The following disclosure relates to a flow rate measuring device and method capable of correcting a gradient. More particularly, the following disclosure relates to a flow rate measuring device and method capable of correcting a gradient that may measure a flow rate having a minimized measurement error by including a sensor unit provided on a bottom surface of an inner portion of a fuel tank in order to accurately measure a flow rate value remaining in the fuel tank to obtain distance information between the bottom surface of the fuel tank and an oil surface and gradient information of the fuel tank itself.