Compound Interest Calculator with Monthly Contributions
Calculate compound interest with monthly contributions. See projected savings growth, total deposits, and interest with a yearly chart and CSV schedule.
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How monthly compound interest works
Each month earns interest on the previous balance before the next contribution is added. Depositing earlier gives money more periods to grow. This calculator uses one constant nominal annual rate, divided into 12 monthly rates; it does not treat the input as APY.
Future balance = P × (1 + r)n + C × ((1 + r)n − 1) ÷ r, where P is the starting amount, C is the month-end contribution, r is annual rate ÷ 1200, and n is years × 12. At a zero rate: balance = P + C × n.
Example: saving every month
Starting with 1,000 and adding 100 monthly for one year at a nominal 6% rate produces approximately 2,295.23. Deposits total 2,200 and projected interest is 95.23, before tax or fees.
What does the projection exclude?
Markets and savings rates change. This illustration excludes taxes, charges, withdrawals, and inflation. Negative assumptions model a steady decline rather than realistic market volatility. Use the inflation calculator to explore purchasing power separately.
Reference and methodology
Background reading: Investor.gov compound interest calculator. Calculations use the formulas and assumptions explained on this page.
About this calculator
Prepared by the UtilitiesWeb team at The Alpha 1 Company. Estimates are educational illustrations, not personalised financial advice, lending decisions, or guaranteed outcomes. Verify important decisions against provider documents and qualified advice where needed.
Methodology updated: 1 November 2026. Example rates and costs are assumptions, not live market quotes. Amounts are calculated at full precision and rounded for display.