Bid -Ask spread

Dear All:

The Bid-Ask spread is (Ask-Bid)/Ask. Why devided by Ask , not Bid? what is the logic behind this formular?

Thank you so much for your time.

Where are you seeing this? The bid-ask spread is typically the absolute difference (just ask - bid).

CFAI says that the spread is sometimes calculated as a % of the ask or the mid point. Schwesersays to always use the ask. Niether text gives a reason why though.