Correlation between radius and diameter, help please. - Lemmy.world

I recently discovered I could split a piece of 4 inch PVC pipe lengthwise heat it up with a heat gun then weight the pvc between some sheets of plywood and end up with a flat PVC sheet. This was a revelation to me as I am a tinkerer and have sourced pieces of plastic from other sources for years. Once I split the 4 inch pipe up the middle and flattened it I measured it and realised it was exactly 13 inches. I left school 20 years ago and I do realise there is a direct correlation between radius and diameter but I was never very good at mathematics. I currently have a sheet of rubber on my workbench to protect the surface but I think the pvc sheet would be great to protect it if it was big enough. My workbench currently measures just over 24 inches. What diameter of pipe would I need to create a PVC protective top for my work bench. I’m in UK if that helps. There are standard sizes of 4,6,8,10 inches of PVC pipe available from what I can see. I am no Mathematician so any help is greatly appreciated.