Free Simple Interest Calculator
Work out simple interest on a loan or saving by days, months, years or exact dates. You can also solve backwards for the rate, the time or the principal.
Simple Interest Calculator: Work out simple interest on a loan or saving by days, months, years or exact dates. You can also solve backwards for the rate, the time or the principal. It is free, needs no sign-up and runs in your browser. How to use it: 1) choose what to find: the interest, the rate, the time or the principal; 2) enter your numbers and pick the time unit. For days, choose actual/365 or actual/360; 3) read the interest, total and per-day amounts, then copy, download a CSV or save a PDF. Simple interest is charged or earned only on the original principal. Earlier interest never earns more interest, so the balance grows in a straight line.
How the simple interest calculator works
Simple interest is charged or earned only on the original principal. Earlier interest never earns more interest, so the balance grows in a straight line. This tool turns your time period into years, applies the yearly rate, and shows the interest per year, per month and per day, plus a period schedule and a growth chart.
Day count
When the time is in days or between two dates, the days are divided by 365 (actual/365) or 360 (actual/360). Savings accounts and most personal loans use 365. Money market deals and many business loans use 360, which gives slightly more interest for the same number of days.
Formulas
- Interest:
I = P x r x t, where P is the principal, r the yearly rate as a decimal and t the time in years. - Total amount:
A = P + I - Rate:
r = (A - P) / (P x t) - Time:
t = (A - P) / (P x r) - Principal:
P = A / (1 + r x t) - Days to years:
t = days / 365ordays / 360
Example: 10,000 at 8% for 2 years earns 1,600, so the total is 11,600. For 90 days the interest is 197.26 with actual/365 and 200.00 with actual/360.
Limitations
Most savings accounts and many loans use compound interest, which this tool does not apply. Use the fixed deposit calculator for that. Months are treated as one twelfth of a year. Results are estimates from your inputs, not financial, tax or legal advice, so confirm the figures with your bank or lender.
Simple Interest Calculator FAQs
What is the formula for simple interest?
Simple interest equals principal times rate times time, written I = P x r x t. The rate is yearly and written as a decimal, and the time is in years. For example, 5,000 at 6% for 3 years earns 5,000 x 0.06 x 3 = 900. Add the interest to the principal to get the total amount you receive or repay.
How do I calculate simple interest for days?
Divide the number of days by 365, or by 360 if your bank uses actual/360, to get the time in years. Then multiply principal, yearly rate and that fraction. For 10,000 at 8% for 90 days, the interest is 197.26 with 365 days or 200.00 with 360 days. The calculator does this for you and shows the daily amount.
What is the difference between actual/365 and actual/360?
Both count the real number of days between dates. Actual/365 divides by 365, so a full year earns exactly the quoted rate. Actual/360 divides by 360, so a year of 365 days earns about 1.4% more interest than the quoted rate. Money market accounts and many commercial loans use actual/360, while savings usually use 365.
Can I find the interest rate or time from the final amount?
Yes. Choose interest rate or time period in the first box. Enter the principal and the final amount, plus the time or the rate you know. The tool solves r = (A – P) / (P x t) or t = (A – P) / (P x r) and shows the answer in years, months and days.
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