1.7 Effective and Equivalent Interest Rates
Learning Objectives
- Calculate effective interest rates
- Calculate equivalent interest rates
Formula & Symbol Hub
Symbols Used
or = Effective interest rate = Future value or maturity value = Periodic interest rate or = Nominal interest rate per year or = Number of compounds per year or compounding frequency or = Total number of compound periods for the term = Present value of principal
Formulas Used
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Formula 1.1 – Total Number of Compounds
-
Formula 1.2 – Periodic Interest Rate
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Formula 1.3 – Future Value
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Formula 1.4 – Present Value
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Formula 1.5 – Effective Interest Rate
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Formula 1.6 – Equivalent Interest Rate
Introduction
How can you compare interest rates posted with different compounding? For example, let’s say you are considering the purchase of a new home, so for the past few weeks you have been shopping around for financing. You have spoken with many banks as well as onsite mortgage brokers in the show homes. With semi-annual compounding, the lowest rate you have come across is
When considering interest rates on loans, you clearly want the best rate. If all of your possible loans are compounded in the same manner, selecting the best interest rate is a matter of picking the lowest number. However, when interest rates are compounded differently the lowest number may in fact not be your best choice. For investments, on the other hand, you want to earn the most interest. However, the highest nominal rate may not be as good as it appears depending on the compounding frequency.
To compare interest rates fairly and select the best, they all have to be expressed with the same compounding frequency. This section explains the concept of an effective interest rate, and you will learn to convert interest rates from one compounding frequency to a different frequency.
Effective Interest Rates
The effective interest rate is the interest rate compounded annually that has the same future value of a given present value for a fixed term as an interest rate compounded at some other (non-annual) frequency. In other words, the amount of interest accrued at the effective interest rate once in an entire year exactly equals the amount of interest accrued at the periodic interest rate successively compounded the stated number of times in a year. For example, if you invest
The corresponding effective interest rate (
Effective Interest Rate
Example 1.7.1
Convert
Solution
Step 1: Calculate the periodic interest rate.
Step 2: Calculate the effective interest rate.
Using the TI BAII Plus Calculator to Find and Effective Interest Rate
The TI BAII Plus calculator has a built-in effective interest rate converter called ICONV. The interest rate converter involves three variables.
Variable | Meaning |
NOM | Nominal interest rate, I/Y. |
C/Y | Compounding frequency, C/Y. |
EFF | Effective interest rate. |
To find the effective interest rate:
- Press
(the number key). - At the NOM screen, enter the nominal interest rate
and press ENTER. The interest rate is entered as a percent (without the percent sign). For example is entered as . - Press the up arrow.
- At the
screen, enter the compounding frequency for the nominal interest rate entered in the previous step and press ENTER. - Press the up arrow.
- At the EFF screen, press CPT to calculate the effective interest rate.
The interest converter allows you to solve for any of these three variables, not just the effective interest rate.
Video: Nominal and Effective Rate Conversions by Joshua Emmanuel [5:50] (Transcript Available).
Example 1.7.2
Convert
Solution
NOM | |
I/Y | |
EFF | ? |
- Press
. - At the NOM screen, enter
and press ENTER. - Press the up arrow.
- At the
screen, enter and press ENTER. - Press the up arrow.
- At the EFF screen, press CPT.
Example 1.7.3
You are offered loans at two different interest rates:
Solution
Step 1: Convert
NOM | |
I/Y | |
EFF | ? |
Step 2: Convert
NOM | |
I/Y | |
EFF | ? |
Step 3: Write as a statement.
You should select the loan at
Paths to Success
To compare interest rates that compound at different frequencies, you must convert the rates so that they compound at the same frequency. As in the above example, both interest rates were converted to their effective interest rates and then their effective rates are compared.
- When choosing an interest rate for a loan, you want to pay the least amount of interest. So, select the rate with the lowest effective interest rate.
- When choosing an interest rate for an investment, you want to receive the most interest. So, select the rate with the highest effective interest rate.
Try It
1) If your investment earns
Solution
NOM | |
I/Y | |
EFF | ? |
Try It
2) As you search for a car loan, all banks have quoted you monthly compounded rates (which are typical for car loans), with the lowest being
Solution
Convert the bank’s rate to it’s effective rate:
NOM | |
I/Y | |
EFF | ? |
The credit union’s
Equivalent Interest Rates
Equivalent interest rates are interest rates with different compounding frequencies that result in the same future value for the same given present value and term. For example, if you invest
The convert a given nominal interest (
Equivalent Interest
Key Takeaways
Because Formula 1.5 only gives the periodic interest rate for the new nominal interest rate, the nominal interest rate must be calculated from the periodic interest rate:
Using the TI BAII Plus Calculator to Find an Equivalent Interest Rate
To find an equivalent interest rate:
-
- Press 2nd ICONV (the number
key). - At the NOM screen, enter the nominal interest rate
for the old interest rate and press ENTER. The interest rate is entered as a percent (without the percent sign). For example is entered as . - Press the up arrow.
