Reamer & conduit bundle size calculator

A common rule is about 1.5 times the outside diameter of what you are pulling, so a single 2 inch product reams to roughly 3 inches. Step up toward 2 times for long shots, rock, or tight clay that swells, and hold nearer 1.25 to 1.5 in clean, stable soil where you want to save fluid and spoil.

For equal conduits pulled as a tight cluster, the bundle diameter is a fixed multiple of one conduit's OD: two side by side is 2 times, three is about 2.15 times, four is about 2.41 times, and seven (a hex around a center) is 3 times. The calculator uses these packing ratios, then applies your backream factor to that envelope.

Friction and clearance. A hole cut to the exact product size binds, because the pipe has nowhere to move and the drilling fluid has no annulus to carry cuttings out. Over-reaming gives room for the pull and for spoil to flow back, which is what keeps the product from getting stuck.

Easier, but not free. A larger hole lowers pull friction, but it also uses more drilling fluid, makes more spoil to manage, and takes longer to ream. The 1.5 times rule is the usual balance; go bigger only when the pull length or ground calls for it.