The world's most advanced compound interest calculator — Monte Carlo, 5-way solve, fee drag & FIRE milestones.
| Year | Balance | Interest | Principal | Added |
|---|
Find any unknown to hit your target. Select what to solve for:
Conservative, Base, and Aggressive returns — using your current principal, contributions & time horizon.
See exactly what you lose by waiting — using your current rate, contributions & time horizon.
Final balance at every combination of rate and time, using your current principal & monthly contributions.
Based on your current inputs, here's when you'll hit key wealth milestones.
Final balance at different monthly contribution levels, using your current rate & time horizon. Your current contribution is highlighted.
Your rate vs. S&P 500, Real Estate, Bonds & HYSA — same principal & contributions over time.
Enter your starting principal and regular contributions. Use the Solve For pills to find any unknown — future value, required contribution, rate, time, or starting amount.
Expand Advanced Settings to include inflation, tax drag, annual advisory fees, and volatility for a Monte Carlo simulation with confidence bands.
Use Scenario Analysis for goal seeking & Power of Starting Early, or Wealth Projector for FIRE milestones and benchmark comparisons.
Compound interest is the addition of interest to the principal sum of a loan or deposit — interest on interest. It results from reinvesting interest rather than paying it out, so interest in the next period is earned on the principal sum plus previously accumulated interest.
The Solve For pills at the top of the input panel let you find any unknown. Select "Find Contribution" and enter a target amount to see how much you need to save monthly. Select "Find Rate" to discover what return you need to reach your goal. The solved value appears highlighted in the corresponding input field.
Monte Carlo runs 500 scenarios where each year's return varies randomly around your entered rate (using the volatility σ setting). The confidence band chart shows the p10, p25, p50 (median), p75, and p90 outcome ranges. The success rate badge shows what percentage of scenarios hit your target goal.
An annual advisory fee (e.g., 0.5% for an index fund ETF vs 1.0% for an actively managed fund) is deducted from your balance each year. At $500/month over 30 years at 7%, a 1% fee costs over $150,000 in lost compound growth — nearly the same as 3 years of contributions.
APR (Annual Percentage Rate) is the annual rate without compounding effects. APY (Annual Percentage Yield) is the effective annual rate including compounding — it's always higher than APR when compounding is more frequent than annually. The calculator shows live APY based on your rate and compounding frequency.
Inflation erodes the purchasing power of your money. Enable "Inflation" in Advanced Settings to see the real value of your future wealth in today's dollars. At 2.5% inflation, $1M in 30 years has the purchasing power of only about $477K today.
This chart shows the difference between daily, monthly, quarterly, and annual compounding — all other inputs equal. It reveals that while daily compounding is mathematically superior, the practical difference vs monthly is often surprisingly small (usually under 0.1% of the final balance).
FIRE (Financial Independence, Retire Early) milestones show how many years until your balance reaches key thresholds ($100K, $500K, $1M, $2M, $5M) based on your current inputs. These appear in the Wealth Projector tab and update live as you change your inputs.
What is compound interest?
Compound interest is interest earned on both your initial principal and on previously accumulated interest. Unlike simple interest (calculated only on the principal), compound interest grows exponentially over time. The frequency of compounding (daily, monthly, annually) affects how quickly your money grows.
Compound interest calculates growth on both the initial principal and previously accumulated interest, creating exponential wealth over time.
A = P(1 + r/n)^(nt) Where: A = final amount, P = principal, r = annual rate (decimal), n = compounds per year, t = years
FV = P(1 + r/n)^(nt) + PMT × [((1 + r/n)^(nt) − 1) / (r/n)] PMT = periodic contribution amount, adjusted for contribution frequency
After-Tax Return = r × (1 − tax_rate) Applied annually to interest earned; reduces the compounding base each year
Real Value = Nominal Value / (1 + inflation_rate)^t Years to Double ≈ 72 / Annual Interest Rate Scenario: $10,000 principal, 7% annual rate, monthly compounding, 10 years, no contributions
A = 10,000 × (1 + 0.07/12)^(120) = $20,097 | Interest: $10,097
Scenario: $10,000 + $500/month, 7% rate, monthly compounding, 20 years
Final balance: $301,354 | Interest earned: $171,354 (57% of final balance)
Scenario: $10,000 + $500/month, 7% rate, 30 years — with vs without 1% annual fee
No fee: $632,408 | With 1% fee: ~$475,700 | Fee drag: ~$156,700
$10,000 at 7% APR over 10 years with different compounding frequencies:
| Frequency | Compounds/Year | APY | Final Balance | Extra vs Annual |
|---|---|---|---|---|
| Annually | 1 | 7.00% | $19,672 | — |
| Quarterly | 4 | 7.19% | $20,016 | +$344 |
| Monthly | 12 | 7.23% | $20,097 | +$425 |
| Daily | 365 | 7.25% | $20,138 | +$466 |
Albert Einstein is often credited with calling compound interest the eighth wonder of the world. Whether or not he actually said it, the math behind compounding is genuinely remarkable. Unlike simple interest, which only grows based on your original deposit, compound interest generates returns on your accumulated returns, creating an exponential curve that accelerates over time.
The most powerful variable in the compound interest equation is time. An investor who starts at age 25 with modest contributions will almost always outperform someone who starts at 35 with larger contributions. The "Power of Starting Early" section in Scenario Analysis quantifies exactly what each year of waiting costs you in future wealth.
Investment fees are the silent wealth destroyers. A seemingly small 1% annual advisory fee sounds innocuous, but compounding that fee drag over 30 years on a growing portfolio can cost more than $150,000 in lost final wealth. The Fee Impact chart in this calculator makes that cost viscerally clear. Passive index funds with expense ratios of 0.03–0.05% are vastly superior to actively managed funds at 1–2% from a compounding perspective.
Real markets don't deliver smooth average returns — they fluctuate wildly. The Monte Carlo simulation in this calculator models 500 potential futures where annual returns vary randomly around your expected rate (using the volatility setting). The confidence bands show you the range of realistic outcomes, helping you understand not just the expected result but the risk around it.
Compounding is most powerful in tax-advantaged accounts like 401(k)s, IRAs, and Roth accounts where returns are not diminished by annual capital gains taxes. The tax rate input in Advanced Settings lets you compare tax-advantaged vs taxable account growth and see exactly how much the tax drag costs you over decades.