360 Day Year In Interest Expense

I was approached by a client asking me why does your bank use 360 days instead of 365 days when calculating the interest payment of a loan.

I told him it's some sort of a convention, but he wasn't fully convinced.

I looked up Wikipedia, but it doesn't provide much detail.

Why 360 Days Instead of 365 Day?

The simple answer is that 360 days is used because of its simplicity.

User @whateverittakes", a private equity associate, shared the simple explanation:

whateverittakes - Private Equity AssociateFolks didn't have calculators back in the day. Much easier to use "round numbers," so "360 days in a year and 30 days in a month" became the convention of choice. It stuck.

alexpasch - Corporate Strategy DirectorThere's actually many different ways. 360 days is used because it's far simpler. 30/360 is the best method in my opinion because of how you can divide up payment frequencies and because you don't have to count the actual number of days between dates.

You can learn more about this concept with the below video.

Read More About Interest Expense on WSO

Preparing for Investment Banking Interviews?

The WSO investment banking interview course is designed by countless professionals with real world experience, tailored to people aspiring to break into the industry. This guide will help you learn how to answer these questions and many, many more.

Investment Banking Interview Course Here

5 Comments
 

Folks didn't have calculators back in the day. Much easier to use "round numbers," so "360 days in a year and 30 days in a month" became the convention of choice. It stuck.

 

There's actually many different ways. 360 days is used because it's far simpler. 30/360 is the best method imo because of how you can divide up payment frequencies and because you don't have to count the actual number of days between dates.

How can you say wikipedia does not provide much detail? How much more detail do you want? lol (see below, maybe you went to a wrong link?)

http://en.wikipedia.org/wiki/Day_count_convention

 

It's a normalized number, a la 30.25 days per year. Pretty straight forward, sorry your clients an idiot.

Ace all your PE interview questions with the WSO Private Equity Prep Pack: http://www.wallstreetoasis.com/guide/private-equity-interview-prep-questions
 

Et debitis qui blanditiis est sed repellat fuga. Et quis commodi dignissimos tempora maxime numquam. Vitae neque earum natus. Doloribus sed in amet asperiores.

Nostrum alias consequatur odit. Fugit consequatur temporibus quae esse laudantium. Officiis nihil eos corporis quos non consequatur unde. Explicabo officiis culpa est aspernatur aspernatur est voluptatum.

Ace all your PE interview questions with the WSO Private Equity Prep Pack: http://www.wallstreetoasis.com/guide/private-equity-interview-prep-questions

Career Advancement Opportunities

August 2026 Investment Banking

  • Evercore 01 99.4%
  • Moelis & Company 01 98.9%
  • JPMorgan 01 98.3%
  • Morgan Stanley 08 97.8%
  • Goldman Sachs 02 97.2%

Overall Employee Satisfaction

August 2026 Investment Banking

  • Moelis & Company No 99.4%
  • Evercore No 98.9%
  • Morgan Stanley 01 98.3%
  • Banco Santander 02 97.8%
  • BMO Capital Markets 12 97.2%

Professional Growth Opportunities

August 2026 Investment Banking

  • Evercore 01 99.4%
  • Moelis & Company 01 98.9%
  • Morgan Stanley 06 98.3%
  • Goldman Sachs 01 97.8%
  • JPMorgan 01 97.2%

Total Avg Compensation

August 2026 Investment Banking

  • Vice President (16) $429
  • Associates (48) $259
  • 3rd+ Year Analyst (8) $210
  • 2nd Year Analyst (25) $178
  • Intern/Summer Associate (14) $159
  • 1st Year Analyst (83) $151
  • Intern/Summer Analyst (75) $101
notes
16 IB Interviews Notes

“... there’s no excuse to not take advantage of the resources out there available to you. Best value for your $ are the...”

Leaderboard

1
redever's picture
redever
99.2
2
BankonBanking's picture
BankonBanking
99.0
3
kanon's picture
kanon
99.0
4
Secyh62's picture
Secyh62
99.0
5
CompBanker's picture
CompBanker
98.9
6
GameTheory's picture
GameTheory
98.9
7
dosk17's picture
dosk17
98.9
8
Betsy Massar's picture
Betsy Massar
98.9
9
DrApeman's picture
DrApeman
98.9
10
bolo up's picture
bolo up
98.8
success
From 10 rejections to 1 dream investment banking internship

“... I believe it was the single biggest reason why I ended up with an offer...”