Simple Interest Calculator
Calculate interest without compounding. Your work stays on this device whenever possible.
How to use Simple Interest Calculator
- Enter the amount in Principal amount ($) and the yearly percentage in Annual interest rate (%).
- Type a number in Time period and pick Months, Years or Days in Time unit.
- If you chose Days, set Day convention to "Actual / 365 (Standard)" or "Ordinary / 360 (Commercial)".
- Leave "Compare with compound interest (monthly compounding)" ticked to see how much extra compounding would add, or untick it to hide the comparison.
- Press "Calculate simple interest", then check the accrual schedule or use "Download Schedule (CSV)".
Worked example
Enter Principal 4,000, rate 5%, Time period 90, unit Days, convention Actual / 365. Time = 90 ÷ 365 = 0.2466 years, so interest = 4,000 × 0.05 × 0.2466 = 49.32 and the balance is 4,049.32. The comparison shows monthly compounding at 4,049.52, which is 0.20 more, about 0.4% more growth.
When to use this tool
- Working out the interest on a 90-day bridging note between two parties.
- Checking a certificate of deposit quoted in simple interest for 18 months.
- Showing a student why compounding beats simple interest even over a short period.
Important limitations
Assumes simple linear interest without intra-period reinvestment or compounding. For savings accounts, bonds with reinvestment, or credit card balances, use the compound interest calculator.
Frequently asked questions
What formula does it use?
Interest equals principal times annual rate times time in years. Months are divided by 12, and days by 365 or 360 according to your choice, then the interest is added to the principal for the maturity balance.
Why do the 360 and 365 day options differ?
Some commercial loans count a 360-day year, which makes each day's interest slightly larger. Standard practice for many savings products is 365. The setting only matters when the unit is Days.
What does the compound comparison show?
It recalculates the same principal at the same rate with monthly compounding for the same period, then displays the extra interest and the percentage more growth. Over short periods the gap is small.
Does the schedule show every payment?
It spreads the total interest evenly over up to 30 years or 36 months and lists a row for each. It is an accrual illustration, not a payment plan, so it caps long terms at those limits.
Can I use it for a loan with monthly repayments?
No. This calculates interest on a fixed principal for the entire period. A loan repaid in instalments needs the loan calculator, which reduces the balance each month.
Can it work out the rate, principal or time instead?
Yes. In "Solve for any value", pick the unknown and fill in the other three, and the steps show I = P x r x t with your numbers substituted. The same panel counts days between two dates, charts simple against monthly compound growth, and converts a flat loan fee into an APR.