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Home/Simple vs Compound
Mathematical Comparison

Simple vs Compound Interest Calculator

Visually compare linear growth against exponential growth. See exactly when and how compound interest overtakes simple interest to build generational wealth.

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The Difference Between Simple and Compound Interest

If you want to understand how wealth is built, you must understand the simple vs compound interest dynamic. While simple interest pays you a flat rate on your original money, compound interest pays you interest on your original money and on the interest you've already accumulated.

1. The Simple Interest Model

Your return is flat. If you invest $10,000 at 5% simple interest, you earn exactly $500 every single year. Year 1 = $500. Year 20 = $500.

Example: $500 + $500 + $500...

2. The Compound Interest Model

Your return accelerates. If you invest $10,000 at 5% compound interest, Year 1 earns $500. But in Year 2, you earn 5% on $10,500 (earning $525). By Year 20, you might earn $1,200 in a single year.

Example: $500 → $525 → $551 → $578...

When Compound Overtakes Simple

If you enter 1 or 2 years into the calculator, you'll see almost no difference between the simple vs compound values. For short-term loans or savings, the difference is mathematically negligible.

The magic of compounding is exponential growth, which means the variable of time (t) is an exponent in the formula. Around the 7-to-10 year mark (depending on the interest rate), the compound interest curve begins to arc upward drastically, leaving the flat, linear trajectory of simple interest far behind.

Where do we see them in the real world?

  • Simple Interest: Auto loans, short-term personal consumer loans, and certain types of non-reinvesting bonds.
  • Compound Interest: 401(k) accounts, IRA accounts, ETF stock portfolios, high-yield savings accounts, and unfortunately, credit card debt.

Common Questions

Simple interest only pays you on the money you originally put in. Compound interest pays you on your original money PLUS the interest you've already earned. Compound interest grows much faster over long periods.
Lenders use simple interest for car loans because the principal balance decreases quickly as you pay it off, making it straightforward to calculate. Investments use compounding because you are accumulating value over decades.
Usually around year 7 to 10. In the early years, simple and compound amounts look very similar. But once the 'interest on the interest' becomes a large number, the compound curve arcs upward sharply while the simple interest line stays entirely flat and straight.
Savings accounts always use compound interest (typically compounding daily and paying out monthly). If you leave your interest in the account, it starts earning its own interest the next day.
Simple Interest: A = P(1 + rt). Compound Interest: A = P(1 + r/n)^(nt). For most investment comparisons, n (frequency) is 1 for annual compounding.

About This Calculator & Financial Disclaimer

This tool was built to help users mathematically project their financial goals using standard formulas. The default variables provided are for educational purposes only and do not represent guaranteed future market performance.

Not Financial Advice: We are not certified financial planners (CFP) or investment advisors. The stock market involves risk, and inflation can vary drastically. Please consult a licensed professional before making major financial decisions, executing a 72(t) early withdrawal, or rebalancing your portfolio.

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