Allowable stress check of a plain stone lintel: bending, shear, bearing and deflection. Wall load is taken from arching action where the masonry above is deep enough to develop it, and in full where it is not. Allowable bending stress is the modulus of rupture divided by the factor of safety entered; use test data for the stone actually specified.
Use and liability: While I believe these tools to be free of error, I cannot be held liable for their results. They are to be used for instruction and should not be used to design a building or its components. There almost certainly are assumptions made for this tool that do not apply to your scenario. If you do something interesting with one of my tools, please tell me about it!
Will McCartney, PE — mccartneyeng.com