How to use the compound interest calculator

What this calculator does

It estimates the future value of money that starts with an initial balance and grows through regular contributions and compound interest.

Who should use it

Savers, investors, small business owners, and anyone planning for a future purchase or reserve fund.

Input definitions

• Starting amount: the amount already invested or saved
• Contribution per period: the amount added each compounding period
• Annual interest rate: the yearly rate expressed as a percentage
• Term: how long the money grows
• Compounding frequency: monthly, quarterly, or yearly

Formula

• Periodic rate: annual rate ÷ periods per year
• Future value: FV = P(1 + r)^n + C × [((1 + r)^n - 1) / r]
• Interest earned: future value - total contributions

Assumptions

• The example uses USD, but the math works for any currency
• Contributions happen at the end of each compounding period
• The same period is used for contributions and compounding in this simplified model

Quick example

A bakery starts with $5,000, adds $500 each month, and earns 6% per year for 10 years.

Future value is about $91,037.

Total contributions are $65,000.

Interest earned is about $26,037.

How to interpret the result

The future value is the projected balance at the end of the selected term. The interest earned shows how much growth came from compounding rather than from your own contributions.

The schedule table shows how the balance changes each period after interest and contributions are added.

Common mistakes

• Using the annual interest rate as if it were already a periodic rate
• Forgetting that contributions are added every period
• Ignoring taxes, fees, or changing returns in the real world