- At the
screen, enter the compounding frequency for the old interest rate entered in the previous step and press . - Press the up arrow.
- At the
screen, press to calculate the effective interest rate. - Press the down arrow.
- At the
screen, enter the compounding frequency for the new interest rate and press . - Press the down arrow.
- At the
scree, press to calculate the new interest rate.
- Press 2nd ICONV (the number
Video: Nominal and Effective Rate Conversions by Joshua Emmanuel [5:50] (Transcript Available).
Note
Two equivalent interest rates have the same effective rate. The calculator uses this fact to find an equivalent rate. The first step to finding an equivalent rate on the calculator requires the calculation of the effective rate for the old interest rate. Using this effective rate, the calculator finds the new interest rate.
Example 1.7.4
What rate compounded quarterly is equivalent to
Solution
Step 1: Convert
NOM | |
I/Y | |
EFF | ? |
Step 2: Convert the effective rate from the previous step to the new interest rate compounding quarterly.
Make sure that you keep all of the decimal places from the effective interest rate found in the previous step to avoid any round off error in calculating the new interest rate.
NOM | ? |
I/Y | |
EFF |
Things to Watch Out For
When converting interest rates, the most common source of error lies in confusing the two values of the compounding frequency, or
Example 1.7.5
You are looking at three different investments bearing interest rates of
Solution
Notice that two of the three interest rates are compounded semi-annually while only one is compounded quarterly. Although you could convert all three to effective rates (requiring three calculations), it is easier to convert the quarterly compounded rate to a semi-annually compounded rate. Then all rates are compounded semi-annually and are therefore comparable.
Step 1: Convert
NOM | |
I/Y | |
EFF | ? |
Step 2: Convert the effective rate in the previous step to the new interest rate compounding semi-annually.
NOM | ? |
I/Y | |
EFF |
Step 3: Write as a statement.
Try It
3) What nominal interest rate compounded semi-annually would result in the same financial position as
Solution
NOM | ||
C/Y | ||
EFF |
Try It
4) You are offered three different investment opportunities at three different interest rates:
Solution
Convert
NOM | ||
C/Y | ||
EFF |
Convert
NOM | ||
C/Y | ||
EFF |
Select the investment at
Note: There are other ways to solve this problem. For example, all of the interest rates could be converted to rates that compound quarterly, or all the interest rates could be converted to their effective rates.
Section 1.7 Exercises
- For each of the following, find their effective rates.
compounded quarterly. compounded monthly. compounded semi-annually.
Solution
a.
, b. , c. - Convert
effective to the equivalent rate compounded semi-annually.
Solution
- Convert
effective to the equivalent rate compounded monthly.
Solution
- Convert
effective to the equivalent rate compounded quarterly.
Solution
- Convert
compounded monthly to the equivalent rate compounded semi-annually.
Solution
- Convert
compounded quarterly to the equivalent rate compounded monthly.
Solution
- Covert
compounded semi-annually to the equivalent rate compounded quarterly.
Solution
- The HBC credit card has a nominal interest rate of
compounded monthly. What effective rate is being charged?
Solution
- What is the effective rate on a credit card that charges interest of
per day?
Solution
- RBC offers two different investment options to its clients. The first option is compounded monthly while the latter option is compounded quarterly. If RBC wants both options to have an effective rate of
, what nominal rates should it set for each option?
Solution
compounded monthly, compounded quarterly - Louisa is shopping around for a loan. TD Canada Trust has offered her
compounded monthly, Conexus Credit Union has offered compounded quarterly, and ING Direct has offered compounded semi-annually. Rank the three offers and show calculations to support your answer.
Solution
Rank Company Rate ING TD Conexus - Your three-year monthly compounded investment just matured and you received
, of which was interest. What effective rate of interest did you earn?
Solution
- The TD Emerald Visa card wants to increase its effective rate by
. If its current interest rate is compounded daily, what new daily compounded rate should it advertise?
Solution
- Five investors reported the following results. Rank the effective rates of return realized by each investor from highest to
lowest and show your work.- Investor
: principal earned of interest compounded monthly over four years. - Investor
: maturity value including of interest compounded quarterly for two years. - Investor
: principal maturing at after ½ years of semi-annually compounded interest. - Investor
: of interest earned on a principal after five years of daily compounding. - Investor
: maturity value including of interest compounded annually over four years.
Solution
Investor Rate - Investor
Attribution
“1.7 Effective and Equivalent Interest Rates” from Financial Math – Math 1175 by Margaret Dancy is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.