My research came up with 2 numbers 28 and 30 ounces. I converted my R-12 to EnviroSafe-12 (ES-12a). The conversion is 1:3 so I went with the smaller # and effectively 9.33 ounces of ES-12a provides same cooling capacity as 28 ounces of R-12. Nice thing with ES-12a is there is no flushing or other special system work to be done. Just seal up the system, I used Air Dry and Pro Seal as a precaution.