Compound Interest Calculator
See how compound interest grows your money over time with regular contributions.
Watch your money compound
See how initial principal plus regular contributions grow exponentially over time.
Set 0 for nominal, 3% for inflation-adjusted (real) returns.
How to use this calculator
- Enter your initial investment—the principal you're starting with today.
- Add a monthly contribution—set to 0 if you're only compounding the initial principal.
- Input the annual interest rate—use realistic long-term returns (7–8% stocks, 4–5% savings).
- Set your time horizon—compounding gets dramatically more powerful over longer periods.
- Choose compounding frequency—monthly is common for savings; daily is typical for some bank accounts.
- Optionally set inflation—to see the real (purchasing-power-adjusted) value.
- Click Calculate to see your projected balance, breakdown, and growth trajectory.
How compound interest works
Compound interest is interest earned on both your initial principal and the accumulated interest from prior periods. Unlike simple interest (calculated only on principal), compounding creates exponential growth—your money grows faster with each passing year, because each year's interest becomes part of the next year's principal. Albert Einstein is often quoted as calling it "the eighth wonder of the world"; whether or not he actually said it, the sentiment captures why compounding is the most powerful force in personal finance.
The compound interest formula
For a single initial principal with no additional contributions:
A = P × (1 + r/n)^(n × t)
Where:
- A = final amount
- P = principal (initial investment)
- r = annual interest rate (decimal)
- n = number of times interest is compounded per year
- t = number of years
Adding regular contributions
When you make ongoing monthly contributions, the future value combines the compound interest formula above with the future value of an annuity:
A = P(1 + r/n)^(nt) + PMT × [((1 + r/n)^(nt) − 1) / (r/n)]
The first term compounds your initial principal; the second term compounds your stream of monthly contributions. Both grow exponentially, but the contribution term benefits especially from time—even modest monthly additions can exceed the original principal over long horizons.
The Rule of 72
The Rule of 72 is a mental shortcut for estimating doubling time: divide 72 by your annual return rate. At 8%, money doubles every 9 years (72 ÷ 8). At 6%, every 12 years. At 10%, every 7.2 years. This reveals why even a small return advantage matters enormously over decades: a 2% difference (8% vs 6%) over 30 years means roughly 50% more wealth.
Compounding frequency: does it matter?
More frequent compounding means slightly more growth—daily beats monthly, which beats annually. But the difference is modest. At 8% over 30 years, daily compounding yields about 0.1% more per year than annual compounding. The biggest lever by far is the interest rate itself, not the compounding frequency. Don't let banks advertise "daily compounding" distract you from comparing actual APYs.
Why starting early beats saving more
Time is the most powerful variable in compound growth. Consider two investors: Investor A saves $5,000/year from age 25–35 (10 years, $50,000 total) and then stops. Investor B saves $5,000/year from age 35–65 (30 years, $150,000 total). At 8% returns, Investor A has $787,176 at age 65, while Investor B has only $611,729—despite contributing three times as much. The first 10 years of compounding, repeated for 30 more years, overwhelmed triple the savings. This is why retirement advisors plead: start now.
Worked example
Scenario: Marcus invests $10,000 initially and adds $250/month for 30 years at 8% annual interest, compounded monthly.
- Principal (P): $10,000
- Monthly contribution (PMT): $250
- Monthly rate (r/n): 0.08 ÷ 12 = 0.00667
- Compounding periods (n × t): 12 × 30 = 360
- FV of principal: $10,000 × (1.00667)^360 = $110,021
- FV of contributions: $250 × [((1.00667)^360 − 1) / 0.00667] = $250 × 1490.34 = $372,585
- Final balance: $110,021 + $372,585 = $482,606
Of that $482,606: $10,000 was initial principal, $90,000 was monthly contributions ($250 × 360), and $382,606 is pure compound interest—about 79% of the final balance. Marcus's money earned nearly four times what he contributed.
By the Rule of 72, his initial $10,000 doubled roughly every 9 years—so it doubled 3+ times over 30 years: $10K → $20K → $40K → $80K → $110K (with the contribution stream layered on top). That's compounding in action.
Glossary
- Compound Interest
- Interest calculated on the principal plus all previously earned interest—producing exponential growth.
- Simple Interest
- Interest calculated only on the original principal. Over long periods, compound interest dramatically outperforms simple interest.
- Rule of 72
- Mental shortcut: divide 72 by your annual return rate to estimate doubling time. At 8%, money doubles in 9 years.
- APY (Annual Percentage Yield)
- The effective annual rate including compounding. APY is what you actually earn, vs. the nominal rate which doesn't reflect compounding.
- Effective Annual Rate (EAR)
- Same as APY—the true annual return after accounting for compounding frequency. Higher compounding frequency slightly increases EAR.
- Real Return
- Nominal return minus inflation. An 8% nominal return with 3% inflation equals a ~5% real return—your actual gain in purchasing power.
Frequently asked questions
Quick answers to the most common questions about compound interest calculator.
This calculator is provided for informational and educational purposes only and does not constitute financial, legal, tax, or professional advice. Results are estimates based on the inputs you provide and standard assumptions. Actual figures may vary. Please consult a qualified professional before making financial decisions. Read our full disclaimer.