How many lbs does it take to lose a pant size?

5-7 for me.

for me about 5 kg which is roughly 10lb.

Another thing i haven't seen mentioned so maybe you don't have this as much, but someone mentioned a size 14 is loose, but a 12 won't do up - here in australia clothing sizes, especially ladies, fluctuate wildly - even within the same brand but different styles I might take different sizes, and I've had the same style fit in different sizes (and they feel exactly the same). So now I don't go so much by what size a tag says - just by the fit and how it feels on - it's a pain if you're in a hurry as you can't grab and run, but until sizing is standardised ..... I used to love fitting a small size and feel depressed if something was a bigger size, but I've come to realise that what size something says it is has no relationship with how it relates to anything else of the same "size", and I've come to realise that my body measurements and changes are the best measure of my all my hard work.

example: I have three pairs of jeans - sizes 10,12,14 - the 10's are a little loose, the 12's are so loose they need a belt, and the 14's are tight but especially on my thigh. So go by fit not tag size