Investment Growth Calculator
Free Compound Interest Calculator 2026
See exactly how your money grows over time. Get year-by-year breakdowns, total returns, and ROI — with or without monthly contributions. 100% free, no signup.
Instant Results
Year-by-Year Breakdown
Monthly Contributions
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How to Use This Compound Interest Calculator
6 simple steps to project your investment growth
1
Enter Your Principal
Type your starting investment — any amount from $1 upward.
2
Set the Annual Rate
Enter your expected annual return %. The S&P 500 has historically averaged ~10% per year.
3
Choose Your Time Horizon
More years = exponentially more growth. Even a few extra years makes a massive difference.
4
Pick Compounding Frequency
Daily compounding earns slightly more than monthly, which earns more than annually.
5
Add Monthly Contributions
Optional but powerful. Even $100/mo can add hundreds of thousands over 30 years.
6
Click Calculate & Analyze
Instantly see future value, total interest earned, ROI %, and your year-by-year breakdown table.
The Compound Interest Formula Explained
A = P(1 + r/n)nt
Standard compound interest formula
A
Future Value
The total amount after interest compounds over time
P
Principal
Your initial investment or deposit amount
r
Annual Rate
Annual interest rate as a decimal (e.g. 7% = 0.07)
n
Compounding Periods
How many times per year interest compounds (12 = monthly)
t
Time
Number of years the investment is held
Typical Interest Rates by Account Type (2026)
| Account / Investment Type | Typical Rate | Compounding | Notes |
|---|---|---|---|
| S&P 500 Index Fund | ~10% | Annual | Historical long-term average; returns vary annually |
| High-Yield Savings (HYSA) | 4–5% | Daily | FDIC insured; rates change with Fed policy |
| US Treasury Bonds | 3–5% | Semi-annual | Low risk; backed by US government |
| Certificate of Deposit (CD) | 4–5.5% | Daily / Monthly | Fixed term; penalty for early withdrawal |
| Traditional Savings Account | 0.01–0.5% | Daily | Very low yield; consider HYSA instead |
| Diversified ETF Portfolio | 6–9% | Annual | Moderate risk; diversification reduces volatility |
The Rule of 72 — Doubling Time Shortcut
Want to quickly estimate when your money doubles? Divide 72 by your annual interest rate. At 8%, your money doubles in approximately 9 years (72 ÷ 8 = 9). At 6%, it takes 12 years.
4% rate → doubles in 18 yrs
6% rate → doubles in 12 yrs
8% rate → doubles in 9 yrs
10% rate → doubles in 7.2 yrs
12% rate → doubles in 6 yrs
72
Frequently Asked Questions
Everything you need to know about compound interest
What is compound interest and why does it matter?
Compound interest is interest earned on both your original principal and the interest you've already accumulated. Unlike simple interest, it grows exponentially — meaning the longer you invest, the faster it accelerates. Albert Einstein reportedly called it "the eighth wonder of the world." A $10,000 investment at 7% annual compound interest becomes $76,122 in 30 years without adding a single dollar more.
What is the compound interest formula?
The standard formula is A = P(1 + r/n)^(nt), where A is the future value, P is the principal, r is the annual rate as a decimal, n is how many times interest compounds per year, and t is time in years. With monthly contributions, this becomes a more complex summation formula — which our calculator handles automatically.
How do monthly contributions affect my final balance?
Dramatically. Each monthly deposit starts earning compound interest immediately. Over 30 years, adding just $200/month at 8% annual interest adds over $270,000 to your final balance compared to a lump sum alone. The earlier and more consistently you contribute, the more powerful the compounding effect becomes.
Which compounding frequency is best?
More frequent compounding yields more interest. Daily compounding earns slightly more than monthly, which earns more than quarterly, which earns more than annually. The difference becomes more significant over longer time periods. Most savings accounts and CDs compound daily or monthly, while bond yields are often semi-annual.
Is this calculator accurate for retirement planning?
Our calculator gives accurate mathematical projections based on the inputs you provide. However, real-world returns fluctuate year to year, and inflation erodes purchasing power. For actual retirement planning, use this as a starting reference and consult a certified financial planner (CFP). This tool is for educational and estimation purposes only.
What's the difference between APY and APR?
APR (Annual Percentage Rate) is the nominal interest rate without accounting for compounding. APY (Annual Percentage Yield) reflects the actual return after compounding is applied. If a bank advertises 5% APR with daily compounding, the APY is actually ~5.13%. Our calculator uses APR — enter the rate your bank or investment quotes you.