About the Simple Interest Calculator
Simple interest is calculated only on the original principal, never on accumulated interest. It is the arrangement used for most instalment loans, short-term borrowing, and some bonds, and it is the baseline against which compounding should be understood.
The formula
I = P · r · t and A = P(1 + rt)I is the interest, P the principal, r the annual rate as a decimal, and t the time in years. The interest is the same in every period, which is what makes it simple.
How to use this calculator
- 1Enter your Principal ($). The field starts at
10000, which you can overwrite. - 2Enter your Annual Interest Rate (%). The field starts at
5, which you can overwrite. - 3Enter your Time (Years). The field starts at
3, which you can overwrite. - 4Read the result straight away — it recalculates as you type, so there is no button to press. Use Share to copy a link that reopens the page with your exact numbers filled in.
Worked example
| Input | Value |
|---|---|
| Principal ($) | 10000 |
| Annual Interest Rate (%) | 5 |
| Time (Years) | 3 |
Result
Simple Interest: $1500.00
Total Amount: $11500.00
Monthly Interest: $41.67
Effective Rate: 15.00% total
Those are the values the page loads with, so you can reproduce this result yourself and then change one field at a time to see what drives the outcome.
Understanding your result
Because interest never accrues on interest, the total grows in a straight line. Ten thousand dollars at 5 percent earns exactly 500 every year, whether it is year one or year twenty. That predictability is the appeal for borrowers and the limitation for savers.
Real-world simple interest loans usually recalculate on the declining balance rather than the original principal, which is subtly different. As you repay, the balance falls and each period's interest falls with it — which is why extra payments on a car loan reduce total interest immediately rather than merely shortening the term.
Things worth knowing
- Keep the rate and the time in matching units. An annual rate needs time in years.
- For periods under a year, express time as a fraction: six months is 0.5.
- On a simple interest loan, paying early always reduces total interest, since interest accrues by the day.
- Precomputed interest loans are different — the interest is fixed at origination, so early payoff saves less.
- Bonds pay simple interest on face value through their coupon, though yield calculations then compound.
Frequently asked questions
How do I calculate simple interest?+
Multiply principal by rate by time. 10,000 dollars at 5 percent for three years earns 10,000 × 0.05 × 3 = 1,500 dollars in interest.
Where is simple interest actually used?+
Most car loans, personal loans, and student loans, along with short-term commercial paper and bond coupon payments. Savings accounts and credit cards compound instead.
Does paying early save money on a simple interest loan?+
Yes. Interest accrues daily on the outstanding balance, so reducing the balance sooner reduces the interest charged from that point onward.
What is precomputed interest?+
Interest calculated for the entire term at origination and added to the balance upfront. Paying such a loan off early saves much less than on a true simple interest loan, and some carry prepayment penalties.