Understanding Compound Interest (And Why Starting Early Matters)

Compound interest is the phenomenon where the interest you earn also starts earning interest. Einstein reportedly called it the eighth wonder of the world; whether or not he did, the math is genuinely astonishing.

The formula

A = P × (1 + r/n)^(nt) — where A is the final amount, P the principal, r the annual rate, n the number of compounding periods per year, and t the number of years.

Three scenarios that show why time matters most

Assume a steady 8% annual return (roughly the historical average of the S&P 500):

1. Alex invests $200/month from age 22 to age 32, then stops. Total invested: $24,000.
2. Bailey invests $200/month from age 32 to age 62. Total invested: $72,000.
3. Cameron invests $200/month from age 22 to age 62. Total invested: $96,000.

At age 62, Alex has about $393,000; Bailey has about $293,000; Cameron has about $686,000. Alex invested one-third what Bailey did — and ends with more. Time in the market beats timing the market, again and again.

What this means for you today

Even $50 or $100 a month, started ten years earlier than you would otherwise, is worth more at retirement than $500/month started ten years later. The single best financial decision most people can make is to start saving today, not next year when things settle down.

Where compound interest shows up in your life

• Retirement accounts (401k, IRA, NPS): decades of tax-advantaged growth.
• Index-fund investing: broad, low-cost, and mathematically the highest expected long-run return for most people.
• High-yield savings: not spectacular, but risk-free and better than 0.05%.
• Credit-card debt: compound interest working against you at 22–28% APR. Kill it first.

Modelling it on your dashboard

Use the Goals module in Expense & Save to set a target amount and target date, then let the daily-saving assistant calculate the amount you need to set aside. Watching the graph curve upward is a genuine motivator — small numbers really do become large.