MATLAB code that determines the scaling factor to convert an SI unit from one to another prefix

prefixscale.m

function scale = prefixscale(old, new)

% PREFIXSCALE determines the scaling factor to convert an SI unit from one to another prefix
%
% For example
%   prefixscale('kg', 'mg')
% returns 1000000.

% Copyright (C) 2019, Robert Oostenveld

prefix = {
  'Y'
  'Z'
  'E'
  'P'
  'T'
  'G'
  'M'
  'k'
  'h'
  'da'
  '_'
  'd'
  'c'
  'm'
  'u'
  'n'
  'p'
  'f'
  'a'
  'z'
  'y'
  };

multiplier = [
  1e24
  1e21
  1e18
  1e15
  1e12
  1e9
  1e6
  1e3
  1e2
  1e1
  1e0
  1e-1
  1e-2
  1e-3
  1e-6
  1e-9
  1e-12
  1e-15
  1e-18
  1e-21
  1e-24
  ];

oldindex = find(strcmp(prefix, '_'));
newindex = find(strcmp(prefix, '_'));

for i=1:numel(prefix)
  if startsWith(old, prefix{i}) && ~isequal(old, prefix{i})
    oldindex = i;
  end
  if startsWith(new, prefix{i}) && ~isequal(new, prefix{i})
    newindex = i;
  end
end

scale =  multiplier(oldindex) ./ multiplier(newindex);