Simple Interest Calculator

How to Calculate Simple Interest

Simple interest is calculated only on the original principal — never on interest that's already accumulated — which makes it the most straightforward interest formula there is: $$I = P \cdot r \cdot t$$

Simple vs. Compound Interest

Simple interest grows in a straight line — the same dollar amount of interest every year, since it's always calculated on the same original principal. Compound interest grows faster over time because each period's interest gets added to the balance that the next period's interest is calculated on. Over short periods the difference is small; over many years it becomes large — this calculator's "Extra If Compounded Annually Instead" figure shows exactly how much bigger your own numbers would be under compounding, and the Compound Interest Calculator lets you carry the same principal, rate, and time over and run the full year-by-year math.

Is My Loan or Account Actually Simple Interest?

Not sure which formula actually applies to your situation? A quick way to check: does the interest you owe or earn ever get added back into the balance that future interest is calculated on? If yes, it compounds, and this calculator will understate the real number. A few rules of thumb — not guarantees, since terms vary by lender and product:

• Usually simple interest: many short-term personal loans, some subsidized student loans while in school, add-on auto loans, and short-term promissory notes.
• Usually compound interest: credit cards, most mortgages, most savings and money market accounts, and most long-term investment or retirement projections.
• When in doubt: check the loan or account's own disclosure documents for the words "simple interest" or "compounded [daily/monthly/annually]" — those documents govern, not a general rule of thumb.

Worked Examples

1. Basic case. A 10,000-dollar principal at 5% for 3 years: $I = 10{,}000 \times 0.05 \times 3 = 1{,}500$ dollars in interest, for a total of 11,500 dollars.
2. Short period. A 2,000-dollar principal at 8% for 6 months (0.5 years): $I = 2{,}000 \times 0.08 \times 0.5 = 80$ dollars.
3. Long period. A 5,000-dollar principal at 4% for 10 years: $I = 5{,}000 \times 0.04 \times 10 = 2{,}000$ dollars — exactly double the 5-year figure, since simple interest scales linearly with time.

A Brief History of Simple Interest

Simple interest is the older of the two interest concepts by a wide margin. Documented interest charges on loans go back to some of the earliest known legal codes — the Code of Hammurabi, in ancient Babylon around 1750 BCE, set rules and limits on interest charged on loans of grain and silver. Interest calculated this way was simple by necessity as much as by choice: without the mathematical tools to easily compute compounding over many periods, basic loans throughout the ancient and medieval world were almost always priced as a flat charge on the original amount, for the length of the loan.

Compound interest existed in principle far earlier than it became common in practice — it was understood mathematically well before it was widely used, partly because charging "interest on interest" was viewed with suspicion or outright banned under usury laws in many historical and religious traditions across different periods and regions. Simple interest, by contrast, was rarely controversial in the same way and remained the default method for everyday lending for most of recorded history. Compound interest only became the standard for savings accounts, mortgages, and most long-term lending much more recently, as formal banking systems and the mathematics of compounding became commonplace — simple interest survives today mainly in short-term loans and certain bonds, exactly the kinds of uses covered in the FAQ below.

Common Simple Interest Mistakes

Applying the simple interest formula to a loan or account that actually compounds — most credit cards, mortgages, and savings accounts — will understate how much interest actually accrues, sometimes significantly over a long enough period. Mismatching the rate and time units is another common error: the formula's rate is an annual rate, so a term given in months has to be converted to a fraction of a year (six months is 0.5, not 6) before plugging it in. It's also easy to assume simple interest is always the "safer" or more conservative estimate, but that's only true when comparing it to compounding growth — as a way to project real investment returns over many years, simple interest actually understates growth, since it never accounts for interest earning its own interest.

Simple Interest Terms You Should Know

Principal — the original amount of money, before any interest is added.

Interest Rate — the percentage of the principal charged or earned per year.

Term — the length of time the interest accrues over, typically expressed in years for this formula.