Fall damage is suffered when The Spelunker falls eight or more tiles. He is then stunned for a length of time and may take further damage from traps or enemies, making long falls one of the more dangerous hazards in Spelunky.
Depending on the length of the fall, the Spelunker may take more than one point of damage. A fall of at least eighteen tiles will deal the Spelunker two damage, while a fall of at least twenty-eight tiles will cause ten damage, which is usually instantly fatal.
Fall damage does not occur if the Spelunker lands in water or has his fall otherwise interrupted, for example by landing on an enemy or by proper use of the jetpack, cape or climbing gloves to reset your fall distance and speed.
As a quick rule of thumb, if the Spelunker is on the center of the screen and the target platform can be seen without looking down, it is safe to drop onto.