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Compound Interest Calculator

See how your initial investment and regular contributions could grow over time. Explore different rates, time horizons and compounding frequencies.

Detailed optionsStep-by-step guideWorked examples
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USD

The amount already invested at the start of the first month.

USD

The amount you add every month during the first year.

%

A constant hypothetical annual rate, before taxes and investment fees.

years

The number of whole years in the projection. A longer term changes both the payment and total interest.

Controls the equivalent monthly growth factor. This model spreads the selected compounding rate across monthly deposits.

Advanced options4 more ways to explore

A beginning-of-month deposit earns one additional month of growth compared with an end-of-month deposit.

%

Raises the monthly deposit once every 12 months; 5% makes a 200 monthly contribution become 210 in year two.

%

Used to express the future balance in today’s purchasing power. It does not change the nominal balance.

%

Compare the selected annual rate with a lower and higher scenario. Rates are bounded between 0% and 100%.

Private by design. Calculated in your browser.
Your future balanceLIVE RESULT
$54,713.58
Total contributions$34,000.00
Interest earned$20,713.58
In today’s money$42,742.16
Your savings over timeHover to explore each year
Start5 years10 years
Illustrative model. Check the assumptions below; real outcomes may differ.
YOUR ANIMATED GUIDE

Follow your numbers, step by step.

These steps use your current inputs. Choose a step or play the walkthrough.

STEP 1 / 4
$10,000.00
1234

Start with what you have

Your starting balance is separate from the deposits you make later.

Starting principal = $10,000.00
Explore at your own pace
ONE ASSUMPTION, A DIFFERENT PERSPECTIVE

Compare your options

These rows update with your inputs. Comparisons illustrate assumptions, not forecasts.

Scenario comparison · USD
Hypothetical annual rateFuture balanceInterest earned
5%$47,526.55$13,526.55
7%$54,713.58$20,713.58
9%$63,216.43$29,216.43

Change an input above to explore a different scenario.

How to use this compound interest calculator

A compound interest calculator connects four decisions: the money you start with, the money you add, the return you assume and the time you allow. Use the main estimate for a first scenario, then open advanced options to explore deposit timing, rising contributions and inflation. The comparison table holds your contributions and timeline constant while changing the rate, making the assumptions easier to compare.

What is compound interest?

Simple interest is calculated on the original principal. Compound interest also applies growth to interest already accumulated. For example, 1,000 earning 10% annually becomes 1,100 after one year and 1,210 after two years when nothing is added or withdrawn. The second year earns 110 because the starting balance for that year is larger.

How regular deposits change the projection

A deposit made earlier spends longer in the account. This calculator models monthly contributions and lets you choose beginning- or end-of-month timing. Annual contribution increases change the deposit amount after each block of 12 months. A 200 monthly deposit with a 5% annual increase becomes 210 per month in the second year, rather than increasing by 5% every month.

Nominal growth, purchasing power and uncertainty

A future account balance and its purchasing power answer different questions. The inflation-adjusted result discounts the final balance using the inflation assumption; it does not deduct money from your account. The lower and higher rate scenarios are sensitivity illustrations, not confidence intervals or predicted investment returns. This model excludes taxes, transaction charges and investment fees.

Compound Interest formulas, explained

Growth without additional deposits

A = P × (1 + r / n)^(n × t)

P is principal, r is the annual nominal rate as a decimal, n is compounding periods per year and t is years.

Monthly deposits

End: Bnext = B × (1 + i) + D; beginning: Bnext = (B + D) × (1 + i)

i = (1 + r/n)^(n/12) − 1. D is that month’s deposit.

Inflation-adjusted value

Today’s value = A ÷ (1 + inflation)^t

Use a decimal inflation rate and the same number of years as the projection.

EVERY OPTION, EXPLAINED

Know what goes into your result.

Use this reference to check units, assumptions and advanced settings.

Compound Interest calculator input reference
Input or optionWhat it means
Initial investmentThe amount already invested at the start of the first month.
Monthly contributionThe amount you add every month during the first year.
Annual nominal interest rateA constant hypothetical annual rate, before taxes and investment fees.
Investment periodThe number of whole years in the projection. A longer term changes both the payment and total interest.
Compound frequencyControls the equivalent monthly growth factor. This model spreads the selected compounding rate across monthly deposits.
Contribution timingA beginning-of-month deposit earns one additional month of growth compared with an end-of-month deposit.
Annual contribution increaseRaises the monthly deposit once every 12 months; 5% makes a 200 monthly contribution become 210 in year two.
Annual inflationUsed to express the future balance in today’s purchasing power. It does not change the nominal balance.
Rate comparison ±Compare the selected annual rate with a lower and higher scenario. Rates are bounded between 0% and 100%.
PUT THE FORMULA TO WORK

Worked examples you can try.

Load an example into the calculator, then change one value to explore.

EXAMPLE 01

A clean compounding example

Start with 1,000 at 10% compounded annually for 2 years, with no deposits. The future balance is 1,210; 210 is modeled interest.

EXAMPLE 02

Build with monthly contributions

Start with 10,000, add 200 at each month-end and assume 7% nominal interest compounded monthly for 10 years. The balance is about 54,713.58.

MAKE THE NUMBERS MAKE SENSE

Common mistakes to avoid

  • Entering 0.07 in the rate field when you mean 7%. Enter 7; the calculator converts it to a decimal.
  • Comparing a nominal rate with an effective annual return without checking compounding.
  • Treating a constant-rate illustration as a guaranteed outcome.
A LITTLE MORE CLARITY

Frequently asked questions

How does compound interest work?

Compound interest earns returns on your original investment and on accumulated interest. Over time, this can accelerate growth. Returns here are hypothetical and remain constant.

When are monthly contributions added?

The default adds deposits at month-end. Advanced options also support beginning-of-month deposits and an annual contribution increase. The nominal annual rate is converted to a consistent monthly growth factor.

Why does beginning-of-month timing give a different result?

Each monthly deposit receives one additional month of modeled growth. At 0% interest, deposit timing makes no difference.

Do contributions increase every month?

No. The optional annual increase is applied after each 12-month period. Deposits remain constant within each modeled year.

Is more frequent compounding always better?

For a positive nominal rate, more frequent compounding increases effective growth when other assumptions stay the same. Real products may quote effective rates or apply different crediting rules.

How are inflation and the rate comparison used?

Inflation discounts the final balance into today’s money. The comparison changes the annual rate by your chosen variance while keeping the other inputs fixed.

Methodology & assumptions

Calculations run in your browser using the formulas and assumptions explained on this page. Intermediate values retain full floating-point precision; displayed amounts are rounded. Financial results are illustrations with constant assumptions, not quotes or guarantees. Sharing an input link places those inputs in the URL.

Further reading:Investor.gov compound interest tool
KEEP EXPLORING

A little more perspective.

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