How much radiation is deadly?

Lethal radiation dose varies, depending on physical condition and care given after receipt of dose. There is a grey area (no pun intended) between 6 and 8 Grays (a unit of ionizing radiation), equivalent to about 600 to 800 rads (in the old nomenclature), where 95% to 100% of those untreated will die, and 50% to 100% of those treated will die. Above 8 Grays, 800 rads, 100% will die.

For gamma radiation, one Gray is approximately equal to one Sievert, which is a unit of dose equivalent for biological tissue, so these numbers are comparable, i.e. 8 Sieverts or 800 rem.