Clothes weigh somewhere between 2 and 2 1/2 lbs. on average depending upon the type of outfit worn and skewing heavier for male-gendered clothing. This number climbs with heavy winter gear, but is fairly consistent for most basic outfits in the western world and includes in its calculations shoes, sweaters, pants and other basic garments.
Factoring clothing into a personal or medical weigh in is a common mental adjustment tactic known as "clothing discount" or "the clothing effect." Most people tend to estimate higher, near to 5 lbs. or a little less, while the actual number is in fact just half of that.
Stepping onto the scale unclothed is an easy way to combat the impulse to adjust readouts mentally without recourse to reliable math. Those people weighing themselves can simply strip down first in order to receive an unvarnished report from the scale and remove the necessity for mental math and justifications.
Without accounting for coats, theoretically heavier winter wear often weighs little or no more than summer wear. The difference in warmth provided can be explained by the materials used in creating winter and summer clothing, which breathe differently but are not substantially divergent in terms of how much they weigh.