Gas exchange measurement is the gold standard method for determining leaf CO ₂ assimilation rate ( A _n). However, conventional systems for measuring A _n often require considerable time and/or effort to collect numerous samples in the field owing to their high weight and large size. Here, we present an efficient and convenient method for estimating A _n using a handheld porometer with a chlorophyll fluorometer, facilitating on-the-go assessment of A _n in the field. This method integrates the measured stomatal conductance and quantum yield of photochemistry in PSII into a biochemical photosynthesis model, incorporating model uncertainties into a single calibrated parameter. Using this method, we successfully estimated A _n variations in 12 species under field conditions, with a root mean square error of 2.0 mol m ⁻² s ⁻¹, despite using the common parameter set. In contrast, without calibration (i.e., with the often-assumed parameter value), this method greatly overestimated A _n. These results highlight the importance of appropriate calibration depending on prevailing conditions, particularly the light source. In summary, this method demonstrates the potential for accessible, high-throughput, and accurate estimation of A _n in diverse plants, thereby addressing a key bottleneck in field-based phenotyping of photosynthesis. However, further studies are required to reduce the uncertainties imposed on the calibrated parameter.