How to calculate force from explosion on a rigidbody

Rigidbody.AddExplosionForce already do the calculation for you. You just have to specify the center of the explosion and call that function on the RB.

The explosion framework, which has been mentioned by Yoerick above, does probably use this function and is a good example how to use it.

Generally You just need to find all objects that are in "range" and call AddExplosionForce on all of them with exact the same parameters:

 // C#
 float range = 10.0f;
 float force = 100.0f;
 
 Collider[] objs = Physics.OverlapSphere(transform.position, range);
 foreach(Collider C in objs)
 {
     var R = C.rigidbody;
     if (R == null)
         continue;
     R.AddExplosionForce(force, transform.position, range);
 }

edit
Since your question isn't very detailed what you actually want, most people wait for a clarification what you want to do and for what purpose.

If you need to know some kind of damaging value for each object, well You can't see how AddExplosionForce calculates the distance. It could be linear, it could be quadratic or logarithmic. If you really want to know how it calculates the force, you might do some tests to figure it out.

The other possibility is that you just calculate a damage-value yourself, the way you want.

You usually start by determine the distance of each object to the explosion source and linearize the distance within the range:

 // C#
 float CalculateExplosionValue(Vector3 aSource, Vector3 aTarget, float aRange)
 {
     float dist = (aTarget - aSource).magnitude;
     if (dist > aRange)
         return 0.0f;
     return 1.0f - dist / aRange; 
 }

This function will return 1.0f if the object "sits" on the explosion and 0.0 when it's at max range or further away. Any position within the radius will be linearly between 1.0 and 0.0.

This can now be made a quadratic falloff or any other function graph you like.