Compound Interest Calculator
See exactly how your savings and investments grow with compound interest. Live growth chart, year-by-year breakdown, Rule of 72. Works for HYSA, 401(k), Roth IRA, S&P 500 investing.
Calculator
How it works
Calculation methodology
How Compound Interest Actually Builds Wealth
Compound interest is simple to describe and counterintuitive to feel: you earn returns on your returns. Year one you earn 7% on $10,000. Year two you earn 7% on $10,700. By year 30 you’re earning 7% on $76,000. The math gets wild fast.
The real formula — with monthly contributions
A = P(1 + r/n)^(nt) + PMT × [((1 + r/n)^(nt) − 1) / (r/n)]
P = starting amount, r = annual rate, n = compounding periods/year, t = years, PMT = monthly contribution. The calculator above handles all of this — just plug in your numbers.
2026 US account benchmarks
| Account | 2026 rate/return | Tax treatment |
|---|---|---|
| HYSA (online banks) | 4.00%–4.27% APY | Taxable |
| I-Bonds | 3.11% (May 2025 — check TreasuryDirect.gov for current rate) | Federal only |
| 401(k) target-date | 6%–7% projected | Tax-deferred |
| Roth IRA (index funds) | 7%–10% projected | Tax-free |
| S&P 500 (historical) | ~10%/yr average | Taxable |
2026 contribution limits
Roth IRA: $7,500/year ($8,600 if 50+). 401(k): $24,500/year ($32,500 if 50+). HSA: $4,400 individual, $8,750 family. These limits reset every January — max them out before investing in taxable accounts.
The cost of waiting one year
If you’re 30 and invest $6,000 this year vs waiting until next year at 7% return: that one-year delay costs you roughly $45,000 by age 65. Not $6,000. $45,000. That’s compound interest working against procrastination.
Ready to model your retirement number? Our Retirement Calculator pairs 401(k) projections with employer match and Social Security estimates for a complete picture.
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