Understanding “interest”

“Interest” is an interesting word.

In common usage, it suggests that a person is at least curious about something that they take an “interest” in, like a hobby.  But in a political context, it can mean that someone is an advocate for a certain policy position that benefits them in a specific way, as in an “interest” group.

In a financial context, “interest” also has multiple meanings.  For example, it can mean that someone has an ownership stake or an “interest” in a company or an asset.  But in these videos from Khan Academy, interest is understood to be the cost of money — or the “rent” (per unit time) that a borrower must pay to a lender for the right to use the lenders money.

Understanding the units that are used to express interest rates (remember the importance of time) is very important.  Typically, interest rates are expressed as a dimensionless ratio, in percent.  For example, you might have to pay 13% per year on the balance of a credit card.

In this case, “per cent” means the amount of money that must be paid per 100 dollars borrowed in the time period specified.  The amount borrowed is the balance, or the principal.  So the ratio is actually $i / $b / time, or dollars of interest ($i) per 100 dollars of balance ($b) per unit time.

Where no unit of time is specified, then it is customary to assume that interest rates are expressed per year (sometimes called per annum).  But it is not so unusual to express interests rates in terms of months.  Unscrupulous lenders (called usurers, or loan sharks) may express interest rates on a per day basis (in which case the interest is called the vigorish, or “vig” for short).

In addition to the unit of time, it is important to understand the compounding period.  As interest accumulates, the balance of a loan increases.  When interest is compounded, the interest is added to the balance.  At the next compounding, the new balance on which the interest is computed will include the interest added at the last compounding.  So, unless payments are being made, the borrower now owes interest on the interest.  This way of computing an interest is called compound interest, which is different than simple interest.  This next video explains the difference between simple and compound interest.

The compounding period and the units of time can be different.  For example, the interest rate on a loan might be expressed as 12%/yr, compounded monthly.  In this case, there is a significant difference between the nominal interest rate and the effective interest rate.  The effective rate includes the interest paid on the interest after compounding.

By convention, an effective interest rate can be calculated from the nominal rate according to the formula:

effective = (1 + i/n)^n – 1

where i = the nominal interest rate, and  n = the number of compounding periods per unit of time expressed in the interest rate (in this case, 12 months per year).

Compounding periods can be yearly, monthly, daily, or continuous.  The formula for an effective interest rate in continuous compounding is a little different than the formula above, because it has to be evaluated at the limit of n as n -> infinity.  For continuous compounding:

effective (continuous) = e^(r) -1

where r = the nominal interest rate per year.

4 thoughts on “Understanding “interest”

  1. Hannah Dixon

    This is such an important topic especially for young adults. Being raised in a family that doesn’t believe in credit cards or loans to think people agree to pay more for something than it is worth. Teaching theses types of topics in high school and in college are valuable because many people fall into the trap that is other people do it then it is okay when in reality the best solution is to save the money to make the purchase. I personally have to save for school and had to save to buy my first truck. I understand loans can be helpful for many people but they can also be very dangerous if you do not fully understand them.

  2. Mawi Shahin (@Mawii_s)

    It’s crazy to see a friend of mine buy a $16,000 vehicle for almost $22,000. Although this may seem irrelevant, but I think there’s a better solution than drowning yourself in all these interest rates. My beliefs do not allow receiving or giving interest fees, and for that reason, I along with a group of my friends jumped in on a system that provides us all with that lump sum of cash.
    The system is simple. Grab a group of friends (12 for example) that will all put in $1000 per month, for example, into a large pool of money.
    For the first month, the first $12,000 (1000 * 12 friends) will go to the individual that needs it the most. Second month, it’ll go to the next person on the list and so on. Therefore, at the end of the year, everybody invests 12,000 and receives 12,000. Nothing is lost and there are no extra payments!

  3. jonathanedgington

    They should start teaching finance in high-school too students so they better understand how money works.
    Especially since in college this is usually the first time they will have to start keeping track of how much they spend so they can eat, shop, live, and sleep somewhere on their own.
    Plus credit cards can be a huge problem if you don’t understand variable interest rates.


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