Hello I am doing some self plumbing and need a little help. I have a 500 litre tankon the roof 13 meters up from ground floor I am planning on putting in a 8cm diameter vertical line. Now Iunderstand that pressure will be basically a function of height as in 1300cm cubed meaning pressure = 1.3 kg /cmsquared now the full weight of the water in the line will be roughly 65kg using volume formulas 4cm*4cm*3.14*1300 and 1l=1kg. This is very low water pressure but if I reduce pipe sizeat entry to my floor to say 2cm diameter will the full weight of the water aply to the new area and sky rocket my pressur.

No. The pressure is a function of the heigth only. Without converting your measurements in metric to something I understand, the pressure will be the same for a small pipe as for a large pipe. The volume will be different but the pressure will remain the same.

I am having a hard time rationalizing that say with a garden house when you constrict the end your pressure increases but flow decreeses right same logic ? Ill put in imperial roughly a 3 inch line 50 feet high full of water and then at bottom I conc down to 1 inch doesn't gain pressure?