About this calculator
The Compound Interest Calculator estimates how money grows when interest or investment returns are reinvested. It is useful for savings goals, pensions, ISAs, long-term investing, FIRE planning, and understanding the effect of regular contributions. Compound growth means returns can earn further returns over time, so the same annual rate can produce very different outcomes depending on the time period, contribution pattern, and compounding frequency.
Compound interest formula
For a single lump sum, compound interest grows the starting balance by the rate for each compounding period. Regular contributions are added separately.
- Future value = principal x (1 + r / n)^(n x t)
- With regular contributions, each contribution compounds from the date it is made
How the calculator works
- Enter the starting balance.
- Add the annual interest rate or expected return.
- Choose the time period and contribution amount.
- The calculator compounds growth and adds contributions over time.
- The result separates contributions from growth where possible.
Worked examples
Lump sum growth
Input: GBP 10,000 at 5% for 10 years
Calculation: 10,000 x 1.05^10
Result: Future value is about GBP 16,289
Regular monthly saving
Input: GBP 200 per month at 5% for 10 years
Result: The final value is higher than the GBP 24,000 contributed because returns compound along the way.
Longer time horizon
Input: Same rate and contribution, but 20 years instead of 10
Result: The difference is more than double because later years include growth on earlier growth.
Why compound interest matters
Compound interest is powerful because growth is added to the balance and can then generate more growth. Over short periods the difference can look small, but over decades compounding can become the main driver of the final value.
This is why time is often more important than the starting amount. Money invested or saved earlier has more years to compound, while delays can require much larger contributions later.
Main compound growth inputs
The calculator result is shaped by a small number of assumptions. Changing any one of them can materially change the final value.
- Starting balance
- The amount already saved or invested at the beginning of the calculation.
- Regular contributions
- Monthly or yearly additions can become a large part of the final value, especially when started early.
- Growth rate
- The assumed annual return or interest rate. For investments, this is not guaranteed and can vary widely year to year.
- Time horizon
- The number of years the money has to grow. Longer periods increase the effect of compounding.
- Fees and inflation
- Platform fees, fund charges, tax, and inflation can reduce the real value of the result.
Simple vs compound growth
Simple interest pays interest only on the original principal. Compound interest pays growth on the principal and on previous growth. The longer the time period, the more important this difference becomes.
Compound interest calculator components
The final balance depends on four main inputs: the starting amount, the regular contribution, the return assumption, and time. A small difference in return or contribution can become a large difference over decades.
- Principal
- The initial amount that begins compounding. A larger starting balance gives growth more money to work on from day one.
- Contribution frequency
- Monthly contributions compound for different lengths of time depending on when they are added. Early and consistent contributions matter.
- Compounding frequency
- Interest may compound daily, monthly, quarterly, or yearly. The difference can be modest at low rates but still matters over long periods.
- Return assumption
- Savings interest may be predictable for a fixed period. Investment returns are uncertain and can be negative in some years.
Different compounding frequencies
Interest can compound yearly, monthly, daily, or on another schedule. More frequent compounding means interest is added to the balance more often, so future interest is calculated on a slightly larger amount. The difference is small over short periods but can matter over long periods or large balances.
| Frequency | Periods per year | Where users may see it |
|---|---|---|
| Annual | 1 | Some savings and investment projections |
| Quarterly | 4 | Some savings products or business calculations |
| Monthly | 12 | Loans, mortgages, and many savings examples |
| Daily | 365 | Some savings accounts and credit products |
| Continuous | Infinite limit | Mathematical comparison rather than ordinary product wording |
Inflation and real returns
A future balance may look large in pounds but buy less than expected if inflation is high. For long-term planning, compare nominal growth with real growth after inflation. A 6% return with 3% inflation is not the same as a 6% increase in purchasing power.
How to interpret long-term projections
Compound interest projections are scenario tools. They are useful for comparing contribution levels, start dates, and return assumptions, but they should not be read as promises. For investing, fees, asset allocation, tax wrappers, and market volatility all affect the final result.
AER, APR, and quoted rates
Savings products in the UK often quote AER, which shows what the rate would be over a year after compounding. Borrowing products often quote APR, which is designed for comparing loan costs and may include certain fees. A headline rate can therefore mean different things depending on whether you are saving or borrowing.
When comparing products, check whether the rate is gross, net, AER, APR, fixed, variable, before tax, or after fees. The same nominal rate can produce a different effective result if compounding frequency differs.
Rule of 72
The Rule of 72 is a quick way to estimate how long it takes money to double at a fixed compound return. Divide 72 by the annual percentage return. At 6% per year, money doubles in roughly 12 years. At 8%, it doubles in roughly 9 years.
The rule is only an approximation. It works best for moderate positive rates and does not replace a full compound interest calculation.
Common mistakes and edge cases
- Treating expected investment returns as guaranteed.
- Ignoring inflation when looking at long-term results.
- Using annual returns while contributions are monthly.
- Forgetting platform fees, taxes, and product charges.
- Comparing simple and compound interest as if they behave the same.
Limitations
This calculator provides an estimate only and is not financial or tax advice.
- Investment returns vary and can be negative.
- The calculator does not recommend investments or savings products.
Frequently asked questions
What is compound interest?
Compound interest means interest or returns are added to the balance and can then earn further returns.
Why does time matter so much?
More time gives earlier growth more opportunities to compound.
Is monthly compounding better than annual compounding?
At the same nominal rate, more frequent compounding usually gives a slightly higher effective return.
Does this include inflation?
Only if you enter an inflation-adjusted or real return assumption yourself.
Can I use this for investments?
Yes for planning scenarios, but investment returns are not guaranteed.
Related calculators
- Simple Interest Calculator
- Savings Rate Calculator
- Interest Rate Calculator
- Inflation Calculator