I think it is two sizes up. I recently bought some boots from the states. I'm a size five and my hubby bought a size seven. They are a tiny bit tight, but that could just be the fit. i wear them and all tho. i think I would have got a seven and half.
I didn't find the shoe conversion charts very useful in America. I had a chart of my own and the lady in the shop had one, but the shoes that were supposed to fit me were tiny. I'm a size 5 and I ended up buying size 8 shoes.