What is simple interest and how is it calculated?
Simple interest is calculated only on the original principal — the interest earned doesn't earn additional interest. Formula: I = P × R × T. Where P = principal, R = annual rate (decimal), T = time in years. The interest accrual is linear — the same amount is earned each year. Example: $10,000 at 8% for 3 years. Year 1: $800. Year 2: $800. Year 3: $800. Total interest: $2,400. Total amount: $12,400.
When is simple interest used?
Simple interest is most common in: car loans (interest accrues on declining balance daily, which approximates simple interest if payments are on time), short-term personal loans and payday loans, some bank savings accounts and CDs, Treasury bills and some bonds, and academic calculations. Most mortgage interest, credit card interest, and savings account interest compounds — meaning interest earns interest. Understanding whether a financial product uses simple or compound interest is important for comparing true costs.
How do I find the principal using simple interest?
Rearrange I = P × R × T to solve for P: P = I ÷ (R × T). Example: you paid $1,200 in interest on a loan at 6% for 4 years. P = $1,200 ÷ (0.06 × 4) = $1,200 ÷ 0.24 = $5,000. Alternatively, if you know the total amount paid: P = A ÷ (1 + R × T).
What is the difference between simple and compound interest over time?
The difference between simple and compound interest grows exponentially with time. At 8% on $10,000: after 1 year, they're equal ($800 each). After 10 years, compound ($11,589 interest) generates $3,589 more than simple ($8,000). After 30 years, compound ($90,627) generates $66,627 more than simple ($24,000). This is why Einstein reportedly called compound interest 'the eighth wonder of the world' — it works powerfully for savers (returns compound) and against borrowers (debt compounds).
Is a car loan simple or compound interest?
Most car loans use 'simple interest' in the sense that interest accrues daily on the outstanding balance — it doesn't compound (interest on interest). The daily rate = APR ÷ 365. Each payment first covers accrued interest, with the remainder reducing principal. If you pay on time, this behaves identically to the standard amortization formula. If you pay late, more days of interest have accrued, meaning less of your payment goes to principal. The practical difference from compound interest is small for typical loan terms.