Refer to figures.
EASA learning objective 022.01.04.01.02 says: 'State that fuel flow may be measured by volume or mass per unit of time.'

Fuel Flow (FF) sensors provide an indication of the mass or volume of fuel being used per unit of time. Measurment of fuel flow provides the pilots and the Flight Management System (FMS) with essential information about the performance of the engines and the state of the fuel system.

Fuel flow data is used to calculated total fuel used which can be compared with the fuel remaining in the tanks to check for leaks! Fuel Flow guages are calibrated as follows:

• Mass per unit of time: kgs per hour or lbs per hour.
• Volume per unit of time: Imperial or US gallons per hour, litres per hour.

Commercial air transport aircraft use mass for fuel quantity and flow measurements because it's a more consistent and accurate method of measurement. Volume measurements tend to be used by light aircraft with piston engines.