im looking to replace the button to my wf-501b but am unable to find out what size button i need too order. Im sure many of you know the answer. and no i dont have a ruler… found some replacement buttons on www.fasttech.com/category/1608/diy-kits-parts-tailcaps. i would like red or a gitd blue i think. also open to any other links to replacement buttons especially ones that sell red. they seem to be harder to find. thanks in advance 
Should be 16mm. How can someone not have even a simple ruler? :P
Dont know about red rubber buttons though..
I never felt the need to buy a ruler idk…never really needed one before
I just measured my 501A and the button cover is 14mm.
Do the 501A and 501B have same button? Ive never looked into a 501A
501a is just a shorter version of the 501b. Tailcaps are the same.
Sweet now i can order a new 14mm button thanks rojos