How to Calculate Present Values Using Timelines

In economy it’s important to talk about present values, as the value of money changes over time. $1 is not worth the same today as $1 will be worth in a year’s time. Inflation and the general economic development in the world affects the value of money. This is important to consider when you look at monetary values from different time periods.

Theory

Present Value

When we talk about present value, we are describing what future money would have been worth today.

Present values are often linked to loans in order to find the actual value of the loan, and has a quotient

k = 1 growth factor = 1 (1 + p 100 ) n.

The present value K0 of a sum Kn that will be paid back in n periods of time is given as

K0 = Kn (1 + p 100 ) n,

where p is the interest in percentages.

Example 1

You have decided to buy a car in 7 years, and you’re saving up money to do so. The car value of the car is $30000, and you get 3% interest on the money. How much do you have to set aside today to have $30000 in seven years?

You insert the numbers into the formula above and get

K0 = 30000 (1 + 3 100 ) 7 = $24392.75.

That means that if you put $24392.75 in the bank today, you will have enough money to buy the $30000 car in seven years.

Loan

When looking at loans, it’s useful to draw a timeline. A timeline helps you remember how many periods the money is split across and how large the payments should be. Here’s an example concerning the present value of a loan:

Example 2

How much mortgage can you take? You’re able to pay $800 each month, and are set to make annual repayments. The interest is 5%, and you pay the loan back over a period of 20 years, with the first payment being after one year.

In these kinds of exercises, it’s particularly smart to use timelines! But first you need to find out how much you can pay annually,

800 12 = $9600,

where 12 is the number of months in a year.

That makes the timeline look like this:

Timeline showing the present value of annual payments for each year

This gives you the geometric series

9600 1.05 + 9600 1.052 + + 9600 1.0519 + 9600 1.0520,

9600 1.05 + 9600 1.052 + + 9600 1.0519 + 9600 1.0520,

with

a1 = 9600 1.05 ,k = 1 1.05,n = 20.

a1 = 9600 1.05 ,k = 1 1.05,n = 20.

We can then put this directly into the formula for the sum of a geometric series:

S20 = 9600 1.05 ( 1 1.05 ) 20 1 1 1.05 1 $119637.22

S20 = 9600 1.05 ( 1 1.05 ) 20 1 1 1.05 1 $119637.22

This means you can take up a mortgage of approximately $119637 today.

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