Compound interest is interest calculated on both your original principal and the interest that's already accumulated from prior periods. Unlike simple interest (which only applies to the principal), compound interest grows exponentially over time — the longer your money compounds, the faster it accelerates.
Compound interest follows the formula A = P × (1 + r/n)^(n×t), where A is the final amount, P is your starting principal, r is the annual interest rate (as a decimal), n is how many times per year it compounds, and t is time in years. Use the Compound Interest Calculator on this site to run your own numbers, including monthly contributions, without doing the math by hand.
Time is the single biggest lever in compound interest. Someone who starts investing years earlier, even with smaller contributions, often ends up ahead of someone who starts later with larger contributions, simply because their money has more time to compound.
This is the core argument for contributing to retirement accounts as early as possible, even in small amounts — see the retirement savings guide on this site for how 401(k)s and IRAs let that compounding happen with less tax drag along the way.
Interest can compound daily, monthly, quarterly, or annually — more frequent compounding produces a slightly higher return at the same nominal rate, but the difference is usually small compared to the impact of the rate itself and the length of time invested. Don't let compounding frequency distract you from the two variables that matter far more: how much you contribute, and how long you stay invested.
Informational content, not financial, tax, or legal advice. Verify amounts, limits, and current conditions directly with official sources (IRS, FDIC, SEC, CFPB) before making a decision.
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