Only on light airplanes is the flowmeter calibrated to read only volume. Most larger or more complex aircraft use temperature-adjusted mass flow based on weight. I want to know how many pounds per hour, or Kg per hour, of fuel I'm burning. Same for fuel in the tank. The number of gallons is really irrelevant, because the energy per unit of fuel is by weight not by volume. Additionally, for weight and balance purposes, volume isn't relevant...but the weight is.
We fuel by weight, keep track of burn by weight, and plan our reserves by weight.
This is usually accomplished using sensors in one or more fuel tanks, which are then used to apply a correction factor to fuel information in the tanks and in the system.