Walk into any hardware shop in Uttar Pradesh and ask for a door lock, and the first question back will be “kitne inch ka?” Most customers guess. Some say ten because it sounds stronger. Some say eight because it sounds cheaper.

Neither is a great reason. The size of a door lock matters, but for reasons that have more to do with your door than with security.

What the inches are actually measuring

When people talk about an 8 inch or 10 inch door lock in India, they are usually talking about the length of the handle plate, the long metal plate that holds the handle and the key hole on each side of the door. In some shops the number refers to the overall set.

It is worth confirming at the counter, because a 10 inch door handle lock in one brand may not line up with the holes left by a 10 inch lock from another. If you are replacing an old lock, take it with you or measure the plate and the distance between the screw holes.

When a 10 inch door lock makes sense

Bigger doors look better with bigger hardware. A tall main door, a heavy wooden door, or a door with a wide stile looks balanced with a 10 inch plate. A small plate on a big door looks like it was borrowed from another house.

A longer plate also covers more of the area around the lock pocket, which helps hide old holes or marks when you are upgrading. And on a heavy door, a longer handle gives a little more leverage, which makes the door easier to open.

When an 8 inch door lock is the better fit

Interior doors in most apartments and many houses are narrower and lighter. On a bedroom, study or kitchen door, an 8 inch lock looks proportionate and does the job perfectly well.

Slim doors, flush doors and doors with glass panels often do not have the width for a big plate. Forcing a 10 inch plate onto them can mean the plate overhangs the edge or clashes with a glass panel.