When I was diagnosed in the USA, I learned all about blood sugar in mg/dl. I was taught that a low blood sugar was anything under 70 mg/dl (a nice round number).
I moved to Hungary and after spending hours converting my blood sugar logs from mg/dl to mmol/L before each doctor's appointment, I finally decided to switch to using mmol/L in my daily life last year. I got used to the new measurements pretty quickly (though I still do sometimes convert the numbers). I was treating lows as anything below 3.9 mmol/L (since 70/18.05=3.87).
At one point my new CDE told me that lows are anything below 3.5 mmol/L (which is 63.17 mg/dl). Magically after that I started not feeling low at 3.7 :-) So I stopped treating the lows unless they are below 3.5. My endo still tries to keep my blood sugars above 4, but considers below 3.5 as the real lows.
But I was wondering about the rest of you who measure in mmol/L -- what is your cut-off for a low blood sugar?
agree, 4.0 - 7.0 is my "target range" that I try to stay in, but 3.5 and below is a real low to me. If I am, say, 3.8 before a meal I will often just eat the meal rather than going, "Oh no, must get sugar now!!" Same if I wake up at 3.8 or 3.7, my endocrinologist doesn't mind this and will not consider it low, except of course I usually would adjust pump settings anyway because it's getting a bit too close to low for me. During the night I will treat a 3.8 or so, but I also treat anything below 5.0 at night, as well as after meals, because there's a good chance of dropping lower. So I guess my definition of "low" varies depending on the circumstances, time to next meal, and so on.