Letterbox Reach: The Minimum Distance Between a Mail Slot and the Inside Latch That Defeats a Hooked Tool

A mail slot through a front door is also a hole through which a hooked wire can be worked toward the inside latch, and on many doors the latch sits within easy reach of the slot. This study models the geometry from stated assumptions: a 700 mm hooked tool, after losses at the flap, the door thickness and its own bends, commands an effective radius of 520 mm from the slot centre, while an internal cover box behind the slot reduces that radius to 180 mm. A latch is taken as vulnerable when its distance from the slot is within the effective radius, and a safety margin of 80 mm is added for the design rule. On that basis an unprotected door wants at least 600 mm between slot and latch, a distance many doors, where the slot sits in the mid-rail a few hundred millimetres from the latch, do not have. With a cover box fitted, the required separation falls to 260 mm, which almost any door satisfies. The cheapest robust rule is therefore not to move the latch but to cage the slot, and to keep keys and thumbturns out of the unprotected radius in the meantime. SIMULATED, NOT MEASURED.