Long rounded log

Hello, I’m new to sketchup and am passionate about timberframing and general woodworking without screws or nails.
So I’m having fun making up plans for projects I plan to make this summer. But right now for one of my plans I’m just wondering if anybody has a rule of thumb on how a long log’s diameter reduces as you get higher.
So in my situation I need a 20’ long round log and am wondering what dimensions I should make it at the base and at the top (just to be safe). I’m aware that every tree grew differently and that the species is relevant to this outcome.

Just wondering if anybody can come up with a rule of thumb for this reduction in diameter. ’

thank you.

I don’t know that there is a rule of thumb for that. It depends on the species and where it is growning. You might try contacting the USDA Forest Products Lab to see if they have any information.

Since this isn’t specifically a SketchUp question, it’s going to the Corner Bar.

I understand that at least here in Finland there is, for the most common species. Timber today is mostly felled and collected by semiautomatic harvesting machines that take measurements of the trees as they go, using some kind of average ratios to determine the volume according to a thickess measurement taken in a standardized way and the length of the trunk. The landowner is then paid according to the volume harvested.