A method of providing a volume-mass law for determination of a fuel flow rate of an engine, particularly providing a fuel flow rate to a helicopter turbine, comprising the steps of: determining a sample type of fuel and a start density ρo of said sample type of fuel in said fuel tank using an equation ρ0=aT+b0, with a and b0 being known for said sample type of fuel and calculating real time offset parameters bn from an algorithm to determine real time densities ρ of the fuel.