Three numbers are all NOOR sizing asks for. Here is how to take them so they are true - the way a seamstress would, in your own mirror.
Around the fullest part, tape level across your back. Breathe out, then breathe normally - take the number while breathing, not holding. The dress will be worn while breathing.
Where you actually bend, not where jeans sit. Tie a string snug, bend side to side - the string settles at your waist. Measure over the string, then untie it.
Around the fullest part, usually [seven to nine inches] below the waist string. Feet together, tape parallel to the floor all the way around.
Measure over what you will wear under it. A slip or a shell changes the number - measure over that, not over a sweater.
Between sizes, go by the piece. A wrap forgives, a fitted knit does not. Each product page says which way its fabric leans.
Write the three numbers down once. In your account they sit saved [if accounts launch] - otherwise a note on your phone does the same job.