Compound Interest by Decade: $500/mo at 7% Nominal

This fixed-rate illustration follows $500 deposited at each month-end at a 7% nominal annual rate, with monthly compounding and no fees, taxes or inflation adjustment. Total contributions over 40 years are $240,000 and the arithmetic ending balance is about $1,312,407. It explains the formula; it is not a return forecast.

After 10 years: about $86,542

You've contributed $60,000 and the arithmetic growth is about $26,542. A short horizon makes the contribution line look more prominent than the growth line; changing the rate, timing or contribution changes the result.

After 20 years: about $260,463

You've contributed $120,000 and the arithmetic growth is about $140,463. Under this fixed-rate case, growth has become larger than the deposits, but actual returns can vary from year to year.

Our 10,000-scenario simulation pegs the median crossover year — when annual interest first exceeds annual contributions — at around year 13–17 for realistic inputs. The "decade 2" milestone in this guide is the structural reason most savers give up early: years 1–10 look almost linear before the curve takes off.

After 30 years: about $609,985

You've contributed $180,000 and the arithmetic growth is about $429,985. The result is sensitive to the selected rate and deposit timing; it does not describe a predictable market path.

After 40 years: about $1.31 million

You've contributed $240,000 and the arithmetic growth is about $1,072,407. The final decade adds more in dollars than earlier decades under this constant-rate model, because the modeled balance is larger.

The doubling progression

At 7%, the Rule of 72 gives a doubling period of about 10 years. Deposits make the balance path differ from a single lump sum: this scenario reaches about $86,542 after 10 years, $260,463 after 20, $609,985 after 30 and $1,312,407 after 40. Each extra period contributes both another deposit and another period of modeled growth.

How the horizon changes the arithmetic

A 30-year case has $180,000 of deposits and a modeled balance of about $609,985; the 40-year case has $240,000 of deposits and about $1,312,407. The difference comes from the extra deposits and the extra periods of modeled growth, not from a guaranteed market effect.

The opposite case: a later start

Starting the same fixed-dollar plan at 35 instead of 25 and stopping at 65 changes the modeled horizon from 40 to 30 years. Under this scenario the balances are about $609,985 and $1,312,407 respectively; a different rate, inflation path or contribution schedule changes the comparison.