An isothermal sensor is developed to quantify the atomic hydrogen flux on a surface, which can be located at any distance from the molecular hydrogen cracking unit. This flux is determined from the measured heat effect due to recombination of atomic hydrogen at the sensor surface. The temperature of the sensor was kept constant at 350°C to keep the heat losses constant during the measurement. Other heat flows due to radiative, conductive, and convective phenomena were quantified with targeted measurements. The design of the sensor allows ample area for the atomic hydrogen recombination reaction; thus enabling the flux values to be determined with high accuracy (errors were between ±8.3×1015 and ±3.3×1016atcm-2s-1). The atomic hydrogen flux, generated with a commercial atomic hydrogen source was measured as a function of the filament temperature in the range of 1400-1950°C and H2 gas flow in the range of 7.44×10-6 to 7.44×10-5mol/s (10–100 sccm). These measurements showed that the atomic hydrogen flux increases with both filament temperature and H2 flux.