Simple Interest
WEBWork out simple interest and the total on a principal at a fixed rate.
Simple interest: I = P × r × t. Interest is charged only on the principal, not on interest already earned.
What is the Simple Interest tool?
This simple interest calculator works out the interest earned or owed on a fixed principal, plus the total amount at the end of the term. It uses the classic formula I = P × r × t: interest equals the principal times the annual rate times the number of years. Enter your amount, the yearly interest rate as a percentage, and the length of the term, and the calculator shows the interest and the total instantly, with a breakdown bar splitting the total into principal and interest.
Simple interest is charged only on the original principal — never on interest that has already accrued. That makes it the basis for many short-term personal loans, car loans, some bonds and Treasury bills, and the way interest is often quoted on a fixed deposit. It is the honest, easy-to-check counterpart to compound interest, where interest itself earns interest and the total grows faster over time.
Everything runs in your browser: the calculator is client-side, nothing you type is uploaded, and no account is needed. It is free, works on your phone, and updates live as you change any field — handy when you are comparing a couple of loan offers or checking a lender's quoted figure. For long-term saving or borrowing where interest compounds, pair it with the compound interest calculator to see how much the two methods diverge.
How to use the Simple Interest
- Enter the principal — the amount you are lending, borrowing or investing.
- Enter the annual interest rate as a percentage (for example 5 for 5%).
- Enter the term in years — decimals are fine, so 1.5 means eighteen months.
- Read the interest earned and the total amount, split into principal and interest on the breakdown bar.
- Copy or share the result, then tweak any field to compare scenarios instantly.
Frequently asked questions
How is simple interest calculated?
Simple interest = principal × rate × time, where the rate is the annual rate as a decimal and time is in years. For example, £1,000 at 5% for 2 years earns 1000 × 0.05 × 2 = £100 in interest, for a total of £1,100. The calculator applies this formula automatically as you type.
What is the difference between simple and compound interest?
Simple interest is charged only on the original principal, so it grows in a straight line. Compound interest is charged on the principal plus any interest already added, so the balance grows faster over time. For short terms the two are close; over many years compound interest pulls well ahead.
Can I use it for months or days instead of years?
Yes — enter the term as a fraction of a year. Six months is 0.5, ninety days is roughly 0.25, and eighteen months is 1.5. The formula scales linearly, so a fractional term simply gives a proportional amount of interest.
What counts as the principal?
The principal is the starting amount: the sum you deposit, invest, or borrow before any interest is added. In this calculator the interest is always figured on that original figure and never on interest that has already accrued.
Is my data private?
Yes. The amounts you enter are processed entirely in your browser — they are never uploaded, stored or sent to any server. Once the page has loaded, the calculator even works offline.
Is this calculator free?
Yes — free, with no sign-up, no account and no limits. Open the page, enter three numbers, and read the interest and total in seconds.