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The general formula

So we know how to convert a monthly interest rate into an effective annual interest. Similarly, we can convert a quarterly interest, or a semi-annual interest rate or an interest rate of any frequency for that matter into an effective annual interest rate.

For a quarterly interest rate of say 3% per quarter, the interest will be paid four times per year (every three months). We can calculate the effective annual interest rate by solving for i :

P ( 1 + i ) = P ( 1 + i 4 ) 4

where i 4 is the quarterly interest rate.

So ( 1 + i ) = ( 1 , 03 ) 4 , and so i = 12 , 55 % . This is the effective annual interest rate.

In general, for interest paid at a frequency of T times per annum, the follow equation holds:

P ( 1 + i ) = P ( 1 + i T ) T

where i T is the interest rate paid T times per annum.

De-coding the terminology

Market convention however, is not to state the interest rate as say 1% per month, but rather to express this amount as an annual amount which in this example would be paid monthly. This annual amount is called the nominal amount.

The market convention is to quote a nominal interest rate of “12% per annum paid monthly" instead of saying (an effective) 1% per month. We know from a previous example, that a nominal interest rate of 12% per annum paid monthly, equates to an effective annual interest rate of 12,68%, and the difference is due to the effects of interest-on-interest.

So if you are given an interest rate expressed as an annual rate but paid more frequently than annual, we first need to calculate the actual interest paid per period in order to calculate the effective annual interest rate.

monthly interest rate = Nominal interest rate per annum number of periods per year

For example, the monthly interest rate on 12% interest per annum paid monthly, is:

monthly interest rate = Nominal interest rate per annum number of periods per year = 12 % 12 months = 1 % per month

The same principle applies to other frequencies of payment.

Consider a savings account which pays a nominal interest at 8% per annum, paid quarterly. Calculate (a) the interest amount that is paid each quarter, and (b) the effective annual interest rate.

  1. We are given that a savings account has a nominal interest rate of 8% paid quarterly. We are required to find:

    • the quarterly interest rate, i 4
    • the effective annual interest rate, i
  2. We know that:

    quarterly interest rate = Nominal interest rate per annum number of quarters per year


    P ( 1 + i ) = P ( 1 + i T ) T

    where T is 4 because there are 4 payments each year.

  3. quarterly interest rate = Nominal interest rate per annum number of periods per year = 8 % 4 quarters = 2 % per quarter
  4. The effective annual interest rate ( i ) is calculated as:

    ( 1 + i ) = ( 1 + i 4 ) 4 ( 1 + i ) = ( 1 + 2 % ) 4 i = ( 1 + 2 % ) 4 - 1 = 8 , 24 %
  5. The quarterly interest rate is 2% and the effective annual interest rate is 8,24%, for a nominal interest rate of 8% paid quarterly.

On their saving accounts, Echo Bank offers an interest rate of 18% nominal, paid monthly. If you save R100 in such an account now, how much would the amounthave accumulated to in 3 years' time?

  1. Interest rate is 18% nominal paid monthly. There are 12 months in a year. We are working with ayearly time period, so n = 3 . The amount we have saved is R100, so P = 100 . We need the accumulated value, A .

  2. We know that

    monthly interest rate = Nominal interest rate per annum number of periods per year

    for converting from nominal interest rate to effective interest rate, we have

    1 + i = ( 1 + i T ) T

    and for calculating accumulated value, we have

    A = P × ( 1 + i ) n
  3. There are 12 month in a year, so

    i 12 = Nominal annual interest rate 12 = 18 % 12 = 1 , 5 % per month

    and then, we have

    1 + i = ( 1 + i 12 ) 12 i = ( 1 + i 12 ) 12 - 1 = ( 1 + 1 , 5 % ) 12 - 1 = ( 1 , 015 ) 12 - 1 = 19 , 56 %
  4. A = P × ( 1 + i ) n = 100 × ( 1 + 19 , 56 % ) 3 = 100 × 1 , 7091 = 170 , 91
  5. The accumulated value is R 170 , 91 . (Remember to round off to the the nearest cent.)

Nominal and effect interest rates

  1. Calculate the effective rate equivalent to a nominal interest rate of 8,75% p.a. compounded monthly.
  2. Cebela is quoted a nominal interest rate of 9,15% per annum compounded every four months on her investment of R 85 000. Calculate the effective rate per annum.

Formulae sheet

As an easy reference, here are the key formulae that we derived and used during this chapter. While memorising them is nice (there are not many), it is the application that is useful. Financial experts are not paid a salary in order to recite formulae, they are paid a salary to use the right methods to solve financial problems.


