These rods work for all heat-based plastic welders, both the hot-tip and the hot-air style. As a general rule, you can estimate 1' of rod per 8" of welding.

Since you must match the rod-type to the material (ABS, PCV, or Nylon, etc.) you can do so by looking for the international stamp somewhere on piece to be repaired (it looks very much like the 'recycle' symbol you see around... three arrows going in a triangle). By international standards, every separate piece of plastic will have that stamp to indicate its plastic type (ABS, TPO, Polyethelene, etc.). For example, on an ATV each fender piece, cover panel, and headlamp cover have that stamp on their underside.