You might need this calculator if you’re in the construction industry. You would need to measure walls, doors, windows, woods, floors, cabinets, screws and bolts. You might have the tape to measure in inches, but your products would be in millimeters. if you have say, 2 inch wood thickness, and have 35 mm screws. You don’t want them to get through the wood.
So, if you convert 2 inches to mm, it will be around 50.80 mm, which is definitely viable for 35 mm screws as they will not cut through the wood. This 2 inch can be easily converted in this calculator. That’s where it can be helpful.