Simple Interest Calculator

What Is Simple Interest and How Is It Calculated?

Simple interest charges a fixed percentage of the original principal for each time period, with no compounding. The formula is I = P x R x T, where P is principal, R is the annual rate as a decimal, and T is time in years. Enter your principal, rate, and time in the calculator above to see the total interest earned, the final amount, and monthly and daily interest breakdowns. Unlike compound interest, the base amount never changes because earned interest is not added back to the principal.

Simple Interest Calculation Examples

$5,000 at 6% for 3 years: I = $5,000 x 0.06 x 3 = $900 total interest. Monthly interest: $25. Final amount: $5,900. $10,000 at 4.5% for 18 months (1.5 years): I = $10,000 x 0.045 x 1.5 = $675. Monthly interest: $37.50. Final amount: $10,675. $25,000 at 8% for 6 months (0.5 years): I = $25,000 x 0.08 x 0.5 = $1,000. Monthly interest: $166.67. Final amount: $26,000. The linear relationship makes simple interest easy to calculate mentally and predictable over any time period.

Where Is Simple Interest Used in Practice?

Auto loans in the US typically use simple interest calculated on the remaining principal balance. As you make payments and reduce the principal, the interest portion of each payment decreases. Personal loans from banks and credit unions commonly use simple interest. Treasury bills (T-bills) and some short-term bonds use simple interest conventions for periods under one year. Promissory notes between individuals or businesses usually specify simple interest. Simple interest is less common for long-term instruments because the difference from compound interest grows dramatically over extended periods.

Simple Interest vs Compound Interest: Side-by-Side

$10,000 at 5% for 10 years. Simple interest: $10,000 x 0.05 x 10 = $5,000. Total: $15,000. Compound interest (annually): $10,000 x (1.05)^10 = $16,289. The compound method earns $1,289 more. At 20 years: simple = $20,000, compound = $26,533 ($6,533 more). At 30 years: simple = $25,000, compound = $43,219 ($18,219 more). The gap widens exponentially because compounding generates "interest on interest." For short periods (under 2-3 years) at moderate rates, the difference is modest. For long time horizons, compounding dominates completely.

Calculating Daily Simple Interest on Loans

Many auto loans and personal loans calculate interest daily. The daily rate = annual rate / 365. On a $20,000 auto loan at 5%: daily rate = 0.0137%. Daily interest = $20,000 x 0.0137% = $2.74. If you make a payment that reduces the balance to $19,500 on day 15 of the month: days 1-14 accrue at $2.74/day ($38.36) and days 15-30 accrue at $2.67/day ($42.74). Monthly interest: $81.10. This daily calculation means paying earlier in the month or making extra payments immediately reduces the interest accruing each subsequent day.

How Does Paying Early Save Money on Simple Interest?

Because simple interest accrues on the current balance, every extra payment directly reduces the principal and immediately lowers the daily interest charge. On a $25,000 auto loan at 6% for 60 months (normal payment $483): an extra $100/month reduces the loan to 49 months and saves $1,120 in total interest. An extra $200/month shortens it to 42 months and saves $1,890. Unlike some mortgage structures with prepayment penalties, most simple interest loans accept extra payments without penalty. Paying biweekly instead of monthly (26 half-payments per year instead of 12 full payments) effectively adds one extra full payment per year, cutting a 60-month loan to approximately 54 months.

Promissory Notes and Personal Lending

Private loans between family members, friends, or business associates typically use simple interest documented in a promissory note. The note should specify: principal amount, annual interest rate, payment schedule, maturity date, and consequences of default. A $10,000 family loan at 4% simple interest for 2 years: total interest = $800. Monthly payments of $450 (principal + interest spread evenly). The IRS sets Applicable Federal Rates (AFR) as minimum interest rates for private loans. Charging less than the AFR on loans above $10,000 can trigger gift tax implications. The current AFR varies monthly but typically ranges from 3-5% depending on the loan term.

Converting Between Simple Interest Rate Formats

Annual rate to monthly: divide by 12. A 6% annual rate = 0.5% monthly. Annual to daily: divide by 365. 6% annual = 0.0164% daily. APR (Annual Percentage Rate) on simple interest loans includes fees spread over the loan term, making it slightly higher than the stated interest rate. A $10,000 loan at 5% with a $250 origination fee has an effective APR of approximately 5.5% because you received $9,750 but pay interest on $10,000. Comparing APR rather than stated rates gives a more accurate cost comparison between loan offers from different lenders, especially when one charges fees that the other does not.

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