A system for calculating a mass of fuel in a fuel tank having a geometry that defines a volume, the system comprising: a meniscus tube disposable within the volume defined by the geometry of the fuel tank; a first side-emitting optical fiber having one end and another end; a side-receiving optical fiber that is positioned parallel to and at a distance from the first side-emitting optical fiber and has one end and another end; a first total internal reflection optical fiber having one end and having another end optically coupled to the another end of the first side-emitting optical fiber, wherein the first side-emitting optical fiber, the side-receiving optical fiber and the first total internal reflection optical fiber are disposed within the meniscus tube; an optical source configured to convert input electrical power to output light, wherein the optical source is optically coupled to the one end of the first side-emitting optical fiber; a first optical detector configured to convert impinging light into electrical signals having an amplitude which is a function of a first optical power of the light impinging on the first optical detector, wherein the first optical detector is optically coupled to the one end of the side-receiving optical fiber; a second optical detector configured to convert impinging light into electrical signals having an amplitude which is a function of a second optical power of the light impinging on the second optical detector, wherein the second optical detector is optically coupled to the one end of the first total internal reflection optical fiber; a computer system configured to calculate an estimated level of fuel in the fuel tank based on a difference of the first and second electrical signals output by the first and second optical detectors; a display device electrically coupled to the computer system; and a densitometer locatable in the fuel tank and configured to measure a density of fuel in the fuel tank, wherein the computer system is configured to execute the following operations: storing data representing the geometry of the fuel tank; receiving data from the densitometer representing a measurement of a density of the fuel in the fuel tank; calculating a mass of fuel remaining in the fuel tank based on the geometry of the fuel tank, the density of the fuel and the estimated level of fuel; and outputting an electrical signal representing the calculated mass of fuel in the fuel tank to the display device. a meniscus tube disposable within the volume defined by the geometry of the fuel tank; a first side-emitting optical fiber having one end and another end; a side-receiving optical fiber that is positioned parallel to and at a distance from the first side-emitting optical fiber and has one end and another end; a first total internal reflection optical fiber having one end and having another end optically coupled to the another end of the first side-emitting optical fiber, wherein the first side-emitting optical fiber, the side-receiving optical fiber and the first total internal reflection optical fiber are disposed within the meniscus tube; an optical source configured to convert input electrical power to output light, wherein the optical source is optically coupled to the one end of the first side-emitting optical fiber; a first optical detector configured to convert impinging light into electrical signals having an amplitude which is a function of a first optical power of the light impinging on the first optical detector, wherein the first optical detector is optically coupled to the one end of the side-receiving optical fiber; a second optical detector configured to convert impinging light into electrical signals having an amplitude which is a function of a second optical power of the light impinging on the second optical detector, wherein the second optical detector is optically coupled to the one end of the first total internal reflection optical fiber; a computer system configured to calculate an estimated level of fuel in the fuel tank based on a difference of the first and second electrical signals output by the first and second optical detectors; a display device electrically coupled to the computer system; and a densitometer locatable in the fuel tank and configured to measure a density of fuel in the fuel tank, wherein the computer system is configured to execute the following operations: storing data representing the geometry of the fuel tank; receiving data from the densitometer representing a measurement of a density of the fuel in the fuel tank; calculating a mass of fuel remaining in the fuel tank based on the geometry of the fuel tank, the density of the fuel and the estimated level of fuel; and outputting an electrical signal representing the calculated mass of fuel in the fuel tank to the display device. storing data representing the geometry of the fuel tank; receiving data from the densitometer representing a measurement of a density of the fuel in the fuel tank; calculating a mass of fuel remaining in the fuel tank based on the geometry of the fuel tank, the density of the fuel and the estimated level of fuel; and outputting an electrical signal representing the calculated mass of fuel in the fuel tank to the display device.