Compound Interest Calculator

Worked example

Example: $10,000 starting balance, about $200/month additions, near 7% annual return, and 10 years often projects an ending balance in the mid-$40,000s—actual market returns vary year to year.

When to use this

• Projecting long-term savings with monthly contributions.
• Illustrating how starting earlier beats contributing more later.
• Comparing conservative vs optimistic return assumptions.
• Teaching compound growth for retirement or college funds.

Common examples

• $5,000 starting balance, $100/month contributions, 7% annual return, 10 years → ending balance often in the mid-$20,000s before taxes.
• $0 start, $500/month, 6% return, 25 years → illustrates long-horizon retirement or brokerage growth into low six figures.
• $50,000 lump sum, no contributions, 5% for 15 years → grows to about $104,000 before taxes and annual fees.
• $2,000 start plus $50/month at 6% for 30 years → illustrates long-horizon college or brokerage savings.
• $10,000 lump sum, no contributions, 4% for 20 years → about $21,900 before taxes and fees.

Common mistakes

• Expecting linear growth instead of compounding on prior gains.
• Using nominal returns without considering inflation mentally.
• Adding contributions at the wrong frequency (monthly vs annual).
• Treating projections as guaranteed market outcomes.

How it works

Projects future value from principal and optional recurring contributions at a chosen compounding frequency. An optional annual fee percent is subtracted from the gross rate (expense-ratio style drag), and fee drag is reported versus a no-fee projection.

Limitations

Projects growth using the compounding frequency you select. Market returns, taxes, and fees are not included.

Privacy and file handling

Your data stays on your device and is not uploaded.

Accuracy & methodology

This section documents how the calculator works, what it leaves out, and when results were last reviewed. Figures are educational estimates—not professional advice—and are not labeled "current" unless tied to automatically updated reference data.

Formula source or methodology

Compound growth on principal with optional periodic contributions; compounding frequency follows the selected period (e.g. monthly).

Jurisdiction

General

Unit system

Currency; percent; time in years

Rounding method

Currency amounts round to two decimal places (half up via Math.round × 100 / 100).

Assumptions

• Constant annual rate converted to per-period rate
• Contributions occur each period at the frequency selected

Known omissions

• Not tax, legal, investment, or lending advice. Confirm material decisions with qualified professionals.
• Taxes on interest, account fees, and variable returns
• Inflation-adjusted (real) dollars

Test cases (automated)

• Principal with zero rate grows only by contributions
• Negative rate is rejected

Version & last verified

Logic version 1.0. Content and formulas last verified 2026-07-24.

Important notice

Results are estimates for educational purposes and are not financial advice. Assumed returns are not guaranteed. Consult a qualified financial professional for personal guidance.