ほぼ and ほとんど: What's the difference and how do you use them?

Question

What is the difference between ほぼ and ほとんど?

They both mean almost, don't they? Can they both be used in both positive and negative?

Answer 1

I'm Japanese and I could not figure out the difference by myself. But after I googled it and read the answer, I was convinced of the difference. I realized that I have been using each word on a case-by-case basis.

Well, only about ほとんど, you have to care about the use cases, whereas you can use ほぼ in every cases which means "almost" or "about".
You could use ほとんど only when the goal is clear or the max is 100% or the minimum is 0%. For example, you could say "宿題は、ほとんど終わった" but you could not say "学校まで歩いてほとんど１時間かかる".

With the phrase of "宿題は、ほとんど終わった", the goal is clear. The goal is finishing the homework. But with the phrase of "学校まで歩いてほとんど1時間かかる" there is no goal and neither its max. It just takes about an hour. That's all. In the both of these two cases, you could use "ほぼ" like "ほぼ終わった" or "ほぼ1時間かかる".

”プールの中の水は、ほとんど満杯である” = Good
Because there is the max. The max is fully filled water.

"プールの中の水は、ほとんど1000mlである" = Bad
It just says, it is 1000ml. It doesn't say about the goal.

By the way, they both can be used in both positive and negative. Read @ちょこれーと's answer about this.

Answer 2

1. ほとんど sounds more colloquial than ほぼ to me (when they can be used interchangeably)

2. You can say ほとんど~~ない but not ほぼ~~ない for "almost no~/hardly/rarely"

   彼のことはほとんど知りません。I hardly know him.
   (*彼のことはほぼ知りません。)
   私はほとんど(=めったに)映画には行かない。I rarely go to the movies.
   (*私はほぼ映画には行かない。)
   この町にはほとんど公園がない。The town has practically no parks
   (*この町にはほぼ公園がない。)

3. ほぼ can be used for 約 before a numeral

   郵便局はここからほぼ(=約)100メートルです。
   (*郵便局はここからほとんど100メートルです。)

4. ほとんど can be a noun.
   

   ほとんどの人(most people)/ ほとんどが~ / ほとんどを~ etc.
   (*ほぼの人 / *ほぼが~ / *ほぼを~ --- (ほぼ全ての/が/を would be fine))