What is the main difference between simple and compound interest?

Simple interest is calculated only on the original principal amount of a loan or investment. Compound interest is calculated on the principal plus any accumulated interest from previous periods, meaning your balance grows exponentially over time.

Why do banks use a 360-day year instead of 365 days for simple interest?

This practice, known as the Banker's Rule or Ordinary Interest, dates back to when calculations were done manually. A 360-day year simplified the math because 360 is highly divisible. Today, banks still use it because it yields a slightly higher interest payout from the borrower compared to a 365-day year.

Are car loans simple interest?

Yes, most auto loans use a daily simple interest model. Interest accrues daily based on your outstanding principal balance. This means if you pay your monthly bill early, you pay less total interest over the life of the loan.

Can you have an amortization schedule with a simple interest loan?

Absolutely. If a simple interest loan is paid back in regular installments (like monthly payments), an amortization schedule is used to show how each payment is split between the interest accrued for that month and the principal balance.

Simple Interest Formula: Calculations, Real Examples & Guide

A business owner named Sarah needs $50,000 to secure inventory for the holiday rush. She has ninety days to pay it back. A local lender offers her a short-term bridge loan at a flat 8% simple annual interest rate. Another broker offers her 7.8% interest, but it compounds monthly.

Sarah almost took the 7.8% offer because the number was lower. She assumed a lower rate always means a cheaper loan.

She was wrong.

Because of the short timeframe and the way compounding works, the simple interest loan was actually the cleaner, more predictable, and cheaper option for her business. But without understanding how simple interest functions under the hood—and how to calculate it quickly—she would have thrown money away.

Simple interest is the backbone of short-term debt, promissory notes, treasury bills, and many personal auto loans. Yet, most people gloss over the mechanics. They assume all interest behaves the same way. It does not.

Here is what you need to know to master simple interest, protect your cash flow, and make smarter borrowing or lending decisions.

The Deceptive Simplicity of the �KINL7� Formula

At its core, simple interest is calculated only on the original amount of money borrowed or invested. This original amount is called the principal. Unlike compound interest, where you earn or pay "interest on interest," simple interest remains static. It does not snowball.

To find the interest, you use a classic formula:

�KBLK0�

Where:

�KINL8� is the total interest earned or paid.
�KINL9� is the Principal (the starting balance).
�KINL10� is the annual interest rate (expressed as a decimal).
�KINL11� is the time the money is borrowed or invested (expressed in years).

If you want to know the total amount (�KINL12�) you will owe or receive at the end of the term, you add the principal back to the interest. That formula looks like this:

�KBLK1�

But here is the thing: while the math looks basic, people trip up on the time variable (�KINL13�) constantly.

If a loan lasts for six months, you cannot plug "6" into the formula. If you do, the math assumes you are borrowing the money for six years. You must convert those months into years. Six months becomes �KINL14�, or �KINL15� years.

What if the loan is for 90 days? Now things get interesting—and potentially expensive.

The Banker's Rule: Why a Year Isn't Always 365 Days

In the financial world, time is money, literally. When calculating daily simple interest, banks and lenders have two ways of defining a year. Most people assume every calculation uses a standard 365-day year.

Professionals know better.

1. Exact Interest (The 365-Day Year)

This method uses the actual number of days in a year (365, or 366 in a leap year). It is common in government transactions and many consumer loans.

�KBLK2�

2. Ordinary Interest / The Banker's Rule (The 360-Day Year)

This is a legacy banking practice that remains highly active today. It assumes every month has exactly 30 days, resulting in a 360-day year.

�KBLK3�

Why does this matter? Because dividing by 360 instead of 365 makes the time fraction slightly larger. A larger time fraction means more interest paid to the lender.

Let’s look at the numbers. Imagine you borrow $1,000,000 at a 6% simple interest rate for 90 days.

Using Exact Interest (365 days): �KBLK4�
Using Ordinary Interest / Banker's Rule (360 days): �KBLK5�

By using the Banker's Rule, the lender makes an extra $205.48 on the exact same loan. On multi-million dollar commercial paper transactions, this discrepancy can run into tens of thousands of dollars. Always read the fine print of your loan agreement to see which day-count convention your lender uses.

Simple vs. Compound Interest: A Real-World Comparison

To truly appreciate simple interest, you must contrast it with compound interest.

Let's say you invest $10,000 at an 8% annual rate for 5 years.

With simple interest, your investment grows linearly. You earn exactly �KINL16�10,000 \times 0.08�KINL17�4,000 in interest. Your total balance is $14,000.

With compound interest (compounded annually), your interest is reinvested.

