Compound Interest Calculator

Why Compound Interest Matters

Compound interest lets your money earn money on its earnings, creating exponential growth over time. Unlike simple interest (calculated only on principal), compound interest accelerates growth because each interest payment becomes part of the base for the next period.

The Rule of 72

To quickly estimate how long it takes to double your money, divide 72 by your annual return:

• 6% return: ~12 years to double
• 8% return: ~9 years to double
• 10% return: ~7.2 years to double
• 12% return: ~6 years to double

Starting Early vs. Starting Late

Consider two investors, both earning 7% annually:

• Investor A starts at 25, contributes $200/mo for 40 years = ~$525,000
• Investor B starts at 35, contributes $400/mo for 30 years = ~$489,000

Investor A contributes $96,000 total. Investor B contributes $144,000. Starting 10 years earlier with half the monthly amount produces a larger result.

Compounding Frequency

Interest can compound daily, monthly, quarterly, or annually. More frequent compounding produces slightly higher returns, but the effect is modest. The biggest factors are your rate of return and time horizon.

When to Use This Calculator

• Savings account comparison: Compare 4.5% vs 5.0% APY over 5 years to see the dollar difference.
• Investment planning: Model a Roth IRA or taxable brokerage account growing at a historical market rate.
• Goal setting: Find how much monthly contribution is needed to reach a specific future value.

Real-World Examples

Example 1 — College fund: $5,000 initial, $200/month for 18 years at 7% compounded monthly. Future value: ~$98,000. Total contributed: $48,200. Interest earned: $49,800.

Example 2 — Retirement growth: $50,000 at age 40, $800/month for 25 years at 7%. Future value at 65: ~$735,000. Total contributions: $290,000. Interest earned: $445,000 — more than the contributions themselves.

Data Sources

Formula: FV = P(1 + r/n)^(nt) + PMT × ((1 + r/n)^(nt) − 1) / (r/n). Historical 7% average return reflects S&P 500 inflation-adjusted returns per Vanguard Long-Term Investment Returns data.

Related Guides

• Understanding Compound Interest — how compounding works for both savings and debt
• Financial Planning Basics — emergency funds, debt payoff, and retirement fundamentals

Related Data

Plan your retirement contributions alongside compound growth — see salary benchmarks for 831 occupations at WageDex. Compare savings rates by state and metro at PlainRetire.