Compound Interest Calculator

Inputs and units

Enter starting savings, contributions made at the end of each month, an annual return assumption and years.

Formula

Future value = P(1+r)^m + C((1+r)^m − 1)/r. Here r is annual return / 1,200 and m is years × 12. With zero return, value = P + Cm.

Worked example

$1,000 plus $100 monthly for one year at 0% becomes $2,200.

Assumptions and limitations

Uses monthly compounding and constant contributions. Taxes, investment fees, inflation and variable returns are excluded.

Understanding your result

The final balance combines your starting deposit, monthly contributions and modeled growth. Separate money you contribute from growth when assessing the result. The annual return is an assumption, and the estimate does not adjust for inflation or investment costs.

A practical comparison

Starting with $1,000 and adding $100 at the end of every month for one year contributes $2,200 in total. At a nominal annual return of 6%, compounded monthly, the projected ending balance is about $2,295.23. The $95.23 difference is modeled growth, not a promised return.

Common mistakes

• Enter a monthly contribution, not your full annual savings amount.
• Use the same time horizon when comparing return assumptions.
• A constant positive return hides market losses and the effect of changing returns over time.

Frequently asked questions