How Compound Interest Works in the US

Summary

Compound interest earns returns on both the original principal and on previously earned interest, creating exponential growth over time. In the US, tax-advantaged accounts like 401(k)s, Traditional IRAs, and Roth IRAs allow investments to compound without annual tax drag, significantly boosting long-term wealth. The S&P 500 has returned roughly 10% annually (nominal) over the past 50 years.

How it works

The compound interest formula combines lump-sum growth with regular contributions:

FV = P(1 + r/n)^(nt) + PMT x [((1 + r/n)^(nt) - 1) / (r/n)]

Where P = principal, r = annual rate, n = compounding frequency, t = years, PMT = contribution per period.

US-specific investment vehicles

For taxable brokerage accounts, long-term capital gains (held >1 year) are taxed at 0%, 15%, or 20% depending on income, creating a tax drag that reduces effective compounding.

Cash savings

US savings accounts average 0.39% APY nationally (FDIC), though high-yield savings accounts offer 4-5% in the current rate environment. FDIC insurance covers up to $250,000 per depositor, per bank.

Worked example

$10,000 initial + $500/month at 7% for 20 years (monthly compounding):

1. Monthly rate: 7% / 12 = 0.5833%
2. Lump-sum growth: $10,000 x (1.005833)^240 = $40,387
3. Annuity growth: $500 x ((1.005833)^240 - 1) / 0.005833 = $260,464
4. Total future value: $40,387 + $260,464 = $300,851
5. Total contributions: $10,000 + ($500 x 240) = $130,000
6. Interest earned: $300,851 - $130,000 = $170,851 (131% return on contributions)

In a Roth IRA, the $170,851 in gains would be entirely tax-free on withdrawal.

Key differences from other markets

• Tax-advantaged retirement accounts (401k, IRA, Roth) are a uniquely American structure. The UK uses ISAs and pensions; Australia uses superannuation. The US system offers the highest annual contribution limits ($24,500 + $7,500 = $32,000) among major markets.
• FDIC insurance at $250,000 per depositor is more generous than the UK’s FSCS limit of 85,000 pounds or Australia’s guarantee of A$250,000, though the US average savings rate (0.39%) is lower than many international peers.