You can find several PRT file types that contain 3D parts or components, so there may be several different prt to stp conversions possible, unfortunately no way to convert prt to stp online seems to exist.
One of the prime examples for prt to stp conversion would be an early Solidworks (.prt) file to STEP product data file transfer (.stp). Unless you have access to SolidWorks, you will have to rely on some 3rd party CAD programs or dedicated converters.Open PRT file Open STP file
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