Year 1: You earn 8% on �KINL18�800. Your balance is $10,800.
Year 2: You earn 8% on �KINL19�864. Your balance is $11,664.
Year 3: You earn 8% on �KINL20�933.12. Your balance is $12,597.12.
Year 4: You earn 8% on �KINL21�1,007.77. Your balance is $13,604.89.
Year 5: You earn 8% on �KINL22�1,088.39. Your balance is $14,693.28.

| Year | Simple Interest Total (�KINL23�) | The Difference ($) | | :--- | :--- | :--- | :--- | | 1 | 10,800.00 | 10,800.00 | 0.00 | | 2 | 11,600.00 | 11,664.00 | 64.00 | | 3 | 12,400.00 | 12,597.12 | 197.12 | | 4 | 13,200.00 | 13,604.89 | 404.89 | | 5 | 14,000.00 | 14,693.28 | 693.28 |

Over five years, compounding earned you an extra $693.28. If you are an investor, you want compounding. If you are a borrower, simple interest is your best friend because it keeps your total debt load lower.

Where Simple Interest Actually Rules the Financial World

Many assume simple interest is an outdated concept taught only in high school algebra classes. That is a mistake. It governs several massive financial sectors.

1. Short-Term Business Promissory Notes

When businesses need quick liquidity, they often issue promissory notes to private investors or partners. These are almost always structured using simple interest because they are short-term (under a year) and need to be simple to track.

For example, if a construction firm needs $75,000 for 120 days to buy concrete for a project, they might issue a promissory note at 10% simple interest using the Banker's Rule.

�KBLK6�

At the end of 120 days, the firm pays back $77,500. There are no complex compounding schedules to calculate or audit.

2. Treasury Bills (T-Bills)

T-Bills are short-term debt obligations backed by the US government, with maturities ranging from a few days to 52 weeks. They do not pay regular interest payments like traditional bonds. Instead, they are sold at a discount.

If you buy a �KINL24�10,000 upfront. You pay the discounted price and receive the full $10,000 at maturity. The discount itself is calculated using simple interest math.

3. Auto Loans and Daily Simple Interest

Most modern auto loans are "simple interest loans." This surprise confuses many car buyers.

With a daily simple interest car loan, your interest is calculated daily based on your outstanding principal balance. Every time you make a payment, the money first goes toward the interest that has accrued since your last payment. The rest of the money goes toward reducing the principal.

This structure gives you a secret weapon: you can save money by paying early.

If your payment is due on the 15th of the month, but you pay it on the 10th, fewer days of interest have accrued. More of your payment goes toward the principal. Over a 60-month loan, consistently paying a few days early can shave hundreds of dollars off your total interest bill and help you pay off the car months ahead of schedule.

Conversely, if you consistently pay a few days late—even if you are within the grace period and do not get charged a late fee—more of your payment goes toward interest. Your principal decreases slower, and you might find yourself with a surprise balance due at the end of your loan term.

How a Simple Interest Amortization Table Works

When people hear "amortization," they usually think of complex mortgages. But simple interest loans can be amortized too, especially when they are paid back in regular installments (like an auto loan or a personal loan).

Let's walk through a concrete example.

Principal (�KINL25�): $12,000
Annual Rate (�KINL26�): 6% (0.06)
Term: 12 Months
Monthly Payment: $1,032.80

Even though the loan uses simple interest, the interest is calculated each month on the remaining principal balance, not the original $12,000.

Month 1 Calculation:
- Outstanding Principal: $12,000
- Monthly Interest Rate: �KINL27�
- Interest for Month 1: �KINL28�60.00$
- Principal Reduction: �KINL29�1,032.80 - \�KINL30�972.80$
- New Principal Balance: �KINL31�972.80 = \�KINL32�
Month 2 Calculation:
- Outstanding Principal: $11,027.20
- Interest for Month 2: �KINL33�55.14$
- Principal Reduction: �KINL34�1,032.80 - \�KINL35�977.66$
- New Principal Balance: �KINL36�11,027.20 - \�KINL37�10,049.54$

As you can see, the interest portion of the payment decreases every month because the principal is shrinking. This is why having an amortization table is so crucial. It lets you see exactly where your money is going every single month.

Stop Guessing: Let the Calculator Do the Heavy Lifting

Manual calculations are great for understanding the theory, but they are terrible for making fast business decisions. If you are sitting in a meeting negotiating a contract, you do not want to be scratching formulas on a napkin or trying to remember if your lender is using a 360 or 365-day year.

Our Simple Interest Calculator is designed to eliminate the guesswork.

Instead of wrestling with time conversions and decimal points, you simply input your principal, interest rate, and term (in days, weeks, months, or years). The tool instantly calculates:

The total interest you will pay or earn.
The total accumulated value (principal + interest).
A complete, interactive amortization table showing how your balance behaves over time.
A visual chart that breaks down the ratio of principal to interest.

Whether you are analyzing a short-term business loan, evaluating a promissory note, or trying to figure out how much you can save by paying your car loan early, having instant, accurate data is your best leverage.

Actionable Takeaway: Your Next Steps

Before you sign any loan agreement or investment contract, do three things:

Identify the interest type: Confirm in writing whether the interest is simple or compound.
Ask about the day-count convention: Find out if they use the 365-day Exact Interest method or the 360-day Banker's Rule.
Run the numbers: Plug the terms into our Simple Interest Calculator. Look at the total interest cost and check the amortization table to ensure it aligns with your cash flow goals.

Don't rely on assumptions. Take control of your financial math, use the right tools, and make decisions based on hard data.