Compound Interest Calculator

How Compound Interest Works

Compound interest earns returns on both your original investment and previously earned returns. This creates exponential growth that accelerates over time, making it one of the most powerful forces in building long-term wealth. The effect is modest in early years but becomes dramatic over decades.

The Compound Interest Formula

• Lump sum only: FV = P x (1 + r)^n
• Monthly contributions only: FV = PMT x [(1 + r)^n - 1] / r
• Combined: FV = P(1+r)^n + PMT x [(1+r)^n - 1] / r

Where P = initial investment, r = monthly return rate (annual rate / 12), n = total number of months, PMT = monthly contribution.

$10,000 Initial Investment Growth at 7% Annual Return

• After 5 years: $14,026 (gain $4,026)
• After 10 years: $19,672 (gain $9,672)
• After 20 years: $38,697 (gain $28,697)
• After 30 years: $76,123 (gain $66,123)
• After 40 years: $149,745 (gain $139,745)

Growth is slow at first but accelerates dramatically. The investment grows more in the final decade than in the first 30 years combined.

Rule of 72: Time to Double Your Money

• At 4%: ~18 years
• At 6%: ~12 years
• At 7%: ~10.3 years
• At 8%: ~9 years
• At 10%: ~7.2 years
• At 12%: ~6 years

The Power of Starting Early

Starting early matters more than investing larger amounts later. Consider two investors, both earning 7% annually: Investor A contributes $300/month from age 25 to 65 ($144,000 total). Investor B contributes $600/month from age 35 to 65 ($216,000 total). Despite investing $72,000 less, Investor A ends up with more money (~$791,000 vs ~$729,000) because of 10 extra years of compounding.

Maximizing Compound Growth

• Start as early as possible: Even small amounts benefit enormously from extra time.
• Be consistent: Regular monthly contributions smooth out market volatility (dollar-cost averaging).
• Reinvest all returns: Withdrawing interest or dividends breaks the compounding chain.
• Minimize fees: A 1% annual fee can reduce your 30-year wealth by 25%. Choose low-cost index funds.
• Use tax-advantaged accounts: 401(k), IRA, and HSA accounts let your money compound without annual tax drag.

Important Notes

This calculator assumes a constant rate of return, which does not reflect real market conditions where returns fluctuate annually. Past performance (such as the S&P 500 average of ~10%) does not guarantee future results. Inflation (historically ~2-3% annually) reduces real purchasing power, and taxes on investment gains (15-20% capital gains rate) further reduce net returns. For realistic planning, consider using a conservative estimate of 5-7% that accounts for inflation.