The most powerful force in personal finance. Visualized.

The calculation is built on the standard compound interest formula: balance grows each period by your interest rate divided by the number of compounding periods per year. Daily compounding applies that fraction 365 times a year; monthly compounding applies it 12 times. The practical difference between the two at, say, 7% annual interest is small — daily compounding produces only slightly more than monthly over the same term — but the distinction matters when you're comparing, for example, a savings account that advertises daily compounding against a bond that pays monthly.

On top of the growth calculation, this calculator lets you add regular contributions — monthly, annually, or a lump sum starting balance — and displays each year's result broken into two separate numbers: how much of your ending balance came from money you actually put in (your contributions), and how much came purely from growth. That split is the number most calculators skip, and it's the one that makes compounding make sense intuitively. For the first several years, your contributions dominate. At some point — and you can see it clearly in the chart — investment growth begins exceeding what you contribute each year. That crossover point is when compounding starts doing the heavy lifting.

For example: $10,000 starting balance plus $500 per month at 7% annually, compounded monthly, grows to roughly $606,000 after 30 years. Of that, you contributed approximately $190,000 (starting balance plus 360 months of contributions). The remaining $416,000 — nearly 70% of the ending balance — came from compound growth alone. That ratio flips dramatically with longer time horizons, which is exactly why the cost of starting late is as large as it is.

People tend to think of the cost of starting to invest 10 years later as "10 years of contributions I missed." That framing dramatically understates the real cost. The actual cost of waiting 10 years is the compounded growth on every dollar you would have contributed during those 10 years — not just the dollars themselves, but the decades of compounding those dollars would have had. The later dollars you eventually contribute can never catch up, because they have less time left on the clock.

A concrete example: investor A starts at 25 with $300/month at 7% and stops at 35 — contributing for only 10 years — then leaves the money alone to grow until 65. Investor B starts at 35 with $300/month at 7% and contributes every month until 65 — 30 full years of contributions. Despite contributing for three times as long and putting in three times as many total dollars, Investor B ends up with less money than Investor A at retirement. The reason is purely the 10-year head start on compounding. This is the real insight behind every "start early" argument, stated in numbers rather than in platitudes.

The implication isn't that you should panic if you started late — contributing later is always better than not contributing. But it means that if you're in your 20s and uncertain whether it's worth starting with a small amount, the actual cost of a five-year delay is not "I missed 5 years of $200/month contributions" — it's the full compounded value of those contributions and their growth over your entire remaining investment horizon.

Interest can compound at different frequencies: annually, monthly, weekly, or daily. The stated annual interest rate is the same in each case, but the effective yield — the actual return you end up with — is slightly higher with more frequent compounding because each period's interest earns interest sooner. A 7% annually compounded rate produces a different final balance than a 7% daily compounded rate over 30 years, though the gap is modest at typical interest rate levels.

Where compounding frequency matters more is in debt, not just savings. Credit card APRs (annual percentage rates) that compound daily are one reason why carrying a credit card balance is so expensive — interest is being added to your balance every day, and the next day's interest charge includes yesterday's new balance.

The Rule of 72 is a simple approximation: divide 72 by your annual interest rate to get the number of years it takes your money to double. At 6% annual return, 72 ÷ 6 = 12 years to double. At 8%, about 9 years. At 4%, roughly 18 years. It's a rough estimate, not an exact calculation, but it's useful for quickly gut-checking whether a given interest rate and time horizon seem reasonable without running the full formula — and for appreciating why higher interest rates and longer time horizons compound so dramatically relative to lower rates and shorter ones.