Flue Calculator calculates the composition of Flue gas arising out of Fuel combustion in boilers, heaters, incinerators, etc. Quantity of Flue gas is also calculated. Fuel needs to be specified in terms of molecular formula of components. Air can be specified either as percent excess of stoichiometric or can be even included as components (N2 and O2) in fuel itself. Similarly moisture in fuel can be also specified. Weight percentage of ash/minerals could be also specified. Almost all popular units e.g. mole, lb-mol, scf, Nm^3, kg, etc can be used. Output of the app is represented in a table format but some data are relevant only for the gaseous fuel. Hence, shown in different color. From the given composition of fuel, ultimate analysis of fuel is also calculated for output. If the user wants to specify the ultimate analysis of fuel as input, the same is also possible using weight percent units. Moisture or air, if entered as a fuel component, the ultimate analysis counts them in percentage weight of N, O, H. However moisture, if specified separately, is included straight in the moisture percentage in the ultimate analysis. In case, sum total of percentage fuel compositions is not 100%, then the app redistributes the values to make it total 100%.