I'm trying to have the first while loop run for each iteration of i, but don't know how to implement it, thoughts?

for i=1:length(RET);
  for j=1:length(RelRough);
    %while   %this is the while loop I'm stuck on
    while error > 1e-14
      FCT_cal=1./(-2.0*log10((RelRough(j)./3.7)+(2.51./(RET(i).*sqrt(FCT_guess))))).^2
      error_new=abs(FCT_cal(i,j)-FCT_guess(i,j))
      error = error_new
      FCT_guess = FCT_cal
      d=d+1
      if d>30
        disp('The program failed to converge in the allowable number of iterations.')
        break
      end
      FCT(i(p),j(p)) = [FCT_cal]     
    end
 end

[dpb]

Besides the i loop you've also got the while inside another j loop???

But, presuming the convergence ever does occur, you need a line to reset the value of error to something greater than the convergence before the beginning of the while in order for it to execute.