Compound Interest Calculator
See how a starting amount and regular monthly deposits grow with compound interest, year by year.
What this tool does
A compound interest calculator shows how money grows when interest is added to the balance and then earns interest itself. Enter a starting amount, a monthly deposit, an annual rate, a number of years and how often interest compounds, and it returns the final balance and the interest earned.
Grows a starting amount month by month with an optional regular deposit, applying the interest rate with the compounding frequency you choose.
Splits the final balance into what you put in and what you earned, and lists the balance for every year.
How to use it
- Enter the starting amount you have now.
- Enter what you will add each month, or 0 for none.
- Enter the expected annual interest rate and the number of years.
- Choose how often interest compounds - monthly is typical for savings.
- Read the final balance and the interest earned, and check the year-by-year table.
Why use a compound interest calculator
Includes a monthly contribution as well as a starting amount, which is how most people actually save. Compounding is why starting early matters: the interest on interest builds slowly and then quickly. Seeing the numbers makes the effect of time, rate and regular saving concrete.
Why we built it
We built the compound interest calculator with monthly contributions as well as a starting amount, because that is how most people actually save. If you want the rate that links two values use the CAGR calculator, and for the time to double your money see the rule of 72 calculator.
Handy alongside this: the Simple Interest Calculator, the APY Calculator and the Savings Goal Calculator.
Who this tool is for
- Savers deciding how much to put away each month
- Students learning why starting early matters
- Anyone comparing a lump sum with regular deposits
When to use it
- Planning savings or a pension pot over many years.
- Comparing the effect of 4% versus 6%, or 10 versus 20 years.
- Seeing how much a small monthly deposit adds up to.
- Understanding what a savings account or investment might be worth.
How it is calculated
With no deposits the balance is P × (1 + r ÷ n)^(n × t), where r is the annual rate, n the compounding periods per year and t the years. Deposits are added at the end of each month and also earn the interest that follows.
Worked example: 1,000 at 5% compounded monthly for 10 years is 1,000 × (1 + 0.05 ÷ 12)^120 = 1,647.01. Compounded yearly it is 1,628.89.
Interest rates are not guaranteed. This models a constant rate, but real savings and investments vary, and investments can fall as well as rise.
What you enter, and what you get back
| Item | Details |
|---|---|
| Starting amount | What you have at the beginning. |
| Added each month | A regular deposit at the end of every month. |
| Rate and years | Annual rate and the length of time. |
| Compounded | How often interest is added to the balance. |
| Result | Final balance, total put in, interest earned and a yearly table. |
Issues you may run into
| What you see | Why it happens | What to do |
|---|---|---|
| Result much higher than expected | The rate is higher than a realistic return after fees and inflation. | Try a lower, more cautious rate. |
| Yearly vs monthly differ | Compounding frequency changes the effective rate. | Use the frequency your account states. |
| Does not match the bank | Fees, tax or a variable rate apply. | Use the net rate after fees and tax. |
A worked example
These are the values the calculator opens with, so you can see exactly what it does with them. Change any value on the page and the result updates; the figures below come from the same calculation.
| Input | Value |
|---|---|
| Currency symbol | $ (dollar) |
| Starting amount | 10000 |
| Added each month | 200 |
| Annual interest rate | 5 % |
| Years | 10 |
| Compounded | Monthly |
With those inputs, the calculator returns:
- Balance after 10 years: $47,526.55
- Total you put in: $34,000.00
- Interest earned: $13,526.55
- Interest as share of balance: 28.5%
Assumptions and limitations
- Assumes a constant interest rate for the whole period.
- Does not account for tax, fees or inflation.
- Deposits are assumed to be made at the end of each month.
- Not a forecast or financial advice.
Frequently asked questions
What is compound interest?
Interest that is calculated on the original amount plus the interest already added, so interest earns interest over time.
How do I calculate compound interest?
Use P × (1 + r ÷ n)^(n × t) with the starting amount P, annual rate r, compounding periods per year n and years t. This calculator does it, including monthly deposits.
How often should interest compound?
More often is slightly better for the saver, but the difference between monthly and daily is small. Use whatever your account states.
What is the difference between simple and compound interest?
Simple interest is paid only on the original amount. Compound interest is paid on the growing balance, so it grows faster over time.
Does this include inflation or tax?
No. Use a rate net of fees and tax, and remember that inflation reduces what the final balance will buy.