Compound interest calculator

What compound interest can show you

The result estimates how a starting balance and regular monthly additions could grow if the same nominal annual rate held for the entire period. It is an illustration of compounding, not a quoted bank yield or an investment forecast. Change one input at a time to see which assumption drives your estimate.

How this calculation works

Each month, the model multiplies the balance by (1 + nominal annual rate / 12), then adds the monthly deposit at month-end. It repeats for the rounded number of months in the period. At 0% interest, the result is simply your starting balance plus your monthly additions.

Worked example: Start with $10,000, add $300 at the end of each month for 10 years, and assume a 5% nominal annual rate compounded monthly. The model ends near $63,055. You put in $46,000 ($10,000 + 120 × $300); about $17,055 is modeled interest. Actual returns or rates may differ.

Nominal rate is not APY

Enter a nominal annual rate, which this calculator divides by 12 before monthly compounding. A bank's annual percentage yield (APY) already includes the effect of its compounding schedule. Entering that APY as the nominal rate would double-count some compounding. Check the account's rate disclosure and compounding terms when comparing real products. The CFPB definitions distinguish the stated interest rate from APY.

This model omits fees, taxes, inflation, rate changes, and differences in deposit timing. For investments, returns can be negative and do not arrive evenly each month. A larger modeled ending balance is not a reason by itself to choose an account or investment; compare liquidity, risk, costs and the actual terms.