P Principal (the amount of money at the starting point of the calculation)
i interest rate, normally the effective rate per annum
n period for which the investment is made
i T the interest rate paid T times per annum, i.e. i T = Nominal interest rate T


Simple increase : A = P ( 1 + i × n ) Compound increase : A = P ( 1 + i ) n Simple decrease : A = P ( 1 - i × n ) Compound decrease : A = P ( 1 - i ) n Effective annual interest rate ( i ) : ( 1 + i ) = ( 1 + i T ) T

End of chapter exercises

  1. Shrek buys a Mercedes worth R385 000 in 2007. What will the value of the Mercedes be at the end of 2013 if
    1. the car depreciates at 6% p.a. straight-line depreciation
    2. the car depreciates at 12% p.a. reducing-balance depreciation.
  2. Greg enters into a 5-year hire-purchase agreement to buy a computer for R8 900. The interest rate is quoted as 11% per annum based on simple interest. Calculate the required monthly payment for this contract.
  3. A computer is purchased for R16 000. It depreciates at 15% per annum.
    1. Determine the book value of the computer after 3 years if depreciation is calculated according to the straight-line method.
    2. Find the rate, according to the reducing-balance method, that would yield the same book value as in [link] after 3 years.
  4. Maggie invests R12 500,00 for 5 years at 12% per annum compounded monthly for the first 2 years and 14% per annum compounded semi-annually for the next 3years. How much will Maggie receive in total after 5 years?
  5. Tintin invests R120 000. He is quoted a nominal interest rate of 7,2% per annum compounded monthly.
    1. Calculate the effective rate per annum correct to THREE decimal places.
    2. Use the effective rate to calculate the value of Tintin's investment if he invested the money for 3 years.
    3. Suppose Tintin invests his money for a total period of 4 years, but after 18 months makes a withdrawal of R20 000, how much will hereceive at the end of the 4 years?
  6. Paris opens accounts at a number of clothing stores and spends freely. She gets heself into terrible debt and she cannot pay off her accounts. She owes Hilton Fashion world R5 000 and the shop agrees to let Paris pay the bill at a nominal interest rate of 24% compounded monthly.
    1. How much money will she owe Hilton Fashion World after two years ?
    2. What is the effective rate of interest that Hilton Fashion World is charging her ?

Questions & Answers

what is Nano technology ?
Bob Reply
write examples of Nano molecule?
The nanotechnology is as new science, to scale nanometric
nanotechnology is the study, desing, synthesis, manipulation and application of materials and functional systems through control of matter at nanoscale
Is there any normative that regulates the use of silver nanoparticles?
Damian Reply
what king of growth are you checking .?
What fields keep nano created devices from performing or assimulating ? Magnetic fields ? Are do they assimilate ?
Stoney Reply
why we need to study biomolecules, molecular biology in nanotechnology?
Adin Reply
yes I'm doing my masters in nanotechnology, we are being studying all these domains as well..
what school?
biomolecules are e building blocks of every organics and inorganic materials.
anyone know any internet site where one can find nanotechnology papers?
Damian Reply
sciencedirect big data base
Introduction about quantum dots in nanotechnology
Praveena Reply
what does nano mean?
Anassong Reply
nano basically means 10^(-9). nanometer is a unit to measure length.
do you think it's worthwhile in the long term to study the effects and possibilities of nanotechnology on viral treatment?
Damian Reply
absolutely yes
how to know photocatalytic properties of tio2 nanoparticles...what to do now
Akash Reply
it is a goid question and i want to know the answer as well
characteristics of micro business
for teaching engĺish at school how nano technology help us
Do somebody tell me a best nano engineering book for beginners?
s. Reply
there is no specific books for beginners but there is book called principle of nanotechnology
what is fullerene does it is used to make bukky balls
Devang Reply
are you nano engineer ?
fullerene is a bucky ball aka Carbon 60 molecule. It was name by the architect Fuller. He design the geodesic dome. it resembles a soccer ball.
what is the actual application of fullerenes nowadays?
That is a great question Damian. best way to answer that question is to Google it. there are hundreds of applications for buck minister fullerenes, from medical to aerospace. you can also find plenty of research papers that will give you great detail on the potential applications of fullerenes.
what is the Synthesis, properties,and applications of carbon nano chemistry
Abhijith Reply
Mostly, they use nano carbon for electronics and for materials to be strengthened.
is Bucky paper clear?
carbon nanotubes has various application in fuel cells membrane, current research on cancer drug,and in electronics MEMS and NEMS etc
so some one know about replacing silicon atom with phosphorous in semiconductors device?
s. Reply
Yeah, it is a pain to say the least. You basically have to heat the substarte up to around 1000 degrees celcius then pass phosphene gas over top of it, which is explosive and toxic by the way, under very low pressure.
Do you know which machine is used to that process?
how to fabricate graphene ink ?
for screen printed electrodes ?
What is lattice structure?
s. Reply
of graphene you mean?
or in general
in general
Graphene has a hexagonal structure
On having this app for quite a bit time, Haven't realised there's a chat room in it.
how did you get the value of 2000N.What calculations are needed to arrive at it
Smarajit Reply
Privacy Information Security Software Version 1.1a
Other chapter Q/A we can ask
Moahammedashifali Reply

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Source:  OpenStax, Siyavula textbooks: grade 11 maths. OpenStax CNX. Aug 03, 2011 Download for free at http://cnx.org/content/col11243/1.3
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