What is Compound Interest?
Compound interest is interest calculated on both the initial principal and the accumulated interest from previous periods. Unlike simple interest (which only applies to the principal), compounding creates exponential growth over time. Einstein allegedly called it the "eighth wonder of the world", it's the force behind both wealth building and crushing debt.
The Formula
Future Value = Principal × (1 + Rate ÷ n)^(n × t) Where n = compounding periods per year, t = time in years
Daily compounding (n=365) yields slightly more than monthly (n=12), but the difference is marginal for most business applications.
Worked Example
A business invests $50,000 at 7% annual return, compounded monthly, for 10 years.
- P = $50,000, r = 0.07, n = 12, t = 10
- FV = $50,000 × (1 + 0.07/12)^(12 × 10)
- FV = $50,000 × (1.00583)^120
- FV = $50,000 × 2.0097 = $100,484
📌 The $50,000 doubles to $100,484 in 10 years. The $50,484 in interest includes $35,000 in simple interest plus $15,484 in compound interest (interest on interest).
Why This Matters
Business reinvestment
Reinvesting profits at a consistent rate compounds your business growth. A company growing 10% monthly reaches 3.1x its starting size in 12 months, not 2.2x as linear math suggests.
Debt management
Compound interest works against you on debt. A $50K business loan at 18% compounded monthly costs $9,900 in interest annually, not the $9,000 simple interest suggests.
Long-term planning
Starting to save or invest 5 years earlier can result in 40-60% more wealth at retirement due to compounding. Time is the most powerful variable in the compound interest formula.
Common Mistakes
❌ Using simple interest for long-term projections
Simple interest underestimates growth by an increasingly large margin over time. Over 20 years at 8%, simple interest shows 2.6x growth vs compound's 4.66x, a 79% undercount.
❌ Ignoring compounding frequency
Annual vs monthly compounding matters more at higher rates. At 12%, monthly compounding yields 12.68% effective rate vs 12% with annual compounding.
❌ Not accounting for inflation
A 7% nominal return with 3% inflation yields only ~4% real return. All long-term projections should use inflation-adjusted (real) rates for accurate planning.
Industry Benchmarks
| Category | Good | Average | Poor |
|---|---|---|---|
| Stock Market (S&P 500) | 10-12% nominal | 7-10% nominal | Below 5% nominal |
| Business Savings Account | 4-5% APY | 2-4% APY | Below 1% APY |
| Business Loan Rates | 5-8% | 8-15% | Above 18% |
Source: Investopedia Financial Analysis
Benchmark data sourced from Investopedia Financial Analysis.