There are various formulae that you need to know when it comes to buying airtime, as well as an understanding of what all the jargon means.

If you want to know what all the jargon means, please go to our glossary of TV terms

The equation to calculate cost per thousands is made up of 4 elements: 3 variable measures - money, cpt and ratings and 1 non variable - universe. You will always know the universe of the target audience you are buying, as these are published figures. They do change from month to month though. The network universes do not change much from month to month but the multi-channel universes usually go up a little each month as digital penetration increases. The network universes may change annually as the population profile changes slightly.

If you know 2 out of the 3 variable measures, you will be able to calculate the fourth. The equations you will need are as follows:

How many ratings can I buy?
Ratings = £/cpt/universe x 100,000
For example, if you have a budget of £500,000, the cpt is £6.20 and the universe is 48m, you will be able to buy 168 ratings

How much will it cost?
Money = TVRs x cpt x universe / 100,000
For example, if you want to buy 400 tvrs at a cpt of £6.20 with a universe of 48m, it will cost you £1.19m

What is the cpt?
Cpt = £/universe/tvrs x 100,000
For example, if you want to buy 200 tvrs, you have £600,000 to spend and the universe is 48m, the cpt you need to buy is £6.25

Time Lengths
Please note that for all calculation purposes, it is assumed you are buying a 30 second campaign as this is the industry standard. If you are buying different time lengths, you will need to apply a factor. Time length factors are as follows:

60”  2.0
50”  1.67
40”  1.33
30”  1.0 
20”  0.8 (this varies from broadcaster to broadcaster and sometimes is 0.85)
10”  0.5

Therefore, looking at how many ratings you can buy with different time lengths, you first need to calculate the 30 second equivalent ratings and then convert them to your specific time length by dividing your 30” equivalent tvrs by your factor.

For example, if you are buying 400 30” equivalent tvrs, this would equate to

200  60” tvrs
240 50” tvrs
300 40” tvrs
500 20” tvrs
800 10” tvrs

If you are buying a campaign with a variety of different time lengths, you will need to work out an overall factor.

For example, if you are buying 20”, 30” and 40” in a ratio of 1:1:2 then your factor would be 1.115

For example, if you were buying 10” and 20” in a ratio of 4:1 then your factor would be 0.56

For example, if you were buying 60”, 50” and 40” in a ratio of 2:5:1 then your factor would be 1.71