Nicotine (for vaping purposes) is described and calculated as "milligrams per milliliter" (mg/mL).
Your nicotine concentrate that you are using is 18mg/mL. That means in each and every milliliter; there is 18 milligrams of nicotine. That also means that the strongest vape you can have, using that 18mg/mL concentrate, will be 18mg/mL... and that is only if you vape it undiluted. Whatever you add (VG, PG, flavor concentrates, water, etc.) to your concentrate (or vise versa), is going to result in less/lower nicotine mg/mL.
The math is pretty straight forward:
-- You have a 10mL bottle of 18mg/mL. That means there is a
total of 180mg in that full bottle (18mg X 10mL = 180mg).
If you add 10mL of your plain (no nicotine) glycerin to that full bottle of nicotine:
-- You still have 180mg of nicotine
total.
-- but you now have 20mL of mixed liquid (10mL glycerin + 10mL on nic. concentrate = 20mL total).
-- 180mg of nicotine, divided by the 20mL of total liquid (volume), is 9mg/mL nic. strength (180 ÷ 20 = 9).
Here is a different one:
-- You have a 10mL bottle of 18mg/mL. That means there is a
total of 180mg in that full bottle.
If you add 30mL of your plain (no nicotine) glycerin to that full bottle of nicotine:
-- You still have 180mg of nicotine
total.
-- but you now have 40mL of mixed liquid (30mL glycerin + 10mL on nic. concentrate = 40mL total).
-- 180mg of nicotine, divided by the 40mL of total liquid (volume), is 4.5mg/mL nic. strength (180 ÷ 40 = 4.5).
Or, here is a yet another one:
-- You are combining 3mL of your 18mg/mL nic. concentrate, with 1mL of your plain (no nicotine) glycerin, in your tank.
-- 3mL of 18mg/mL concentrate has a
total of 54mg of nicotine (18 X 3 =54).
-- but you now have 4mL of mixed liquid (1mL glycerin + 3mL on nic. concentrate = 4mL total).
-- 54mg of nicotine, divided by the 4mL of total liquid (volume), is 13.5mg/mL nic. strength (54 ÷ 4 = 13.5).
So, (hopefully) you are seeing that the
"total number of available grams of nicotine" divided by the
"total volume of all liquid" equals the final nicotine concentration as "mg/mL."
You have not yet told us what nicotine strength you want to be vaping; so we can't offer a specific dilution ratio.