Understanding Loan EMI Calculation as a Monthly SIP Investment!

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A simple way to understand a loan EMI calculation in terms of a lump sum investment and a monthly SIP is discussed in this post.  This is applicable for all kinds of loans – home loan, car loan, personal loan etc.

There is a close connection between investment mathematics and loan/mortgage mathematics. In fact, those familiar with the formulae would tell you that they are pretty much identical!

Here is how a home loan EMI can be understood in terms of a lump sum and monthly SIP investment.

Suppose we want a loan of Rs. 10, 00,000  (10L) from a bank at annual interest rate of 10%. This is how the bank will calculate the equated monthly installment or the EMI.

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Photo credit: Got Credit

You want 10L from the bank. Instead of giving this money to you, if the bank had invested it at the rate of 10% a year for 30 years, it would get

1000000 x(1+ 10%/12)^360 =  19837399.4 (198.37 Lakhs)

Here 10%/12 is the monthly interest rate and 360 = duration in months (30 x 12).

If the bank has to lend 10L to you, its aim would be to receive 198.37L from you over 30 years at the same rate of interest, payable each month.

You will not be paying 198.37L to the bank, but you will be paying a monthly amount, which if invested at the same rate would fetch the bank 198.37L after 30 years.

Therefore, as far as the bank is concerned the EMI is a monthly SIP received from you for 30 years, calculated in the following way:

What amount should you invest each month at the rate of 10% a year, so that after 30 years, the investment value is 198.37L? Ring a bell?

This question would be familiar to anyone who has used financial goal calculator. This is the calculation:

19837399.4 = SIP x [(1+10%/12)^360-1]/(10%/12)

This is inverted to calculate the SIP = 877.5.7 ~ 8776.

Therefore, the bank sets the EMI as 8776.

The loan EMI calculation is derived in the following way:

19837399.4 = EMI x [(1+10%/12)^360-1]/(10%/12)

1000000 x(1+ 10%/12)^360 = EMI x [(1+10%/12)^360-1]/(10%/12)

Thus the loan calculation can be viewed as an investment equation:

Lump sum investment value = SIPinvestment value

Now, 1000000 = P = loan amount

10%/12 = R = interest rate per month

360 = n = duration in months

So we have, P x(1+ R)^n = EMI x [(1+R)^n-1]/R

Therefore, EMI =P x R x(1+ R)^n / [(1+R)^n-1]

Credit: This post is based on my answer in Facebook Group Asan Ideas for Wealth.

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About the Author M Pattabiraman author of freefincal.comM. Pattabiraman(PhD) is the author and owner of freefincal.com.  He is an associate professor at the Indian Institute of Technology, Madras since Aug 2006. Pattu” as he is popularly known, has co-authored two print-books, You can be rich too with goal based investing (CNBC TV18) and Gamechanger and seven other free e-books on various topics of money management.  He is a patron and co-founder of “Fee-only India” an organisation to promote unbiased, commission-free investment advice. Pattu publishes unbiased, promotion-free research, analysis and holistic money management advice. Freefincal serves more than one million readers a year (2.5 million page views) with numbers based analysis on topical issues and has more than a 100 free calculators on different aspects of insurance and investment analysis. He conducts free money management sessions for corporates  and associations(see details below). Previous engagements include World Bank, RBI, BHEL, Asian Paints, TamilNadu Investors Association etc. Contact information: freefincal {at} Gmail {dot} com (sponsored posts or paid collaborations will not be entertained)
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6 Comments

    1. I never said that. I recommend taking a home loan once finances are a bit stabilized and at least 10-15% can be invested each month even after paying EMI.

      1. Yeah, it is very difficult thing to handle.. people usually stretch themselves for the EMI.Currently I am in same dilemma

  1. Sir, great article. Nothing can be understood more clearly than mathematics. However I found a catch in above formula. Home loan formula implicitly assumes that compounding frequency is monthly. Also SIP formula is for month end SIP. More importantly in SIP formula monthly rate of return is determined by(1+CAGR)^(1/12)-1 while banks simply divides the annual rate by 12 to arrive at monthly return. Please correct me if I wrong. Thank you.

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