Compound Interest: What Grows It and What Eats It
By The Bureau of Wealth Team 1146 words
This article is general information, not financial advice. Consider your own circumstances or speak to a qualified adviser before acting on it.
Starting ten years earlier can beat doubling your contributions. And a 1% fee can take a bigger bite than you might guess.
Compound interest means your returns start earning returns of their own, so growth speeds up the longer money stays invested. The biggest lever you control is time, not the rate: starting ten years earlier can beat doubling what you put in. But the headline number on any projection overstates what you'll really have, because three things quietly subtract from it every year: inflation, fees and taxes. The goal isn't the highest nominal return; it's the highest return left after those three.
This guide is for savers and new investors who want to know what compounding can and can't do. The growth rates below are illustrations, not forecasts. Investments can fall in value, and no calculator knows what markets will do.
How compounding actually builds up
Say you invest $300 a month and it grows at 6% a year. The compound interest calculator shows that after 35 years you'd have $427,413.09. Only $126,000 of that is money you paid in. The other $301,413.09 is growth, and most of it arrives in the last decade, because that's when the balance is largest.
Now start ten years later. The same $300 a month for 25 years reaches $207,898.19, less than half. Here's the surprise: even if you double your contribution to $600 a month for those 25 years, you end up with $415,796.38. You've paid in $180,000, which is $54,000 more than the early starter, and you still finish behind.
A quick way to see this is the rule of 72, a common approximation: divide 72 by the annual rate to estimate how many years it takes money to double. At 6%, that's roughly 12 years. Every doubling you miss at the start is a doubling you lose at the end.
Don't be distracted by how often interest compounds. Banks advertise daily compounding, but it changes little. On $10,000 at 5% for 10 years, the calculator gives $16,288.95 with yearly compounding and $16,486.65 with daily, a difference of about $198. Dropping the rate to 4% with daily compounding leaves $14,917.92. The rate and the years matter far more than the frequency.
Inflation: the growth you don't really get
A dollar in 35 years won't buy what a dollar buys today. At an assumed 2.5% a year, the inflation calculator shows that $427,413.09 in 35 years has the buying power of about $180,099.51 today. That's still a lot of money, but it's a very different retirement from the one the headline figure suggests.
For cash savers, inflation does more than trim growth. According to the Bureau of Labor Statistics, consumer prices rose 3.4% in the 12 months to August 2026. The FDIC's national average savings rate in August 2026 was 0.38%. At 3.4% inflation, $10,000 held for a year buys only what $9,671.18 buys today, and 0.38% interest recovers just $38.07 of that loss. Compounding at a rate below inflation is shrinking slowly, not growing.
Fees: small percentages, big dollar amounts
Fees are charged on your whole balance every year, whether or not the investment did well, so they compound against you. The SEC's investor bulletin on fees and expenses shows a $100,000 investment growing 4% a year for 20 years: with a 0.25% annual fee it would be worth about $208,000, and with a 1% fee about $179,000.
Run your own numbers in the investment fees calculator. Starting with $10,000 and adding $300 a month for 30 years at a 6% gross return, a 1% yearly fee leaves $284,527.99. A 0.2% fee leaves $335,497.60. The gap is $50,969.61, on total contributions of $118,000. At 1%, fees cost $65,260.81 compared with no fee at all.
Check the expense ratio of every fund you hold, plus any advisory or account fee on top. If you pay an adviser a percentage of assets, ask what you get for it in dollars, not percent.
Taxes: what compounds is what you keep
The IRS treats interest on bank accounts, money market accounts and CDs as taxable income each year. In a taxable account, part of every year's growth goes to tax instead of staying invested, so less is left to compound. The damage grows with your tax bracket and with time.
Tax-advantaged accounts fix much of this. For 2026, the IRS allows up to $24,500 in elective deferrals to a 401(k), 403(b), 457 plan or the Thrift Savings Plan, plus an $8,000 catch-up from age 50 ($11,250 at ages 60 to 63), and up to $7,500 in an IRA. Growth inside those accounts isn't taxed year by year. The retirement savings calculator projects a 401(k) and IRA together, including employer contributions and inflation.
| What erodes growth | How it works | What you control |
|---|---|---|
| Inflation | Reduces what each future dollar buys | Keep long-term money in assets that can outpace it; don't hold decades of savings in low-rate cash |
| Fees | Taken from the full balance every year | Choose low-cost funds; question percentage-of-assets charges |
| Taxes | Take part of each year's growth in taxable accounts | Use 401(k)s and IRAs before a taxable brokerage account |
When chasing compound growth is the wrong move
Compounding needs time. Money you'll need within the next few years, such as an emergency fund or a house down payment, shouldn't be invested for growth, because a fall in value just before you need it can't be waited out. For that money, safety and access beat return.
Compounding also works against you on debt. Paying off a credit card that charges a high interest rate gives you a guaranteed return equal to that rate, which is hard for any investment to beat reliably. Clear expensive debt before investing beyond any employer 401(k) match. If you're close to retirement, the long-horizon examples above overstate what's possible, and a fee-only financial adviser can help you weigh growth against the risk of a late loss.
What to do next
- Next 10 minutes: put your current balance and monthly saving into the compound interest calculator, then rerun it starting five years later to see what delay costs.
- Today: find the expense ratio of each fund you own and enter the average in the investment fees calculator.
- This week: check whether you're using your 401(k) and IRA before any taxable account, and compare your savings account rate with the FDIC's 0.38% national average.
Sources
- U.S. Securities and Exchange Commission (Investor.gov): How Fees and Expenses Affect Your Investment Portfolio: Investor Bulletin
- Internal Revenue Service: 401(k) limit increases to $24,500 for 2026, IRA limit increases to $7,500
- U.S. Bureau of Labor Statistics: Consumer Price Index Summary
- Internal Revenue Service: Topic no. 403, Interest received
