Compound Interest Calculator
Estimate how a starting balance and steady monthly deposits can grow with compound interest over time.
Rule of 72: at 7% your money doubles roughly every 10.3 years.
How compound interest actually works
Compound interest is the reason ordinary savers can build extraordinary balances. Each time your account earns interest, that interest is added to the principal. The next period, interest is calculated on the new, larger balance. Small amounts turn into large amounts because the growth builds on itself.
The three levers that matter most are how much you start with, how much you add on a regular schedule, and how long you leave the money invested. Rate of return matters too, but chasing an extra point of return often adds more risk than it is worth. Time is the variable you should protect most jealously.
Compounding frequency
Interest can compound annually, quarterly, monthly, or daily. More frequent compounding produces a slightly higher effective yield, but the practical difference between monthly and daily is usually tiny. Do not lose sleep over frequency. Focus on rate, contribution, and years.
The Rule of 72
The Rule of 72 is a mental shortcut. Divide 72 by the annual rate of return and you get a rough estimate of how long the balance takes to double. At 6 percent, money doubles about every 12 years. At 9 percent, every 8 years. The rule is not perfect, but it is close enough for planning conversations.
Time in the market
The earliest dollars you invest do most of the work. A dollar invested at age 25 has about forty years to compound before a normal retirement age. A dollar invested at age 45 has only about twenty years. That is why starting early, even with small amounts, tends to beat starting late with larger amounts.
Reality checks
- Real markets do not deliver a smooth annual return. Expect ups and downs.
- Fees, taxes, and inflation all reduce what a projected number actually buys.
- Automating contributions is the single most reliable way to stay on track.
Use this calculator to test scenarios. What happens if you contribute an extra 100 dollars a month? What if you start five years earlier? Small changes, viewed over decades, produce surprisingly large differences.