Compound Interest Calculator
Three modes: Single (full breakdown of one plan), Compare (up to 4 scenarios side by side with one chart and a vs-best diff), and Goal seek (solve for monthly contribution, annual rate or time to hit a target). Models contribution escalation, tax drag, inflation, any compounding frequency and start/end-of-period timing.
All calculations happen in your browser. Nothing is uploaded.
This tool is informational only and does not constitute financial advice. Real returns vary; market investments are not guaranteed.
How to use this compound interest calculator
- Single mode: enter principal, rate, time and compounding frequency. Add optional monthly/yearly contributions, escalation and a tax-drag percentage.
- Compare mode: edit up to 4 plans side by side and read the chart with one line per plan and a vs-best diff.
- Goal seek: enter a target balance, choose what to solve for (monthly contribution, annual rate, or years), and the calculator finds the value with binary search.
- Add inflation to see the real (purchasing-power) value at the end of the term.
- Copy the summary or expand the year-by-year table.
Frequently asked questions
What does goal seek solve for?
Pick one: how much to contribute monthly to hit a target by date X; what annual return you'd need; or how many years it takes given your current contributions and rate.
How does contribution escalation work?
Your contribution grows each year by the percentage you set — useful to model salary increases. Year 1 = base; year N = base × (1 + esc)^(N−1).
What is tax drag and how is it applied?
A flat percentage applied to the gross compounded annual return — a quick way to model the impact of taxable accounts. Set 0% for tax-advantaged accounts.
What is the difference between simple and compound interest?
Simple interest only applies to the original principal. Compound interest also earns interest on previously earned interest. Over long horizons the difference is dramatic.
What is continuous compounding?
It's the limit as compounding frequency goes to infinity, calculated with e^(rt). It's the upper bound — daily compounding is already very close to it.
What is the effective annual rate (EAR)?
The actual yearly return when interest compounds more than once per year. For 6% nominal compounded monthly, EAR ≈ 6.17%.