We report a CO2 emission rate of 8500 metric tons per day (t d-1) for the summit of Kīlauea Volcano, several times larger than previous estimates. It is based on three sets of measurements over 4 years of synchronous SO2 emission rates and volcanic CO2/SO2 concentration ratios for the summit correlation spectrometer (COSPEC) traverse. Volcanic CO2/SO2 for the traverse is representative of the global ratio for summit emissions. The summit CO2 emission rate is nearly constant, despite large temporal variations in summit CO2/SO2 and SO2 emission rates. Summit CO2 emissions comprise most of Kīlauea's total CO2 output (~9000 t d-1). The bulk CO2 content of primary magma determined from CO2 emission and magma supply rate data is ~0.70 wt %. Most of the CO2 is present as exsolved vapor at summit reservoir depths, making the primary magma strongly buoyant. Turbulent mixing with resident reservoir magma, however, prevents frequent eruptions of buoyant primary magma in the summit region. CO2 emissions confirm that the magma supply enters the edifice through the summit reservoir. A persistent several hundred parts per million CO2 anomaly arises from the entry of magma into the summit reservoir beneath a square kilometer area east of Halemaumau pit crater. Since most of the CO2 in primary magma is degassed in the summit, the summit CO2 emission rate is an effective proxy for the magma supply rate. Both scrubbing of SO2 and solubility controls on CO2 and S in basaltic melt cause high CO2/SO2 in summit emissions and spatially uncorrelated distributions of CO2 and SO2 in the summit plume.