Is higher mg/ml always stronger?

Yes, higher mg/ml means more nicotine per milliliter, so it's stronger. But how it feels also depends on whether it's freebase nicotine or nicotine salts. Salts are smoother, so a 50 mg/ml salt can feel less harsh than a 24 mg/ml freebase.

How do I choose my first nicotine strength?

Start with your smoking habit. If you smoked a pack a day, 20–30 mg/ml (salt) is a common starting point. Light smokers might start at 10–15 mg/ml. When in doubt, pick a lower strength—you can always go up.

What does 'nicotine by volume' mean?

It's another way to say mg/ml. On some labels, you'll see nicotine 'by volume' or 'per ml.' It's the same thing: milligrams per milliliter.