Guys, what could be the destruction caused by 1 ton of a nuke and conventional warheads respectively in sq.km?
For nukes, it depends on many things.
The one ton payload of a nuclear missile does not means that the nuclear warhead (the device) weighs 1000kg. Around one-third of the weight (give or take) is of the auxiliary electronics, safety, arming, detonation and self-destruct systems.
If you're talking about a one ton nuclear device, then it depends on the yield of the weapon. Usually modern fission devices are not that heavy.
The altitude of detonation also affects the damage radius. For example the ideal altitude for a 20kT device is ~500m.
However, for the nuclear weapon with the highest yield by India (claimed 60kT), following would be the destruction effects:
Effects radii for 60 kt blast (smallest to largest):
Fireball radius: 0.16 km / 0.1 mi (0.08 km² / 0.03 mi²)
Maximum size of the nuclear fireball; relevance to lived effects depends on height of detonation.
Air blast radius: 1.08 km / 0.67 mi (3.67 km² / 1.42 mi²)
20 psi overpressure; heavily built concrete buildings are severely damaged or demolished; fatalities approach 100%.
Radiation radius: 1.83 km / 1.14 mi (10.51 km² / 4.06 mi²)
500 rem radiation dose; between 50% and 90% mortality from acute effects alone; dying takes between several hours and several weeks.
Air blast radius: 2.85 km / 1.77 mi (25.59 km² / 9.88 mi²)
4.6 psi overpressure; most buildings collapse; injuries universal, fatalities widespread.
Thermal radiation radius: 3.68 km / 2.29 mi (42.55 km² / 16.43 mi²)
Third-degree burns to all exposed skin; starts fires in flammable materials, contributes to firestorm if large enough.
Here is a nuclear destruction effects calculator:
NUKEMAP by Alex Wellerstein