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📈 Compound Interest Calculator
Stick $10,000 in an index fund earning 7% real per year. Don't touch it. In ten years it's about $20,000. Another ten years, $40,000. Forty years from now, somewhere around $150,000. That single deposit grew to more than ten times itself, without you adding a dollar. This calculator shows you that curve for any starting deposit, monthly contribution, rate, and horizon — and tells you what portion is principal versus interest earned, which is the part that makes the whole thing feel like cheating.
Examples
PV $10,000, 7% annual, 30yFuture value: $76,123PV $0, $500/mo, 8% annual, 25yFuture value: $478,316 · Contributions $150k · Interest $328k$10k, 5%, 20 yearsAnnual: $26,533 · Daily: $27,180Continuous-ish compounding adds ~2.4% over annual.
Frequently asked questions
What is the compound interest formula?
A = P(1 + r/n)^(nt) where P is principal, r is annual rate, n is compounding periods per year, t is years. Add ongoing contributions and the formula grows but the principle is the same.
How important is compounding frequency?
Less than people think. At 5% over 20 years, monthly vs annual compounding differs by only ~1.2%. The big drivers are rate and time, not frequency.
Should I expect 7-8% returns from stocks?
Use 7% for planning. The S&P 500 has averaged 10% nominal since 1928 but that includes a long bull run in the postwar era you should not bank on repeating. After inflation it's closer to 7%. Some decades returned 12-15%, others returned 2-3%. Plan for the bad decades, not the good ones.
What is the rule of 72?
A quick estimate: years to double = 72 / interest rate. At 7%, money doubles in ~10.3 years.
Does this account for taxes or inflation?
No. The result is nominal pre-tax. For real spending power, subtract 2-3% per year for inflation, and account for capital gains tax when you withdraw.
About this tool
Compound Interest Calculator runs entirely in your browser using standard Web APIs. No data is sent to any server. The source for this tool is in our public GitHub repository.
Related tools
- 🏖️ Retirement Calculator — 401k / IRA / general retirement projection with employer match.
- 🎯 Savings Goal Calculator — How long to reach a target? Or how much to save monthly?
- 📈 Inflation Calculator — What is $X worth in Y years given Z% inflation? Past and future value.
Why this tool
vs typical free web-tool sites
| 1vw | Typical free site | |
|---|---|---|
| Price | Free | Free |
| Signup required | No | Often (for premium features) |
| Ads inside the tool | No | Yes (banner + video) |
| Data sent to server | No (browser-only) | Yes (inputs analyzed for ads) |
| Total tools | 96 | Often <20 or scattered across sites |
| Bookmarkable URL per tool | Yes | Mixed |
| Loads in under 1s | Yes (static) | Often slow (ad tracking) |
| Multilingual | EN / ES / PT / FR / DE | Usually EN only |
| Open source | Yes | No |