<p>A multicomponent borehole strainmeter is a highly sensitive geodetic instrument capable of detecting small tectonic faulting such as slow slip events in subduction zones. However, in-situ calibration and thorough validation of the calibration results are required to obtain reliable strain data sets from raw observation data. In this study, in-situ calibration of Ishii-type borehole strainmeters deployed in southwest Japan was performed by comparing the tidal linear responses of the strainmeter to the precise theoretical tidal strain. Calibration results were validated using phasor plots, perturbation matrices, and long-period seismic surface wave. We provide calibration matrices for 13 Ishii-type borehole strainmeters that will be useful for future geodetic studies in southwest Japan using these strain data sets.</p> Graphical Abstract <p></p>