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. Calfiny uses the current Bank of England Bank Rate plus 0.75 percentage points as the default example rate; actual savings rates, investment returns, charges, taxes and market performance can differ.
What determines how compound interest grows?
Compound growth is mainly shaped by the amount you start with, the interest rate used and how long the money has to compound.
Starting amount
A larger starting balance gives interest a larger base to build from.
Interest rate
A higher assumed rate increases the amount added during each compounding period.
Time
More time gives repeated compounding more opportunities to build on earlier interest.
How compound interest builds on itself
The defining feature of compound interest is that earlier interest can become part of the balance used to calculate later interest.
Start with your original balance
The initial amount provides the base on which the first period of interest is calculated.
Interest is added
At the end of a compounding period, the calculated interest is added to the balance.
The next period starts from a larger balance
The new balance includes both the original money and the interest already added.
Interest can be earned on earlier interest
As the process repeats, later interest can be calculated on a balance that already includes previous interest.
That repeated process is what distinguishes compound interest from simple interest. Over longer periods, the effect can become increasingly significant because earlier growth becomes part of the balance used for later calculations.
Compounding frequency changes the calculation
Compounding frequency describes how often interest is added to the balance. The frequency can affect the final value even when the stated annual rate is unchanged.
How often does compound interest compound? →Annual compounding
Interest is added once each year, so the balance is updated annually.
Monthly compounding
Interest is added more frequently throughout the year, allowing each addition to become part of the balance sooner.
Why frequency matters
With the same nominal rate and assumptions, more frequent compounding can produce a slightly higher final value because interest is credited earlier.
Compound growth is an illustration, not a guarantee
The mathematics can show how compounding behaves under a chosen set of assumptions. It cannot guarantee what a real savings account or investment will deliver.
The rate is an assumption
The calculator applies the rate you enter consistently unless the calculator itself states that changing rates are being modelled.
Real-world rates and returns can change
Savings rates can move over time, while investment returns can vary from one period to another.
Compound growth is not the same as guaranteed growth
The result shows what would happen if the entered assumptions held. It is not a prediction of the return a particular account or investment will achieve.
Use the result as a planning illustration and test different assumptions rather than treating one projection as a fixed outcome.
Explore your next question
Once you have seen the effect of compounding, the next useful step is to explore the concept, compare it with simple interest or apply it to a different financial question.
Go deeper
Compound interest is a cornerstone concept, so these guides take the calculation further without repeating what the calculator has already shown.
What Is Compound Interest?
Learn the core idea behind compounding, why earlier interest becomes part of later calculations and how the effect develops over time.
Read the guide →Common questions about compound interest
These answers cover the questions that often arise after comparing compound-growth scenarios.
What is compound interest?
Compound interest means interest can be calculated on a balance that already includes interest added in earlier periods. This allows earlier interest to become part of the base used for later calculations.
How is compound interest calculated?
The calculation uses the starting amount, interest rate, length of time and compounding frequency. The calculator applies those assumptions repeatedly across the period entered.
What is the difference between simple and compound interest?
Simple interest is calculated only on the original amount, while compound interest can also be calculated on interest that has already been added to the balance.
Does compound interest work on savings and investments?
The mathematical principle can be used to illustrate both. Savings may earn interest, while investment growth is uncertain and can rise or fall rather than following a fixed interest rate.
How often can interest compound?
Interest may compound annually, monthly, daily or at another frequency depending on the product or calculation. The stated frequency determines how often interest is added to the balance.
Does monthly compounding grow money faster than annual compounding?
With the same nominal annual rate and other assumptions, more frequent compounding can result in a slightly higher final value because interest is added to the balance sooner.
Why does time matter so much with compound interest?
More time means more compounding periods. It also gives interest added in earlier periods more opportunities to become part of later interest calculations.
Can compound interest ever be guaranteed?
The mathematics is predictable when the inputs are fixed, but a calculator does not guarantee that a particular savings rate or investment return will remain available or be achieved in practice.
What happens if the interest rate changes?
The actual outcome will differ from a projection that assumes one constant rate. You can rerun the calculator using different rates to explore how sensitive the result is to that assumption.
Does the calculator include regular contributions?
Use the calculator inputs shown on the page as the source of truth for what is included. If you want to model repeated deposits specifically, the Regular Savings Growth Calculator is designed for recurring contributions.