Compound Interest vs Simple Interest — The Real Difference

The difference between compound and simple interest on $10,000 at 7% over 20 years is $14,697. On the same principal, at the same rate, for the same duration — compound interest produces nearly 61% more money.

Simple interest: $10,000 grows to $24,000.
Compound interest: $10,000 grows to $38,697.

The mechanism is straightforward. Simple interest calculates interest only on your original principal — every year you earn exactly 7% of $10,000, which is $700. Compound interest calculates interest on your principal plus all the interest you have already earned. In year 2, you earn 7% of $10,700, not 7% of $10,000. That difference starts small and becomes enormous over decades.

This guide explains both formulas, shows you a side-by-side comparison table, and covers when each type of interest applies in the real world — from mortgages to savings accounts to credit cards. To compute exact figures for your own scenario, use our free compound interest calculator or simple interest calculator.

Simple interest is interest calculated exclusively on the original principal. The earned interest does not itself earn interest. The formula is:

Example: $10,000 at 7% for 5 years = $10,000 × 0.07 × 5 = $3,500 interest. Final balance: $13,500.

Simple interest is linear — it grows at a constant dollar amount each year. At 7%, $10,000 earns exactly $700 in year 1, $700 in year 2, and $700 in year 30. The graph of simple interest over time is a straight line.

Where it appears: Short-term personal loans, some auto loans, US Treasury Bills (T-Bills), bond interest accrual between coupon payments, and some consumer finance products. Simple interest is generally used for loans with fixed repayment schedules where the balance decreases over time.

Compound interest is interest calculated on the principal plus all previously accumulated interest. The formula is:

A = final amount, P = principal, r = annual rate (decimal), n = compounding periods per year, t = years

Example (annual compounding): $10,000 at 7% for 5 years:
A = 10,000 × (1.07)⁵ = 10,000 × 1.4026 = $14,026

Compare to simple interest: $13,500 — compound interest produces $526 more after just 5 years. After 20 years the gap is $14,697. After 30 years it is $45,123.

Compounding frequency matters: More frequent compounding = more growth. Daily compounding at 7% for 20 years yields $40,177 vs $38,697 for annual compounding — a $1,480 difference on the same $10,000 at the same rate.

Where it appears: Savings accounts, CDs, money market accounts, 401(k) plans, Roth IRAs, mutual funds, mortgages (compounded monthly), credit cards (compounded daily), and virtually all long-term financial products.

The key takeaway for savers: If you are choosing between two savings products and everything else is equal, choose the one with more frequent compounding. Daily compounding at 5% produces more than annual compounding at 5% — even though the stated rate is identical.

The key takeaway for borrowers: Credit cards use daily compounding. A $5,000 balance at 20% APR compounds daily. If you carry this balance for 10 years making only minimum payments, the total paid can exceed $25,000. This is compound interest working against you.