Nuclear Destruction Radius
Here is a nuclear bomb simulator - pick a city and size of bomb. Largest bombs in circulation appear to be around 1 megaton TNT equivalent (MT) and 5 MT. Largest test ever was 50 MT. Nagasaki was 20 kiloton. https://t.co/AnjptMjdLH— Marcin Jakubowski (@marcin_ose) March 13, 2022
- 1.2MT destroys (mostly fatal if exposed) 500 sq km. Ex Kiev, or NYC. Same.
- 200 million square miles total earth area - land area of earth is about 150 million sq km
- To kill everyone (who is exposed to burns, or worse), this would required 300,000 bombs of 1.2MT magnitude
- However, that would be for air explosion. Ground explosion creates lots of fallout (radioactive particles)
- 40000 sq km get a dose of 1 rad/hr. 200 rad gets very dangerous. Person in 1 rad can remain exposed for 200 hours until they have a risk of dying?
- At this rate - the entire land mass will be contaminated with 1 rad from 3750 of 1.2MT bombs.
- At this rate - very hot zone (100 rad/hr) (gives you 2 hours of full exposure before you die) - would be created across the globe land area if about 15,000 bombs of 1.2MT are detonated. One can escape from a very hot zone if they have time to get in a car and drive away (but not right after blast as there will be traffic and fallout travels fast) - so stay indoors and let the main danger of radiation pass (dispersion by wind) 
- Thus, full out nuclear exchange would require 300k bombs for it to potentially kill everyone - if the explosions are air - not ground.