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Finance

Compound Interest Calculator

See how your starting balance and regular contributions could grow with monthly compounding.

Amounts stay on this deviceMethod and assumptions below
Financial workspace
Financial workspace

Check the assumptions, then compare another scenario.

See the method

Build a scenario

Runs on your device

Your financial summary

Your result

Let’s calculate.

Use the Calculate button to see your result.

Estimate based on your inputs and the stated assumptions.

How to use this tool

  1. Enter starting balance, monthly contribution, annual interest rate, time period.
  2. Select Calculate to view the result.
  3. Check the method and assumptions below before using the result.

Input reference and sample values

Use the units and choices shown for each field. Sample values are provided for practice, not as recommendations.

InputFormat and unitsSample
Starting balanceNumeric value. Unit: money. Zero or positive.1000
Monthly contributionNumeric value. Unit: money. Zero or positive. Maximum: 10000000.0
Annual interest rateNumeric value. Unit: %. Zero or positive. Maximum: 100.12
Time periodNumeric value. Unit: years. Minimum: 1. Maximum: 100.1

Result for these sample inputs

1126.82503 currency units
Your contributions
1000
Interest earned
126.82503

Select Try the example in the form to load these values and calculate them again. This replaces the current form inputs. Compare the result with the method and worked explanation below.

The method, explained

Each month: new balance = previous balance × (1 + annual rate ÷ 1200) + contribution. Contributions are added at the end of each month.

A WORKED EXAMPLE

Start with 1,000, contribute nothing, and use 12% nominal annual interest for one year. Monthly compounding gives 1,126.83, including 126.83 interest.

Understanding your result

Compare contribution amounts while holding the rate and term constant. The difference between your contributions and final balance is modeled interest, not a promised return.

What to keep in mind

The interest rate stays constant. This model excludes taxes, fees and inflation, and does not guarantee an investment return. All money inputs must use the same currency.

Reference: Investor.gov savings calculator

Common questions

Are contributions added before interest?

No. Each month earns interest first; your contribution is then added at month end. Beginning-of-month contributions would grow slightly more.

Is the annual rate the same as APY?

No. Enter a nominal annual rate. Monthly compounding creates an effective annual yield of (1 + rate/12)^12 − 1.

Methodology maintained by ClarityKit. How these tools are built and checked.