Compound interest calculator
See how a lump sum grows when interest compounds at a fixed annual rate—choose how often interest is applied (annually through monthly). The math is A = P(1 + r/n)nt. Use any currency; enter amounts in your own units. Results are educational projections, not investment advice. Calculations stay in your browser.
Understand compounding deeply
See how rate assumptions, time horizon, and contribution behavior shape outcomes. The guide explains realistic projections and common planning errors.
Read the full compound interest guideHow to use this result
- Compare conservative and optimistic rate scenarios, not one fixed number.
- Check inflation-adjusted outcomes for real purchasing power.
- Treat output as planning math, not an investment guarantee.
What this projection means
Compound growth means each period earns interest on a slightly larger balance than the last—so the curve bends upward when rates are positive. In real life, banks and bonds pay changing rates; stocks and funds add volatility. This tool holds the rate constant so you can learn the mechanics and stress-test “what if I earned X% for Y years?” before you layer in real-world uncertainty.
Frequency matters: monthly compounding credits interest more often than annual compounding at the same stated APR, so the ending balance is slightly higher. The gap grows with rate and time—compare frequencies with identical inputs to see it.
What you should do next
- Run two scenarios: conservative rate vs long-run historical average you believe is fair—never one number alone.
- Subtract inflation mentally (or use our savings growth predictor) for spending power, not just nominal balance.
- Talk to a licensed advisor before committing large sums; this page does not know your tax situation or risk tolerance.
What-if scenarios
- Rate 1% lower — rerun with the same principal and years; long horizons magnify small rate changes.
- Double the time — compounding is nonlinear; twice the years is more than twice the interest in many cases.
- Add monthly contributions — our savings growth predictor models periodic deposits, not only lump sums.
Common mistakes
- Confusing APR with APY without matching compounding frequency.
- Assuming past returns repeat—markets do not offer fixed rates like this formula.
- Ignoring taxes and fees on real accounts.
- Using nominal growth to plan retirement spending without an inflation check.
Frequently asked questions
What is compound interest?
Growth on both principal and previously earned interest, when those earnings stay invested.
How does compounding frequency matter?
More frequent compounding generally yields a higher ending balance for the same APR and term.
Does this predict investment outcomes?
No—it is a fixed-rate teaching tool. Real portfolios vary year to year.
Method and sources
This page uses standard discrete compounding math with fixed-rate assumptions. For a fuller playbook covering rate assumptions, inflation, and planning mistakes, see our compound interest guide.
- Time value of money and compounding references from core finance education sources.
- Consumer investing guidance on volatility and return uncertainty.
How compound interest is calculated here
We use the standard discrete compounding formula with principal P, annual rate r, compounding periods per year n, and time in years t. The chart data in the engine steps year by year using the same parameters so you can align with tables elsewhere—small rounding differences can appear at the last decimal.
Financial disclaimer: Educational illustration only—not investment, tax, or legal advice. Consult a qualified professional for decisions involving real money.
Related tools & guides
- Savings growth predictor — monthly contributions plus inflation-adjusted view.
- Investment return calculator
- How calculators work — formulas and privacy in plain language.