The median lethal dose for humans appears to be between 6g and 32g depending on the method of administration.[0] Drugs have different median lethal mg/kg dose ratios across species.[1]
edit: After investigating, the source for this claim is the Merck Index from 1989, which is not readily available online. Anyone who would use the above information to determine the appropriate ketamine dose for themselves or someone else is a dangerous fool.
[0] https://erowid.org/chemicals/ketamine/ketamine_faq.shtml [1] https://en.wikipedia.org/wiki/Median_lethal_dose#Examples
edit: After investigating, the source for this claim is the Merck Index from 1989, which is not readily available online. Anyone who would use the above information to determine the appropriate ketamine dose for themselves or someone else is a dangerous fool.