3%, 5% and 7% assumptions, or see what happens when the investment period changes from 15 years to 25 years.
Changing one assumption at a time is usually the clearest approach because it allows you to see which input is responsible for the difference in the result.
Common Compound Interest Calculator Mistakes
The calculator is straightforward to use, but a few common misunderstandings can make a projection appear more certain or more impressive than it really is.
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Using an unrealistic annual return
Why it mattersEntering a very high annual return can produce an attractive projected balance, but the result may depend on assumptions that are difficult to achieve consistently.
A better approachCompare several measured assumptions rather than relying on a single optimistic rate.
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Treating the projection as a guarantee
Why it mattersThe calculator assumes the selected rate continues throughout the entire period. Real savings rates and investment returns can change.
A better approachTreat the result as an illustration of one possible scenario rather than a forecast or promise.
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Ignoring inflation
Why it mattersA future balance may look much larger in pounds, but those pounds may buy less if prices rise over the same period.
A better approachConsider purchasing power separately when the goal is many years away.
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Confusing contributions with compound growth
Why it mattersA large final balance may still contain a substantial amount of money that you contributed yourself.
A better approachCompare total contributions with estimated growth rather than looking only at the final balance.
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Changing several inputs at once
Why it mattersChanging the starting amount, rate, contribution and time period together makes it difficult to understand which factor caused the result to change.
A better approachAdjust one input at a time when you want to understand how that particular assumption affects the projection.
The calculator is most useful as a comparison tool. Sensible assumptions and small controlled changes make it easier to understand how compound growth works.
Calculator results are illustrative and should not be treated as guaranteed savings or investment outcomes.
Learn More About Compound Interest
The calculator shows the numbers. These guides explain the ideas behind them, from the basic principle of compounding through to the long-term effect of time.
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Start with the basics What Is Compound Interest?
Understand what compound interest means, how it works and why earlier growth can begin generating further growth.
Read the guide -
Compare the methods Simple Interest vs Compound Interest
See why simple and compound interest can produce increasingly different outcomes as the time period becomes longer.
Compare simple and compound interest -
Understand the effect Why Compound Interest Is So Powerful
Explore why compound growth may appear modest at first before becoming much more noticeable over longer periods.
Explore compound growth -
Understand the frequency How Often Does Compound Interest Compound?
Compare daily, monthly, quarterly and annual compounding and understand how the frequency can influence the result.
Understand compounding frequency -
Compare starting points Why Starting Early Makes Such a Difference
See how giving contributions more years to grow can create a substantial difference between otherwise similar scenarios.
See why starting earlier matters -
Think long term Why Time Is Your Greatest Investing Advantage
Understand why time gives contributions and previous growth more opportunities to compound over a long-term financial journey.
Explore the advantage of time
If compound interest is new to you, begin with What Is Compound Interest? If you already understand the principle, choose the guide that matches the specific part of your calculator result you want to understand better.
Other Saving & Investing Calculators
The Compound Interest Calculator is useful when you want to see how a balance may grow through compounding. Depending on the question you are trying to answer, one of these calculators may be more appropriate.
Regular Savings Growth Calculator
If regular contributions are the main focus of your plan, the Regular Savings Growth Calculator is designed specifically to show how recurring deposits may build over time.
Future Value Calculator
If you already have a lump sum and mainly want to estimate what it could be worth at a future date, use the Future Value Calculator.
Savings Time Calculator
If you know the amount you want to reach but not how long it could take, the Savings Time Calculator estimates the time required based on your starting balance, contributions and assumed rate.
Interest Rate Calculator
If the future value and time period are already known, the Interest Rate Calculator can estimate the annual rate required to reach that target.
Compound Interest Calculator FAQs
Clear answers to common questions about using the calculator and interpreting compound interest projections.
What interest rate should I enter into the compound interest calculator?
Use a rate that is appropriate for the type of scenario you are modelling. A savings account may have a stated interest rate, while an investment illustration normally requires an assumed return. Investment returns are not fixed, so it can be useful to compare several rates rather than relying on a single projection.
Can I use the calculator for investments as well as savings?
Yes, provided the result is treated as an illustration. Savings accounts may pay a defined interest rate for a period, whereas investment returns can vary and may be negative. An assumed investment return therefore does not represent a guaranteed annual rate.
Does the calculator include regular monthly contributions?
Yes. Regular contributions can be included alongside the starting balance. Each contribution increases the amount available to grow, although contributions made earlier have more time to participate in compound growth than those made near the end of the period.
What does the growth multiple mean?
The growth multiple compares the estimated final balance with the amount contributed. A figure of 1.50× means the projected balance is one and a half times the total amount contributed. It is not the annual interest rate and should not be interpreted as a guaranteed investment return.
Is daily compounding much better than monthly compounding?
More frequent compounding can increase the final value when the stated annual rate is otherwise identical, although the difference may be relatively small compared with changes in the rate, contribution amount or time period.
Are the calculator results guaranteed?
No. The calculator applies the assumptions you enter consistently throughout the selected period. Actual savings rates and investment returns may change, so the projected balance should be treated as an illustration rather than a guaranteed outcome.
Does the result account for inflation?
No. The result is shown in future pounds and does not adjust for changes in purchasing power. Inflation can reduce what a future balance is able to buy even when the nominal amount has increased.
Are fees and tax deducted from the calculation?
No. Unless explicitly stated otherwise, the calculator does not deduct account fees, platform charges, investment costs or tax. These can reduce the amount of growth retained in real-world savings and investment scenarios.
Why does compound growth become larger in the later years?
The balance is usually larger later in the calculation, so the same percentage return is being applied to more money. Previous growth may also have become part of the balance generating further growth, which can make the curve become progressively steeper over longer periods.
What is the best way to compare different scenarios?
Keep most inputs unchanged and alter one assumption at a time. For example, change only the investment period or only the annual rate. This makes it much easier to see which factor is responsible for the difference between two projected results.