formatting a usb stick on a macbook air so windows can read it

ExFAT. Both read and write it, and unlike FAT32 it handles files over 4GB. In Disk Utility go View > Show All Devices first, then select the stick itself (the top level, not the volume under it) and Erase, ExFAT, GUID Partition Map. Selecting only the volume is what trips most people up, it sometimes won't let you change the scheme. Copy anything off it first obviously, erase wipes it.