0.0394 inch diameter; approximately 1mm. The smaller ball stylus' are great for cursive writing lines burning on tagua nuts and other miniatures and a multitude of other uses. It glides over the wood to make writing with this tool a snap. Because of its small size it is welded to the tip rather than having a wire through it. Extra caution needs to be taken when using these tips they stay hotter longer after they are unplugged. The larger sizes can stay hot for up to 5 minutes so please be extra cautious when using them. All ball tips require a longer warm-up time and cool-down time than our other tips.