Key takeaways
- Simple interest is I = P × r × t, where r is a decimal (6% = 0.06).
- It's always calculated on the original principal, never on interest already earned.
- The #1 mistake is entering 6 instead of 0.06 for the rate.
- Total amount owed or saved is I + P, not I alone.
- For time in months, divide by 12 first to convert to years.
In this guide
What is simple interest?
Interest is the price of using money over time — what a bank pays you for keeping your savings with them, or what you pay a lender for borrowing. Simple interest is the plainest version of that idea: it grows by the same fixed amount every year, because it is always calculated on the original amount, called the principal. It never earns interest on interest — that's a different idea called compound interest.
Why does simple interest matter?
Simple interest is the building block for understanding every other kind of interest calculation, from a car loan to a savings bond. It also happens to be the fairest and easiest to predict, because the yearly amount never changes.
Put $1,000 in an account at 5% simple interest, and you'll earn exactly $1,000 in interest after 20 years — the same $50 every single year, no surprises.
The formula
| Symbol | What it means | Example |
|---|---|---|
| I | Interest earned or owed | $360 |
| P | Principal — the original amount | $2,000 |
| r | Annual interest rate as a decimal | 0.06 (for 6%) |
| t | Time the money is borrowed or saved, in years | 3 |
How to calculate simple interest step by step
- 1Write down the principal, P. This is the amount you started with, before any interest.
- 2Convert the rate to a decimal. Divide the percentage by 100. A rate of 6% becomes
6 ÷ 100 = 0.06. - 3Multiply P by r. This gives the interest earned in a single year.
- 4Multiply by t. Multiply the one-year interest by the number of years to get the total interest, I.
- 5Add I to P if you want the total. The final balance or amount owed is
P + I.
Try changing the principal, rate, or time in the calculator below to see how each one affects the interest.
Try it yourself
Pre-filled with the example — change any value.
Common mistakes to avoid
- Mixing up interest with total amount. I is only the interest earned, not principal plus interest — add P yourself if you want the total.
- Using months instead of years for t. If your time period is in months, divide by 12 first. Six months becomes
t = 0.5. - Assuming interest compounds. Simple interest never earns interest on itself; every year's interest is calculated on the same original P.
Tips and tricks
- To estimate quickly, find one year's interest (
P × r) and just multiply it by the number of years — no need to redo the whole formula each time. - A loan or deposit doubles under simple interest when
r × t = 1. At 5%, that takes1 ÷ 0.05 = 20years. - Once this formula feels easy, compare it with the compound interest calculator to see how much faster money grows when interest earns interest too.
Where you'll use it in real life
- Short-term loans: many personal and short-term loans charge simple interest, so the payoff amount is easy to predict in advance.
- Savings bonds and fixed deposits: some basic bonds and bank fixed deposits pay a flat, predictable simple interest.
- Comparing offers: knowing the formula lets you sanity-check a lender's or bank's numbers before you sign anything.
- Planning ahead: the future value calculator and EMI calculator build on the same idea of principal, rate, and time.
Quick summary
Simple interest is I = P × r × t: convert the rate to a decimal, multiply it by the principal and the time in years, and that's your interest. Add it to the principal for the total balance. Use the calculator above to check any scenario with every step shown.
Worked example
You put $2,000 into a savings account that pays simple interest of 6% per year, and you leave it there for 3 years without adding or withdrawing anything.
- 01I = P × r × t
- 02I = 2000 × 0.06 × 3
- 03I = $360
These numbers are pre-filled in the calculator above.
Frequently asked questions
How do I calculate simple interest for months instead of years?
Divide the number of months by 12 to turn it into years, then use that value as t in I = P × r × t. For example, 6 months becomes t = 0.5.
What's the difference between simple interest and compound interest?
Simple interest is always calculated on the original principal, so it grows by the same amount each year. Compound interest adds earned interest back into the principal, so future interest is calculated on a larger base and grows faster.
Does the interest rate go in as a percentage or a decimal?
Use a decimal. Divide the percentage by 100 first — a 5% rate is entered as 0.05.
Can I find the principal if I already know the interest?
Yes. Rearrange the formula to P = I ÷ (r × t), then plug in the interest, rate, and time you know.
