Simple Interest Calculator
Calculate flat (non-compounding) interest for any loan or deposit — principal, rate, and time.
How to use this simple interest calculator
- 1Enter the principal — the original amount borrowed or deposited.
- 2Enter the annual interest rate as a percentage.
- 3Enter the time period in years. Use decimals for partial years (0.5 = 6 months).
- 4The result shows total interest plus daily and monthly breakdowns.
How it's calculated
I = P × r × t (rate as decimal, time in years). Total = P + I.
About the Simple Interest Calculator
Simple interest is the most transparent form of interest calculation — straightforward multiplication of principal, rate, and time with no compounding effects. While it is less common than compound interest in modern financial products, understanding simple interest is fundamental to financial literacy.
The key distinction between simple and compound interest becomes significant over time. At 5% for 10 years: simple interest turns $10,000 into $15,000 (50% total gain). Compound interest (annual compounding) turns $10,000 into $16,289 (62.9% total gain). Over 30 years, the gap is enormous: simple interest produces $25,000; compound interest produces $43,219 — nearly double. This is why compound interest matters so much for long-term investing and why high-rate compound debt (like credit cards) is so destructive.
For short-term calculations — a 6-month loan, an annual CD, or a simple business agreement — simple interest is practical and accurate. Many peer-to-peer lending arrangements and personal loans between friends use simple interest for its clarity and ease of verification.
Frequently asked questions
When is simple interest used instead of compound interest?
Simple interest is used for short-term loans, some auto loans, many installment loans, US Treasury securities, and short-term personal loans between individuals. It is also used to calculate per-day interest on mortgages when you make extra payments mid-month. Most savings accounts and long-term investments use compound interest, which grows faster. The distinction matters most for long periods — over 20+ years, compound interest produces dramatically more growth than simple interest on the same principal and rate.
What is the formula for simple interest?
Simple interest = Principal × Rate × Time, or I = P × r × t, where rate is expressed as a decimal (5% = 0.05) and time is in years. If you borrow $10,000 at 6% simple interest for 2.5 years, interest = $10,000 × 0.06 × 2.5 = $1,500. Total repayment = $11,500. With compound interest at the same rate, the total would be slightly higher — $11,597 compounded annually — because each year's interest earns additional interest.
Is simple interest always better for borrowers?
For borrowers, simple interest loans cost less than equivalent compound interest loans, especially over longer periods. However, most consumer lending uses effective compound rates even when described in simple terms — be sure to check whether interest is calculated on the original principal (true simple interest) or on the outstanding balance. Amortizing loans (mortgage, auto, personal loans) calculate interest on the declining balance each month, which is technically compound interest applied to a reducing principal. True simple interest on the original principal for the full term would be more expensive for installment loans.