Compound Interest Calculator

How to use this compound interest calculator

Enter what you're starting with, what you can add monthly, and a realistic annual return (diversified stock portfolios have historically returned more than savings over long periods but with real ups and downs, while high-interest savings accounts and GICs pay less with far less risk). Set the years, and watch the interest share of the bar overtake your contributions — usually somewhere past year 15.

The compound interest formula

A = P(1 + r/n)^nt

• A — final amount
• P — principal (initial investment)
• r — annual rate (decimal)
• n — compounding periods per year
• t — years

Regular contributions don't fit in that one-liner, so this calculator simulates month by month: each month it applies growth (using the effective monthly rate for your chosen compounding frequency) and adds your contribution.

Why starting early beats saving more

Two savers, both earning 7%: Anna invests $300/month from age 25 to 35, then stops — total contributions $36,000. Ben waits until 35 and invests $300/month until 65 — total contributions $108,000. At 65, Anna has roughly $395,000; Ben has about $366,000. Anna contributed a third as much and ends ahead, because her money had an extra decade to compound. That asymmetry is the entire argument for starting now with whatever you have. Working toward a specific number? The savings goal calculator works backwards from your target.

What this calculator doesn't include

Taxes, investment fees, and inflation are not modeled, and real market returns are volatile — a steady 7% every year is a planning assumption, not a prediction. Expense ratios of even 0.5% meaningfully reduce long-run results; index funds with fees under 0.1% minimize this drag.