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 select iMac or Mac mini models as independent SSD, or upgrade to Apple Fusion config. Not compatible with iMac (Late 2015). 1 Year OWC Fulfilled Limited WarrantyShips: Same Day