Oftentimes, percentages are rounded to the nearest whole number. A $49.99 item may be marked 50% off, even if the price becomes $24.99 (it should be 50.03% off). However, I have come to notice that this rounding rule does not seem to apply with 100% and 0%.
For example, certain soaps will say that they kill 99.99% of bacteria. If a (pretty bad) soap killed 39.99% of bacteria, there would be no problem with advertising as 40%. So how come soaps that kill 99.99% cannot be advertised as 100%? Why is it that the percentages of 0 and 100 are always implied to be exact, and not allowed to be rounded?
Best Answer
"100%" is equivalent to "all". There is no rounding with "all"; either you get all of something or you don't. If a product advertised itself as "kills all bacteria" and then you found that there were 3 bacteria that it didn't kill, it doesn't matter whether that's 3 out of 10 or 3 out of 28 million; it's not all of them.
Even in ordinary conversation, if your child says "I picked up all the blocks" and you find 1 block left on the floor, you can legitimately say that they did not, in fact, pick up all the blocks. Doesn't matter if there were 10 blocks or 20,000; if there's one left on the floor, they did not pick up 100% of them.
(Similarly, "0%" = "none"; if you say "there are none left" and there's one left, you're wrong, regardless of how many there used to be.)