Are you in search to take out a new loan but aren’t sure whether it’s a good deal? Let us help you break down 3 commonly used interest rate formulas to see whether you’re being offered a good loan or not.

What are 30/360, Actual/360, and Actual 365 Formulas Used For?

These are three formulas commonly used by many commercial real estate lenders. They are tools to help lenders calculate loans that help measure risk and the potential return on investment. But you can also use them as a borrower to see how much you will have pay over time.

Each calculation can result in significant payment variations over time, especially since some loans can stretch into decades. Knowing which mortgage is best for you can help you pick the right loan the next time you need to meet with a lender or borrower.

Example Interest Accrual Calculation for 30/360, Actual/360, & Actual/365

Lender	Loan Amount	Interest Rate	Interest Rate Formula	Actual Interest Rate	Total Paid Interest
First	$1 million	4%	30/360	4%	$214,942
Second	$1 million	4%	Actual/360	4.058%	$218,341
Third	$1 million	4%	Actual/365	4.003%	$215,166

As you can see, the minor differences between each interest rate formula result in difference in interest paid on the order of several thousand dollars, even though the loan amount is the same, and the beginning interest rate is also identical. Let’s break down each loan type now.

30/360

You can calculate accrued interest for a loan using the 30/360 formula pretty quickly. It’s calculated by taking:

• the annual interest rate proposed by the loan – in this case, it’s 4%
• divide that by 360. This gives you the daily interest rate: 4%/360 = 0.0111%
• next, take the daily interest rate, then multiply it by 30 – this is representative of the monthly interest rate: 0.0111%/30 = 0.333%

As you can see, the 30/360 interest rate formula assumes that there’s 360 days a year and 30 days every month, which isn’t strictly true. As such, it’s the least accurate out of all three measures, but it’s the easiest to calculate.

To get this number, you can alternatively take the starting 4% interest rate, then multiply it by 30/360. Basic math reduces this to 1/12. Divide 4% by 12, and you get 0.33%

Either way, you can now take the monthly accrual rate (0.33%) and multiply that by your outstanding balance. Since you’re calculating the lifetime interest of a loan, multiply it by $1 million, and you get $330,000: this is your overall interest accrual amount.

Naturally, some of your monthly payment amount (whatever it is) will go towards that interest, and the rest will be applied to the loan principal. The above chart represents how much you’ll pay in total interest over time, given this accrued interest rate. 

Actual/360

This method is also pretty easy to calculate, so let’s break it down. You get it by dividing the annual interest rate by 360 to get a daily interest rate. Then you multiply that number by however many days are in the month. Generally, you’ll see larger interest payments due to getting a larger daily rate by dividing by 360 instead of 365 (as you’ll see below).

• Start with the annual interest rate of 4%
• divide that by 360 to get 0.0111% for a daily interest rate
• multiply that by the days in the month, so 30 on average, for a total of 0.333%

This is pretty similar to the numbers we get above, but remember that your accrued interest changes with certain months that have 31 days. So in total, you’ll pay more interest over the entire term of the loan with Actual/360 loans compared to the others.

Consider, for example, that you calculate the loan for January:

• multiply 0.0111% by 31, and you get 0.3441%
• then multiply that by $1 million, and you get $344,100

Ultimately, you pay the most with this accrual method since it has both the highest daily accrual rate and the highest monthly accrual rate. Interest accrual is calculated over the actual number of days in a given month.

Actual/365

You can calculate the Actual/365 formula by taking the annual interest rate, then dividing that interest rate by 365 (the total days in a typical year). You’ll then multiply that number by how many days are in your current month. This can also accommodate months like February, which only has 28 days in a month.

So:

• Take the annual interest rate of 4%
• Divide that by 365 to get 0.011% (rounded)
• multiply that number by how many days are in the month. Take 30 is an average and you get 0.3287%
• of course, then multiply this by $1 million, and you get $328,700

You’ll notice that you have a smaller daily interest rate (since the above value was rounded). However, you end up paying a slightly higher amount of interest over the whole year since some months have 31 days instead of 30.

Which Interest Calculation Method Is Best?

The overall results of this analysis are pretty clear. You will pay the least with a typical 30/360 loan formula, even over the Actual/365 interest calculation method, since the latter has you pay a little more interest over the entire year. This means that most borrowers will want to go with the loan that requires them to pay the least interest over time.

However, the Actual/365 interest rate formula could be advantageous if you want a loan agreement that has a lower daily interest rate. This may make monthly payments a little easier for some parts of the year. Just remember that the bill will eventually come due, and you’ll have to pay a bit more than if you used the first formula.

The differences described in our above examples are not particularly large, but they can be even more substantial if you need loans for even higher amounts of money (such as loans into the millions of dollars).