Compound Interest Calculator 2026
See how your money grows when you give it time.
Calculate the power of compound interest on any investment in United States. Enter your initial investment, interest rate, and time period. Optionally add monthly contributions. Shows final balance and total interest earned.
Past returns do not guarantee future results. Always consider risk before investing.
The formula behind compound interest growth
Compound vs simple interest
Simple interest calculates interest only on the principal. Compound interest calculates interest on the principal plus previously earned interest — "interest on interest." On $10,000 at 7% for 10 years: simple interest = $7,000 extra. Compound (annual) = $9,672 extra. The difference grows dramatically over longer periods.
Quick way to estimate how long money takes to double
Rule of 72
Divide 72 by the annual interest rate to estimate doubling time. At 6%: 72/6 = 12 years to double. At 8%: 9 years. At 10%: 7.2 years. This works best for rates between 6–20%.
| Rate | Doubles in |
|---|---|
| 4% | 18 years |
| 6% | 12 years |
| 8% | 9 years |
| 10% | 7.2 years |
| 12% | 6 years |
How US tax-advantaged accounts amplify compounding
Compound interest gets crushed by annual tax drag in a regular brokerage account. Every dividend, interest payment, and realized gain is taxed each year, reducing the base that compounds next year. Moving the same dollars into a tax-advantaged wrapper — where growth, dividends, and qualified withdrawals escape The IRS's annual bite — is often the single largest multi-decade return boost available to US savers.
2026 contribution limits
The IRS publishes contribution caps each year. For 2026: 401(k) elective deferrals $24,500 (plus $7,500 catch-up at age 50+, and a new $11,250 super-catch-up at ages 60-63); Traditional and Roth IRA $7,500 combined ($8,500 age 50+); HSA $4,400 self-only / $8,750 family (plus $1,000 catch-up at 55+); SEP-IRA up to 25% of compensation or $70,000; 529 plans follow state rules but typically accept up to the $19,000 annual gift exclusion per donor without filing Form 709. See IRS Retirement Topics — Contributions.
Tax treatment by wrapper
A Roth IRA or Roth 401(k) takes after-tax dollars and delivers tax-free qualified withdrawals after age 59½ plus 5-year rule. A Traditional IRA/401(k) gives an upfront deduction and taxes the whole balance at ordinary rates on withdrawal. An HSA paired with an HDHP is the only triple-tax-advantaged account — deductible going in, tax-free growth, tax-free for qualified medical — and acts like a stealth IRA after 65 (non-medical withdrawals taxed as ordinary income, no penalty). 529s grow federal-tax-free for qualified education, with 30+ states offering a further income-tax deduction on contributions.
Why taxable brokerage loses ~0.5-1% per year
Morningstar's long-running tax-cost ratio study estimates actively managed US equity funds lose 1-2% of annual return to taxes, and even broad index funds lose 0.3-0.7% through distributed dividends and capital gains. Over 30 years at a 7% nominal return, a 0.75% tax drag cuts an ending $100,000 initial balance from roughly $761,000 to $609,000 — a $152,000 IRS-tax-drag penalty that a Roth or HSA would have eliminated entirely.
Nominal vs real compound returns
A compound calculator quoted in nominal dollars always looks bigger than the purchasing power you actually end up with. US inflation, measured by the Bureau of Labor Statistics Consumer Price Index for All Urban Consumers (CPI-U), has averaged roughly 3% since 1926 and about 2.9% over 1995-2025. Subtracting that from your nominal return gives the real (inflation-adjusted) return — the one that actually translates into groceries, rent, and plane tickets decades from now.
$10,000 at 7% nominal, 3% inflation
| Years | Nominal balance (7%) | Real balance (4% after 3% CPI) | Purchasing-power gap |
|---|---|---|---|
| 10 | $19,672 | $14,802 | $4,870 |
| 20 | $38,697 | $21,911 | $16,786 |
| 30 | $76,123 | $32,434 | $43,689 |
| 40 | $149,745 | $48,010 | $101,735 |
Why this matters for retirement planning
Fidelity and Vanguard retirement projections typically assume a 5-6% real return from a diversified 60/40 portfolio, not the 8-10% nominal figure financial media often quote. The 4% withdrawal rule (Bengen, 1994; Trinity Study) is also a real-terms rule — it assumes withdrawals rise each year with CPI. When you run your own compound projection, subtract ~2.5-3% from historical nominal returns, or toggle this calculator's rate to a real-return assumption, to avoid overestimating future spending power. For the latest CPI release schedule see BLS CPI News Release Schedule.
Frequently askedFrequently asked questions
What is a realistic long-term investment return?
US stocks (S&P 500) have historically returned approximately 10% annually before inflation, or 7% after inflation. High-yield savings accounts in 2026 earn approximately 4–5% APY. Individual results vary significantly.
Does compounding frequency matter much?
At 7% over 10 years on $10,000: annual compounding = $19,672; monthly compounding = $20,097; daily compounding = $20,137. The difference between monthly and daily is minimal. The frequency matters much more at higher rates and longer time horizons.
Where these figures come from
Savings and investment figures on this page are drawn from The Federal Reserve (rates), the FDIC (deposit insurance), The SEC (investor protection), and The IRS (tax treatment of retirement vehicles).
- Federal funds rate — Federal Reserve — Open Market Operations.
- Interest rate & economic series — FRED — Federal Reserve Economic Data.
- Deposit insurance (up to $250,000) — FDIC — Deposit Insurance.
- Investor education — SEC — Investor.gov.
- IRA & 401(k) contribution limits — IRS — Retirement Plan Contribution Limits.
Last checked: April 2026. Rates and thresholds are reviewed against the source of record each November, when annual adjustments for the following tax year are published.
Select the question that matches where you are right now.
Your result shows the projected growth or return based on the rate, contribution, and time period you entered — using standard compound or simple interest formulas.
Use this to set savings targets, compare investment options, or understand the impact of starting earlier. Adjust the rate and timeframe to model optimistic and conservative scenarios.
Not a guaranteed return. Actual investment outcomes depend on market conditions, fees, taxes, and timing that cannot be predicted with certainty.
Uses standard financial formulas with the inputs you provided. All calculations run in your browser — no data is sent to any server.
Savings and investment results are dominated by three factors: the rate of return, the time horizon, and regular contributions. Compounding amplifies all three over time.
Returns on returns accelerate growth over time. The difference between 5% and 7% over 20 years is much larger than the 2% gap suggests — compounding is non-linear.
Adding even small regular amounts dramatically increases the final balance. $100/week invested at 7% for 20 years grows to over $110,000 in contributions and $110,000+ in returns.
Starting 5 years earlier often produces a larger final balance than doubling your contribution rate. Time is the most powerful variable in savings calculations.
To improve your savings outcome, focus on starting earlier, increasing contributions, and minimising fees and tax drag on returns.
Starting with a small amount today and increasing over time beats waiting to start with a larger amount. Time in the market matters more than timing the market.
A 1% annual fee on a $100k balance costs $1,000/year and compounds against you. Compare fee structures across savings and investment products.
Super, offset accounts, and tax-free thresholds reduce the drag of tax on your returns — letting more of the growth compound for you.
Savings decisions connect to investment, tax, and retirement planning. Use these calculators to model the broader picture.
Work backwards from a target amount to see how much you need to save each month.
Savings goal →Model how an initial investment grows with regular contributions over different time periods.
Compound interest →See what your future balance is worth in today's dollars after adjusting for inflation.
Inflation calculator →