Question 1

What is compound interest?

Accepted Answer

Compound interest is interest calculated on both your initial deposit and the interest you've already earned. Unlike simple interest — which only accrues on your principal — compound interest means your money grows exponentially over time, often described as "interest on interest".

The formula is: A = P × (1 + r/n)^(n×t)

Where A is the final amount, P is your principal (starting balance), r is the annual interest rate as a decimal, n is how many times interest compounds per year, and t is the number of years. For example, $1,000 at 5% compounded monthly for 10 years: A = 1,000 × (1 + 0.05/12)^(12×10) = $1,647.

Question 2

How is compound interest different from simple interest?

Accepted Answer

Simple interest is calculated only on your original investment. Compound interest is calculated on your principal plus any accumulated interest. Over long periods, the difference is dramatic: $10,000 at 7% simple interest for 30 years grows to $31,000, but with annual compounding it reaches $76,000 — more than twice as much.

Question 3

Does inflation matter when calculating compound interest?

Accepted Answer

Inflation gradually reduces the real purchasing power of money. The US Federal Reserve targets 2% annual inflation, though actual inflation has varied significantly. If inflation runs at 3%, a pot of $100,000 in 10 years will only buy what $74,000 buys today. Use the "Adjust for inflation" toggle to see the real value of your future pot in today's dollars — it's the most honest way to plan for retirement or long-term goals.

Question 4

What is a good rate of return to use?

Accepted Answer

The S&P 500 has historically returned around 10% annually (roughly 7% after inflation). For diversified US investors, a broad index fund like a total stock market fund has averaged 7–10% historically. A conservative estimate for long-term planning is 5–7% per year. Always account for fund expense ratios (typically 0.03–0.50%) and inflation when setting your expectations.

Question 5

How does compound interest work in a 401(k) or IRA?

Accepted Answer

A 401(k) and Traditional IRA allow your investments to compound tax-deferred — meaning you don't pay taxes on gains until withdrawal in retirement. A Roth IRA and Roth 401(k) offer tax-free compounding: you pay taxes on contributions now, but all future growth and qualified withdrawals are tax-free.

For 2024, the 401(k) contribution limit is $23,000 ($30,500 if you're 50+), and the IRA limit is $7,000 ($8,000 if you're 50+). The tax advantages of these accounts can significantly amplify the effect of compound interest over decades.

Question 6

Is compound interest taxable in the US?

Accepted Answer

Yes, in taxable brokerage accounts, interest and dividends are generally taxable in the year earned. Interest from savings accounts and CDs is taxed as ordinary income. Long-term capital gains (assets held over 1 year) are taxed at preferential rates of 0%, 15%, or 20% depending on your income.

In tax-advantaged accounts like 401(k)s and IRAs, you defer or eliminate this tax drag — which is one of the most powerful ways to accelerate compounding.

Question 7

What is the Rule of 72?

Accepted Answer

The Rule of 72 is a quick mental shortcut: divide 72 by your annual interest rate to estimate how many years it takes to double your money. At 6%, your money doubles in roughly 12 years (72 ÷ 6). At 8%, it doubles in 9 years. At 10% (the S&P 500 historical average), it doubles in about 7.2 years. The rule works best for rates between 6% and 10%.

Question 8

How much should I save each month to reach my goal?

Accepted Answer

A common starting benchmark is saving 15% of your gross income for retirement, including any employer 401(k) match. If you're starting late or have specific goals, you may need to save more.

Use this calculator with your current balance, expected return, and time horizon to work backwards: adjust the monthly contribution until the future value matches your target. Many financial planners suggest aiming for 10–12× your final salary by retirement.

Question 9

What are the best compound interest accounts in the US?

Accepted Answer

High-yield savings accounts (HYSAs) from online banks currently offer 4–5% APY and compound daily. Certificates of Deposit (CDs) offer fixed rates for a set term. Money market accounts offer similar rates with check-writing features.

For long-term growth, low-cost index funds in a 401(k) or IRA have historically outperformed savings accounts significantly. The best account depends on your time horizon: HYSAs and CDs for short-term savings, index funds for long-term wealth building.

Question 10

How often should interest compound for the best returns?

Accepted Answer

More frequent compounding means slightly higher returns. Daily compounding beats monthly, which beats annual. However, the difference is smaller than most people think: $10,000 at 5% for 10 years grows to $16,387 compounded annually vs $16,487 compounded daily — a difference of just $100.

The interest rate itself matters far more than the compounding frequency. Focus on maximising your rate of return and contribution amount rather than optimising compounding frequency.

Year	Deposits	Interest earned	Balance
0	1,000	0	1,000
1	0	51	1,051
2	0	54	1,105
3	0	57	1,161
4	0	59	1,221
5	0	62	1,283

Input values

Results

Chart

Table

FAQs

Compound Interest Calculator

Input values

Results

Chart

Table

FAQs