why does a baker's dozen have 13?

Very good question and I give you a star. I found this website that tells a relatively good history of the formulation of the phrase. Just click below and BTW you could have done this same search yourself:

http://rds.yahoo.com/_ylt=A0geu7g93PZHJQEBD01XNyoA...

The oldest known source and most probable origin for the expression "baker's dozen" dates to the 13th century in one of the earliest English statutes, instituted during the reign of Henry IV (r. 1216-1272), called the Assize of Bread and Ale. Bakers who were found to have shortchanged customers could be liable to severe punishment. To guard against the punishment of losing a hand to an axe, a baker would give 13 for the price of 12, to be certain of not being known as a cheat. Specifically, the practice of baking 13 items for an intended dozen was to prevent "short measure", on the basis that one of the 13 could be lost, eaten, burnt or ruined in some way, leaving the baker with the original dozen. The practice can be seen in the guild codes of the Worshipful Company of Bakers in London.

[edit] Modern uses

While modern bakers no longer fear medieval law, they have found other reasons for a baker's dozen, as seen in the tidy way 13 disks (loaves, cookies, biscuits, etc.) can pack a rectangle (baking tray) of appropriate proportions.[citation needed] Modern standard sized packing trays have a 3:2 aspect ratio, and the most efficient two-dimensional array is hexagonal close packing, which has sixfold symmetry, such that each baked item is equidistant from its six nearest neighbors. The corners of a cookie sheet heat up and cool off faster than the edges and interior, so any item placed near a corner will not bake at the same rate as the other items. A 4+5+4 arrangement provides the dense hexagonal packing while avoiding corners, and would have been discovered empirically by bakers with the goal of baking the maximum number per batch with optimal uniformity. Continued use also stems from tradition, and some customers see it as a sign of appreciation from the baker for continued patronage

If you were good friends with the baker, or if you were cute I guess, you'd get an extra thrown in when you bought a dozen. This was done purely at the baker's discretion, hence "Baker's Dozen"

Since people generally don't shop at a bakery any more, this term has been adapted for use in promotions and sales.

a bakers dozen is 13 because every 12 u buy, the baker gives u a free 1.

I heard it was for whoever went to pick up the bread. Like, the family would send their boy out to go pick up the bread and the baker would put an extra roll in for him to eat on the way home.

it something like this... well however many years ago the bakers had to put 12 bread rolls in a bag and if they counted it incorrectly and only put in 11 then they would get sued or something.. i can't remember. So they put an extra bread roll in too make sure that there was at least 12.

first theory it's so that the baker could eat 1 and make sure it tasted good and they'd still have 12 (dozen).

other theory it was so that if a cookie got messed up they'd have 12 others(dozen).

i heard it came from being generous

making them come back for more

i THINK SO THEY COULD SAMPLE THE PRODUCT BEFORE THEY SELL IT AT THE MARKET