What compound growth actually is
Simple interest pays you only on your original principal. Compound growth pays you on principal plus all previously earned returns. The difference seems small in year one and becomes enormous by year twenty: $100,000 at 10% simple interest grows to $300,000 in 20 years, while compounded it grows to roughly $673,000.
The Rule of 72
To estimate how fast money doubles, divide 72 by your annual return. At 6%, money doubles every 12 years. At 10%, every 7.2 years. Over a 36-year career at 10%, one dollar doubles five times — becoming $32. That's why a single $10,000 investment in your twenties can be worth more than $300,000 by retirement without you adding a cent.
Real numbers: $500 a month
Here's what investing $500/month at a 10% average annual return produces over time:
- 10 years: ~$102,000 (you contributed $60,000)
- 20 years: ~$380,000 (you contributed $120,000)
- 30 years: ~$1,130,000 (you contributed $180,000)
- 40 years: ~$3,160,000 (you contributed $240,000)
Why starting early beats investing more
Consider two investors. Anna invests $500/month from age 25 to 35, then stops — $60,000 total. Ben invests $500/month from 35 to 65 — $180,000 total. At 10%, Anna's account at 65 is worth roughly $1.6M; Ben's about $1.13M. Anna invested a third as much money and ended with more, because her dollars had 10 extra years to double.
The lesson isn't "it's too late" — it's that the most expensive thing you can do is wait another year. Whatever your age, the best compounding window you'll ever have starts now.
Compounding works in reverse, too
A credit card at 24% APR is compound growth working against you — debt that doubles in three years if unpaid. This is why eliminating high-interest debt is mathematically identical to earning a guaranteed 24% return, and why it comes before investing in every sound plan.
Frequently asked questions
What return rate should I assume for planning?
The S&P 500 has averaged roughly 10% annually before inflation over the long run (about 7% after inflation). Diversified income portfolios commonly target 7–12%. Plan conservatively at 7–8% and treat anything above as margin of safety.
How often does compounding happen?
It depends on the asset — stocks effectively compound continuously through price growth and reinvested dividends; funds that pay quarterly distributions compound when you reinvest them. The frequency matters far less than the rate and the years you stay invested.
Does compounding still work if markets crash?
Yes — long-run average returns already include crash years. Investors who kept contributing through 2008 and 2020 compounded from discounted prices, which actually accelerated their outcomes.
Put this into practice
Reading builds knowledge. Your number builds urgency. Calculate the exact capital that makes work optional for you.