investment calculator
Compound Interest Calculator
The Compound Interest Calculator helps you estimate how one starting balance could grow when a fixed annual rate compounds once per year. It keeps the numbers visible so you can change one assumption and see why the result moves.
Your scenario
Enter your numbers
Before you calculate
Use it for testing a principal-only future value using starting amount, annual rate, and whole years. It is best for comparing scenarios before asking a lender, adviser, or account provider for official details.
Formula summary
This calculator focuses on annual compounding on the starting principal: future value equals principal times (1 + annual rate) raised to years. The formula references below name the math pieces used by the calculator.
Assumptions
- The result assumes annual compounding on the starting principal only, with no additional deposits, withdrawals, fees, tax, inflation, or live rate changes.
- The calculation uses the numbers entered on the page and does not pull live rates, prices, or account terms.
- Results are planning estimates, not financial, legal, tax, or lending advice.
Common mistakes to avoid
- Do not use this page for monthly deposits or compounding-frequency comparisons; use the savings, interest, or future value calculators when those assumptions matter.
- Check whether each field expects a monthly, yearly, percent, or currency value.
- Change one input at a time when comparing scenarios so the result is easier to explain.
Methodology and sources
This calculator is an original implementation based on documented formulas, app-specific assumptions, deterministic fixtures, edge cases, rounding policy tests, and internal validation. It is not copied from a single source.
Outputs are checked with deterministic fixtures, edge cases, rounding policy tests, and internal independent comparator checks where overlapping outputs are available. The result remains an educational estimate, not a quote, approval, tax answer, or personalised advice.
Formula version 2026.05.20. The version marks the calculation logic and validation fixture set used for this estimate.
This calculator uses generic financial math, so there is no single official source for the formula.
Estimate only
The result is educational and is not financial, tax, legal, lending, investment, or regulated advice. Real provider terms, fees, rates, taxes, and personal circumstances can change the final answer.
Formula and help: read the full compound interest calculator methodology notes.