For strength:
3mg = 0.3%
9mg = 0.9%
12mg = 1.2%
18mg = 1.8%
etc.
Some companies use % while other use mg, so the conversion is good to know, and simple.
Notice how the decimal point moves over one place in the conversion (either way). No need for math.
I agree with Bronze that you should try 12mg (aka 1.2%) strength if you think 9mg (aka 0.9%) isn't doing it for you and 18mg (aka 1.8%) was too strong for you.
If you can't find 12mg (aka 1.2%) strength in your chosen brand/flavor, you can always buy one 10ml bottle each of the 18mg (aka 1.8%) and the 9mg (aka 0.9%) then mix them together in a larger bottle to get 13.5mg (aka 1.35%) strength, which is close enough to 12mg (aka 1.2%).
Personally, as an example, I vape about 10-12ml of 6-8mg (aka 0.6-0.8%) strength per day (I used to use 12mg (aka 1.2%) up until a few months ago).
Lots of people on this forum vape a lot more juice than 10ml per day, however most of those people probably use a much lower strength than me.
How much is too much for you? You just have to experiment.
does the whole 10ml bottle contain 9ml of nicotine?
When you see the strength listed as "9mg", that means
9 milligrams of nicotine per 1 milliliter of juice (also written as "9mg/ml").
That means every 1 milliliter (ml) of liquid contains 9 milligrams (mg) of nicotine.
Therefore, for 9mg (aka 0.9%) strength juice...
- 10ml of juice will contain 90mg of nicotine in total
- 20ml of juice will contain 180mg of nicotine in total
- 30ml of juice will contain 270mg of nicotine in total
- 50ml of juice will contain 450mg of nicotine in total
- 100ml of juice will contain 900mg of nicotine in total
- etc.
BTW: The fact that your bottle says is 0.8% strength (aka 8mg) but then goes on to say 9mg is false and confusing. Disregard the 0.8% and assume that what you have is 0.9% (aka 9mg) strength. That makes 90mg of nicotine for the whole 10ml bottle.
If none of this makes sense, reread it and reread it again.