If you use http://www.royalmail.com/portal/rm/content1?mediaId=53800712&catId=400036 you can check the tables for prices in future :)

For an A4 envelope, the prices are as follows:

20-40 grams £1.35
40-60 grams £1.82

Those stamps (called "second class") are worth 30p therefore you'll need 5 or 7 of them (you'll need to check the exact weight and see if it's 20-40 or 40-60).

Hi,
recently I bought this type of UK Stamps
[img]http://i31.tinypic.com/35d89y0.jpg[/img]
[img]http://i29.tinypic.com/2v94x9c.jpg[/img]

I use a4 envelope and the weight is around 0.30-0.50 grams
how many stamps I have to put on the envelope ?