Compound Interest & Investment Growth Calculator
In short: Compound interest is the return you earn on both your original money and the returns it has already produced. With regular contributions and decades of compounding, growth eventually dwarfs the amount you put in — which is why starting early matters far more than starting big.
Source: IRS Notice 2025-67 (IR-2025-111). Data as of 2026 tax year.
These are estimates for general information, not financial advice. Verify figures with the IRS and a qualified professional before acting.
How it works
Enter a starting balance, a monthly contribution, an expected annual return and a time horizon to see the future value, how much you contributed, and how much is pure compound growth. The result updates as you type and nothing leaves your device — every figure is computed in your browser.
2026 IRS limits used here
Source: IRS Notice 2025-67 (IR-2025-111). Announced November 2025; effective for the 2026 tax year. Data as of the 2026 tax year.
| 2026 limit | Amount |
|---|---|
| 401(k)/403(b)/457/TSP elective deferral | $24,500 |
| 401(k) catch-up (age 50+) | $8,000 (total $32,500) |
| 401(k) catch-up (ages 60–63) | $11,250 (total $35,750) |
| Combined employee + employer (415(c)) | $72,000 |
| IRA contribution (Roth or Traditional) | $7,500 |
| IRA catch-up (age 50+) | $1,100 (total $8,600) |
| Roth IRA phase-out — single | $153,000–$168,000 |
| Roth IRA phase-out — married filing jointly | $242,000–$252,000 |
Frequently asked questions
How does compound interest differ from simple interest?
Simple interest pays only on your original principal. Compound interest pays on the principal plus all previously earned interest, so the balance grows faster and faster over time.
What return should I use for long-term investing?
The S&P 500 has returned roughly 10% per year nominally over the long run, or about 7% after inflation. Using a real (after-inflation) return keeps your projection in today's purchasing power.
How much does starting early matter?
A lot. Because growth compounds, money invested in your twenties has decades to multiply. An investor who starts ten years earlier often ends up ahead even if they contribute less in total.
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Last updated: 2026-06-14