How to calculate the degree of freedom in T test and Z test

How to calculate the degree of freedom in T test and Z test? How to calculate/get the P value?

Degree of freedom for T test

if we use only one variable with n observations, then DF = n-1

if we have two independent variables with n1 and n2 observations, then DF= n1+n2-1

in case of paired variables and each have n observations then DF=n-1

p value of t and z distribution

if T=t1 and value of Z=z1 is positive

then we find (p(T>t1) and p(Z>z1) from standard t and z probabilities tables.

Most of the table provide cummulatie probabilities, in those situations we find above probabilies 1-p(T<t1) , 1-p(Z< z1)

if T=-t1 and value of Z=-z1 is positive

then we calculate p value by p(T<-t1) , p(z<-z1) from standard t and z probabilities table.