Simple interest calculator
Pick what you want to find and fill in the other values. Time can be in years, months or days; days are counted on a 365-day year. The field for the value you are solving is ignored.
Runs in your browser: nothing you enter is sent anywhere.
How to use it
- Choose what to find: interest, principal, rate or time.
- Fill in the other values. The field for the value you are solving is ignored.
- When finding principal, rate or time, enter the interest in the last field.
- Pick years, months or days for the time.
- Interest: I = P × r × t
- Principal: P = I ÷ (r × t)
- Rate: r = I ÷ (P × t)
- Time: t = I ÷ (P × r)
Time is always converted to years first: months ÷ 12, days ÷ 365.
Worked example
You lend 2,000 at 6% a year for 18 months. Time is 18 ÷ 12 = 1.5 years, so interest = 2,000 × 0.06 × 1.5 = 180 and the total is 2,180.
Good to know
- Simple interest is common for short loans, bonds that pay a fixed coupon and some car loans. Savings accounts and credit cards compound instead.
- Days on a 365-day basis: 10,000 at 4% for 90 days gives 98.63.
Frequently asked questions
What is the simple interest formula?
I = P × r × t, where P is the principal, r the yearly rate as a fraction (5% = 0.05) and t the time in years. The total owed or earned is P + I. For 1,000 at 5% for 3 years, I = 1,000 × 0.05 × 3 = 150.
How are months and days converted to years?
Months are divided by 12. Days are divided by 365, so 90 days is 0.2466 years. Banks that use a 360-day year (the ordinary interest method) get a slightly higher figure; this calculator does not.
What is the difference between simple and compound interest?
Simple interest is charged only on the original principal. Compound interest is also charged on interest already added, so it grows faster. After 3 years at 5%, 1,000 earns 150 simple, but 157.63 with yearly compounding.
How do I find the rate or the time?
Choose Rate or Time in the Find menu and enter the interest you paid or earned. The rate is I ÷ (P × t) and the time is I ÷ (P × r). Both need a principal, and a time or rate, above zero.