Compound Interest Calculator

Compound interest means you earn returns on both your original principal and on accumulated interest from prior periods. Each compounding period adds a little more to the base, so growth accelerates over time — especially with regular contributions. Savings accounts, CDs, and bond funds compound; stock and fund returns fluctuate and are not guaranteed at a fixed rate.

This calculator projects future balance from a starting amount, a fixed annual rate, compounding frequency (monthly, quarterly, or annually), and optional monthly contributions. Real investment returns vary year to year; a steady 7% assumption is a common long-term planning shortcut for diversified stock portfolios, not a promise.

Example: $5,000 starting balance, 7% annual rate compounded monthly, $200/month for 20 years grows to roughly $118,000 — about $53,000 in contributions and $65,000 in compounded growth. Actual results depend on market performance, taxes, fees, and whether you stop contributing. This is an estimate only, not investment advice.

Compounding frequency affects results at the same nominal rate. Monthly compounding earns slightly more than annual compounding because interest accrues on interest more often. The difference is modest at typical savings rates but matters over long horizons.

Regular contributions often dominate early balances. In the example above, more than half the final value comes from compounded growth, but only after years of steady deposits — the first decade is mostly principal accumulation.

Fixed rates suit CDs and high-yield savings projections. Stock market planning sometimes uses 7% as a long-run historical equity average, but actual paths include negative years and sequence-of-returns risk near retirement.

Worked example — no contributions: $10,000 at 5% compounded annually for 30 years → about $43,219. Interest earned ≈ $33,219. Adding $100/month at the same rate pushes the total much higher because each deposit also compounds.

Taxes reduce effective compounding in taxable accounts. IRA and 401(k) accounts defer tax on growth until withdrawal (Traditional) or avoid tax on qualified withdrawals (Roth) — account type matters as much as rate.

Inflation erodes purchasing power of the ending balance. Pair nominal projections with our inflation calculator for real-dollar thinking.

Sequence-of-returns risk hits hardest when withdrawals begin — negative early years during retirement drawdowns hurt more than the same negative years during accumulation.

Dollar-cost averaging into a volatile fund produces a different personal return than CAGR on start/end balances because purchases happen at different prices.

Emergency funds belong in liquid accounts with lower rates — do not assume 7% on cash you need within 12 months.

Rule of 72 pairs with compound projections for mental checks — at 7% monthly contributions plus 7% return, doubling time of principal alone understates total balance growth.

Bond ladders and CD ladders produce stepped returns unlike smooth compound curves — laddering reduces reinvestment risk but changes the shape of growth.

Catch-up contributions after age 50 in IRAs and 401(k)s increase the contribution line in real plans — this calculator accepts any monthly amount regardless of IRS limits.

Withdrawal phases in retirement reverse the contribution sign — accumulation calculators overstate retirement sustainability if you begin drawing down without modeling withdrawals separately.

Rate changes mid-plan (CD rollover at new APY) require splitting into segments — one constant rate assumption cannot capture step-down returns when banks cut yields.

Early withdrawal penalties on CDs break smooth compounding assumptions — model each CD term separately if you ladder maturities rather than one blended rate.

Match compounding frequency to your account statement — daily versus monthly compounding changes ending balance slightly at the same nominal APY.

Education savers often model 529 plans with a conservative rate below equity assumptions — match the rate to the actual investment option selected inside the plan, not generic market averages.

HSAs invested for retirement compound tax-free when used for qualified medical expenses — long horizons resemble this calculator’s growth curve if invested in index funds.