We present SketchingWithHands, a 3D sketching system that incorporates a hand-tracking sensor. The system enables product designers to easily capture desired hand postures from a first-person point of view at any time and to use the captured hand information to explore handheld product concepts by 3D sketching while keeping the proper scale and usage of the products. Based on the analysis of design practices and drawing skills in the art and design literature, we suggest novel ideas for efficiently acquiring hand postures (palm-pinning widget, front and center mirrors, responsive spangles), for quickly creating and easily adjusting sketch planes (modified tick-triggered, orientable and shiftable sketch planes), for appropriately starting 3D sketching products with hand information (hand skeleton, grip axis), and for practically increasing user throughput (intensifier, rough and precise erasers)—all of which are coherently and consistently integrated in our system. A user test by ten industrial design students and an in-depth discussion show that our system is both useful and usable in designing handheld products.