1GB = 1,000,000,000 bytes. Total accessible capacity varies depending upon operating environment (typically 5-10% less). The actual total capacity will depend on your own defined file settings/preferences.
Manufacturer provided specifications for Apple p/n MZ-KPW1280
AppleGenuine128GB Solid-State DriveMfr P/N: MZ-KPW1280OWC SKU: APLSSDIF128GB17Add to Mac mini 2018 or iMac 2017 or later model as independent 128GB SSD or upgrade to Apple Fusion config *NOT FOR PRE 2017 MODELS*1 Year OWC Fulfilled Limited WarrantyShips: Same Day