Calculate compound growth with monthly contributions. See your final balance, total contributions, total interest earned, and a year-by-year growth table.
Calculate your future value now · See a worked example · Compare compounding frequencies
This compound interest calculator helps you estimate how savings or investments can grow over time with regular contributions. Enter your starting balance, monthly deposit, expected annual return, and time horizon. The tool shows final balance, total contributions, total interest earned, and a year-by-year growth table so you can compare realistic scenarios.
It is useful for retirement planning, ISA or brokerage growth estimates, long-term savings targets, and checking whether increasing your monthly contribution has a bigger effect than assuming a slightly higher return. Instead of guessing, you can test a conservative, base-case, and optimistic scenario in minutes.
Returns are not guaranteed. For a more realistic plan, test lower return assumptions, include inflation, and compare the effect of fees over time. If you are planning around a real goal, this calculator works well alongside a Savings Goal Calculator, Investment Fee Impact Calculator, and Retirement Planning Calculator.
Use realistic assumptions. Returns are estimates and not financial advice.
Updated instantly from your inputs.
This table shows your balance at the end of each year, based on your inputs. It’s useful for comparing scenarios such as higher monthly contributions, a different return assumption, or a longer time horizon.
| Year | End balance | Contributions | Interest earned |
|---|---|---|---|
| Run a calculation to see results. | |||
Compound interest means you earn returns on both your original money and on the returns you’ve already earned. Over time, this creates a snowball effect where growth can accelerate. The longer your money stays invested, the more times it can compound.
In practice, three variables usually matter most: time, contribution size, and return rate. Many people focus too much on finding a perfect rate and not enough on starting earlier or contributing more consistently.
A common simplified formula is: A = P(1 + r/n)^(n·t). Where P is the starting amount, r is the annual rate, n is the compounding frequency, and t is time in years.
When you add regular monthly contributions, the calculation is usually done period by period rather than with a single simple formula. That is why this calculator models growth over time instead of only showing a one-line estimate.
Here is a simple example of how compound interest can build over time:
In this scenario, your final balance is driven by a mix of contributions and growth. Early on, most progress comes from your deposits. Later, a larger share of growth comes from earned interest compounding on itself. That is why staying invested for longer often matters more than trying to time the market.
A useful comparison is to keep the rate the same but change the monthly contribution from £200 to £250, then compare the result. After that, keep the contribution the same and extend the time horizon from 20 years to 25 years. In many cases, more time produces a bigger jump than a small rate change.
Compounding frequency affects how often returns are added back to your balance. More frequent compounding can produce a slightly higher final value, but the gap is often smaller than people expect.
For most people, the bigger drivers are still contribution amount, return rate, and the number of years invested. Use the compounding dropdown above to compare daily, monthly, quarterly, and yearly results with your exact numbers.
Start with realistic numbers. Enter your current balance, the amount you can add each month, an annual return assumption, and the number of years you expect to stay invested. For most people, the biggest drivers are time and contribution size, not trying to guess the perfect return.
A useful way to plan is to test three scenarios: conservative, base case, and optimistic. For example, you can compare 5%, 7%, and 9% annual returns, or compare £150 per month versus £250 per month. That shows whether increasing your contribution has a bigger effect than chasing a slightly higher return.
You can also add inflation to estimate future purchasing power. That helps turn a headline final balance into a more realistic planning number. If you are comparing investments, fees, or retirement goals, use this calculator together with the Investment Fee Impact Calculator and Retirement Planning Calculator for a fuller picture.
That is why this page works best as a practical planning tool: enter your real numbers, compare a few scenarios, and use the yearly table to see how results change over time.
This compound interest calculator is especially useful when you want to decide between increasing your contribution, changing your time horizon, or comparing savings and investment assumptions. Common use cases include:
If your goal is target-based rather than growth-based, use this page with the Savings Goal Calculator. If you want to compare fee drag, pair it with the Investment Fee Impact Calculator.
Use these tools together to compare scenarios such as contributions, fees, debt payoff, and long-term retirement planning.
A common method compounds your balance each period and adds your regular contribution. Over time you earn growth on both your original money and past growth.
More frequent compounding can slightly increase results over long periods, but your contribution amount, return rate, and time horizon usually matter more.
For long-term planning, many people use a conservative range such as 5%–8% for diversified investments, or a lower rate for cash savings.
It provides educational estimates using simplified assumptions. Real returns vary, fees and taxes may apply, and contributions may not be invested at the same time each month.
Time increases the number of compounding periods. Even small contributions can grow substantially over decades because each year builds on the last.
Daily compounding can produce a slightly higher final balance than monthly compounding when the annual rate is the same, but the difference is usually small compared with contribution size and investment length.
A common simplified formula is A = P(1 + r/n)^(n·t), where P is the starting amount, r is the annual rate, n is the number of compounding periods per year, and t is time in years. Regular contributions are usually modeled period by period.
This calculator uses standard financial formulas and simplified assumptions such as constant rates, regular payments, and rounding. Real-world results can differ due to fees, taxes, rate changes, compounding conventions, and account rules.
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