Compound Interest Calculator

Model growth from a starting balance plus regular contributions.

Assumptions

This calculator assumes an ordinary annuity (contributions credited at the end of each period) and a nominal per-period rate (annual rate ÷ compounding periods — not an effective-rate conversion). Other calculators may use different conventions and show different, also-correct, results.

Methodology details

The periodic interest rate is your nominal annual rate divided evenly by the number of compounding periods per year (e.g. 8% annual compounded monthly = 8%/12 per month) — not converted to a different effective rate. Each contribution is credited at the end of its period, after that period's interest has already been calculated, so a deposit doesn't earn interest during the period it was made. When your contribution frequency differs from your compounding frequency (e.g. monthly contributions with annual compounding), all contributions falling within one compounding period are pooled into a single deposit credited at the end of that period, rather than modeled as separate deposits arriving throughout it — matching your contribution frequency to your compounding frequency avoids this pooling and gives the most precise result.

Starting principal ($)

Annual interest rate (%)

Time (years)

Compounding frequency

Contribution amount ($)

Contribution frequency

Future value
$54,713.58
Total contributions
$34,000.00
Interest earned
$20,713.58

Balance growth over time

Total contributionsInterest earned

What is this tool?

Compound interest grows a balance by applying interest on both principal and accumulated interest. This tool also adds regular contributions using the ordinary annuity future-value formula, then breaks out total contributions versus interest earned.

How to use it

Set principal, annual rate, years, and compounding frequency.
Add a contribution amount and how often you contribute.
Compare future value, total contributions, and interest earned.

Balance growth over time

What is this tool?

How to use it

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