Compound interest by decade — what $500/month becomes in 10, 20, 30, and 40 years

$500/month invested at a 7% real return is the most useful single example in personal finance because almost any household can stretch to that number eventually. The total contribution over 40 years is $240,000 — but the final balance is over $1.2 million. Here's the year-by-year story of how that happens.

After 10 years: $86,000

You've contributed $60,000 and earned about $26,000 in growth. Interest is roughly a third of the balance. The chart looks almost linear — most of what you have is what you put in. Most people quit here, frustrated that "compound interest isn't working." It is. It's just early.

After 20 years: $260,000

You've contributed $120,000 and earned about $140,000 in growth. Now interest exceeds contributions. The chart starts to curve visibly upward. This is the inflection point where momentum takes over and your portfolio starts working harder than you do.

After 30 years: $612,000

You've contributed $180,000 and earned about $432,000 in growth. Interest is now 70% of the balance. Each year, your existing balance throws off more growth than you contribute. The curve is steep — the last decade alone added $352,000 (more than the previous two combined).

After 40 years: $1,228,000

You've contributed $240,000 and earned about $988,000 in growth. Contributions are 20% of the balance; growth is 80%. The final 10 years alone added $616,000 — half the entire 40-year wealth was built in the last quarter. This is the compounding curve's final acceleration.

The doubling progression

At 7%, the Rule of 72 gives a doubling period of about 10 years. So your $86K after decade 1 doubles roughly to $172K — except you keep contributing, so by year 20 it's $260K. By year 30 it doubles again toward ~$520K plus contributions = $612K. By year 40, $1.06M plus contributions = $1.22M. Each doubling is bigger than the previous one in absolute dollars.

Why patience is the entire game

A 30-year investor and a 40-year investor put in only $60,000 difference in contributions ($180K vs $240K) but end with a $616,000 difference in final balance. The extra decade is worth 10× its own contributions because by the time you reach it, your portfolio is gigantic and 7% of "gigantic" is huge. Quitting at year 30 — exactly when compounding becomes most powerful — is the most expensive mistake.

The opposite case: late starts

Starting the same $500/month plan at age 35 instead of 25 means stopping at 65 with $612K instead of $1.22M. You contributed $60K less but ended up $616K poorer. Each early year is worth more than each late year by 5-10×. This is the "compound interest is the eighth wonder" claim made concrete.