Our primary objective in this study was to design and implement the FAME Lab PHS Calculator software (PHS_FL) (www.famelab.gr/research/downloads), a free tool to calculate the predicted heat strain of an individual based on ISO 7933:2018. Our secondary objective was to optimize the practicality of the PHS_FL by incorporating knowledge from other ISO standards and published literature. The third objective of this study was to assess: (i) the criterion-related validity of the PHS_FL by comparing its results against those obtained using the original ISO 7933:2018 code; and (ii) the construct validity of the PHS_FL by comparing its results against those obtained via field experiments performed in human participants during work in the heat. Our analysis for criterion validity demonstrates that PHS_FL provides valid results within the required computational accuracy, according to Annex F of ISO 7933:2018. The construct validity showed that root mean square errors (RMSE) and 95% limits of agreement (LOA) were minimal between measured and predicted core temperature (RMSE: 0.3°C; LOA: 0.06 ± 0.58°C) and small between measured and predicted mean skin temperature (RMSE: 1.1°C; LOA: 0.59 ± 1.83°C). In conclusion, the PHS_FL software demonstrated strong criterion-related and construct-related validity.