How to use it
- Enter the principal. This is the starting amount you invest, deposit or borrow, e.g. 1000. The currency doesn’t matter — just type the number.
- Enter the annual rate. Type the yearly interest rate as a percent, e.g. 5 for 5% a year. Leave it blank (or 0) and no interest accrues.
- Enter the time in years. For example, 3 for three years, or 0.5 for six months.
- Read the results. The Interest card shows what the rate adds over the period; the Total amount card shows the principal plus that interest.
Both cards recalculate the instant you change a number, so you can compare scenarios without pressing a button.
The simple interest formula
Simple interest is charged only on the original principal — it never earns interest on past interest. The formula isInterest = P × r ÷ 100 × t, where P is the principal, r is the annual rate as a percent and t is the time in years. The total you end up with is justTotal = P + Interest. For a principal of 1000 at 5% for 3 years, that’s1000 × 0.05 × 3 = 150 in interest and a total of 1000 + 150 = 1150.
Common real-world uses
- Short-term loans: many car loans, personal loans and instalment plans quote simple interest.
- Fixed deposits & bonds: estimate the flat interest on a deposit that pays a fixed yearly rate.
- Lending to friends or family: set a fair, transparent rate and show exactly what will be owed.
- Homework & study: check simple-interest problems quickly and see the working.
Keep in mind that simple interest grows by the same amount every year, because the rate always applies to the original principal. Over long periods, compound interest — where each year’s interest is added to the balance and then earns interest itself — pulls noticeably ahead. For savings and long-term investments, the compound figure is usually the more realistic one.
Is it private?
Yes. The Simple Interest Calculator is plain JavaScript that runs entirely inside your browser tab. The amounts, rates and time periods you type are never uploaded, logged or stored anywhere — there are no accounts and no tracking of your inputs. Once the page has loaded you can even use it offline.
Frequently asked questions
How do I enter a period of months instead of years?
Convert the months to a fraction of a year. Six months is 0.5, three months is 0.25, and 18 months is 1.5. The time field accepts decimals, so you can be as precise as you like.
Does it handle decimals, like a 4.5% rate or a 19.99 principal?
Yes. Every field accepts decimals, and results are rounded to two decimal places for easy reading — ideal for odd rates and exact amounts.
Why is my total higher than a compound interest result for the same inputs?
It isn’t — simple interest is normally lower over time. Because compound interest adds each period’s interest to the balance, it earns extra interest the simple method never charges, so a compound total is usually higher for the same rate and term.
Results are rounded for display; for contracts or accounting, double-check critical figures.