IF function to work out VAT/ Sales Tax

[Brian Mair]

Hi ,

UK user here.

I have a spreadsheet to track sales and income and we have recently registered for VAT so I want to add a drop down box to mark yes or no for VAT  purposes. 

I've got the formula worked out to calculate VAT but how do I get Excel to work out if it should apply the formula.

In my simple brain I was thinking it would be something like;

IF(C2="yes",M2*0.2,=SUM(D2:L2), but that doesn't seem to work...........

[#2]

Update;

Columns C-G are entries for expense amounts and it will extend over 25-30 rows.

Column I

=SUM(C40:G40)*0.2 (to work out VAT amount)

Column

=SUM(C40:G40)-I40  (to work out NET total)

I want to put an extra column in with the drop down box of yes and no to activate the VAT calculate where applicable