Two complementary analytical methods for quantifying carbon conversion efficiency and species emission rates of gas flares in the form of turbulent nonpremixed flames are derived and tested experimentally. Full mathematical expressions for partial derivative terms necessary to facilitate quantitative uncertainty analysis are also derived and presented as Supporting Information. Key assumptions are individually tested and the resulting generalized expressions are quantitatively compared with several other simplified expressions for calculating flare efficiency found in the literature. The first approach uses a carbon-balance approach to link measured concentrations of diluted combustion products to known flare gas outlet conditions while considering both the dilution of the combustion products and ambient levels of relevant species in the dilution and combustion air. This method is further extended to allow explicit consideration of solid-phase black carbon (soot) that may be present in the products. A second distinct method utilizes a tracer gas injected into the diluted plume to enable quantification of species emission rates from the combustion process directly. Experiments reveal how the two approaches each have advantages in different situations allowing experiments to be better optimized to reduced uncertainties. In addition, the tracer injection method is extensible for use in quantifying efficiencies and liquid-fallout on flares burning a mixed-phase fuel stream.