makeatletter
defconvertto#1#2{strip@ptdimexpr #2*65536/numberdimexpr 1#1}
makeatother

documentclass[a4paper]{article}

usepackage{array}
usepackage[hmargin=2cm]{geometry}

makeatletter
%http://groups.google.com/group/comp.text.tex/msg/7e812e5d6e67fcc5
defconvertto#1#2{strip@ptdimexpr #2*65536/numberdimexpr 1#1}
makeatother

begin{document}

begin{center}begin{tabular}
  {>{defcolunit{pt}}l<{convertto{rowunit}{1colunit}}
   >{defcolunit{mm}}l<{convertto{rowunit}{1colunit}}
   >{defcolunit{cm}}l<{convertto{rowunit}{1colunit}}
   >{defcolunit{ex}}l<{convertto{rowunit}{1colunit}}
   >{defcolunit{em}}l<{convertto{rowunit}{1colunit}}
   >{defcolunit{bp}}l<{convertto{rowunit}{1colunit}}
   >{defcolunit{dd}}l<{convertto{rowunit}{1colunit}}
   >{defcolunit{pc}}l<{convertto{rowunit}{1colunit}}
   >{defcolunit{in}}l<{convertto{rowunit}{1colunit}}
   >{bfseries}l}
multicolumn{1}{l}{bfseries 1pt} & multicolumn{1}{l}{bfseries 1mm} & multicolumn{1}{l}{bfseries 1cm} & multicolumn{1}{l}{bfseries 1ex} & multicolumn{1}{l}{bfseries 1em} & multicolumn{1}{l}{bfseries 1bp} & multicolumn{1}{l}{bfseries 1dd} & multicolumn{1}{l}{bfseries 1pc} & multicolumn{1}{l}{bfseries 1in} & \
gdefrowunit{pt} & & & & & & & & & rowunit\
gdefrowunit{mm} & & & & & & & & & rowunit\
gdefrowunit{cm} & & & & & & & & & rowunit\
gdefrowunit{ex} & & & & & & & & & rowunit\
gdefrowunit{em} & & & & & & & & & rowunit\
gdefrowunit{bp} & & & & & & & & & rowunit\
gdefrowunit{dd} & & & & & & & & & rowunit\
gdefrowunit{pc} & & & & & & & & & rowunit\
gdefrowunit{in} & & & & & & & & & rowunit\
end{tabular}end{center}

end{document}

documentclass{article}
usepackage{printlen}
parindent=0pt

newlengthLength Length=1cm
begin{document}

tabular{p{2cm}p{2cm}}
mbox{--- 1cm ---}

uselengthunit{cm}printlength{Length}
uselengthunit{mm}printlength{Length}
uselengthunit{in}printlength{Length}
uselengthunit{pt}printlength{Length}
uselengthunit{bp}printlength{Length}
uselengthunit{sp}printlength{Length}
uselengthunit{pc}printlength{Length}
&
mbox{--- 1em ---}
Length=1em
uselengthunit{cm}printlength{Length}
uselengthunit{mm}printlength{Length}
uselengthunit{in}printlength{Length}
uselengthunit{pt}printlength{Length}
uselengthunit{bp}printlength{Length}
uselengthunit{sp}printlength{Length}
uselengthunit{pc}printlength{Length}
endtabular
end{document}

documentclass{article}

usepackage{xintfrac}
% http://www.ctan.org/tex-archive/macros/generic/xint
% This code was compiled with the version 1.06a of the xint
% bundle dated 2013/05/09, which should appear soon on CTAN.
% The current CTAN version 1.06 should be also OK for this.

usepackage{array}
usepackage[hmargin=.5cm]{geometry}

% Conversions to the basic dimension, chosen to be the centimeter
% Base dimension: 1cm
defonecm {1}
defonemm {1/10}
defonein {2.54}
%% defonept {xintMul {1/72.27}{onein}}
%% simpler:
defonept {2.54/72.27}
%% defonebp {xintMul {1/72}{onein}}
%% simpler:
defonebp {2.54/72}
defonepc {xintMul {12}{onept}}
defoneex {xintMul {thenumexprdimexpr 1exrelaxrelax}{onesp}}
defoneem {xintMul {thenumexprdimexpr 1emrelaxrelax}{onesp}}
defonedd {xintMul {1238/1157}{onept}}
% 1157 dd = 1238 pt I take this conversion factor from the TeXBook
% Wikipedia has other conversion factors, but of course here we
% have to do it the TeX way.
defonecc {xintMul {12}{onedd}}
defonesp {xintMul {1/65536}{onept}}


% Routines with delimited arguments, the good old TeX way
% (completely expandable)
makeatletter

% exact conversion to an irreducible fraction:
% example: convertexactly 126.2772ptto {bp}
% and `pt' may be a macro expanding to it.
% idem for {bp} which may be a macro expanding to bp
defconvertexactly #1to #2%
    {xintIrr{convertexactly@ #1to {#2}}}%

% Variant with rounding at a number of decimal places
% given by first argument.
defconvertwithrounding #1#2to #3%
    {xintRound {#1}{convertexactly@ #2to {#3}}}%

% routines doing the job:

defconvertexactly@ #1to
{%
    romannumeral0%
    expandafterexpandafterexpandafter
    convertexactly@a
    xintReverseOrder {#1}Z
}%
defconvertexactly@a #1%
{%
    ifcatnoexpand #1relax
       expandafter convertexactly@b
    else
       expandafter convertexactly@c
    fi #1%
}%
defconvertexactly@b #1#2Z #3%
{%
    xintdiv {xintMul {xintReverseOrder{#2}}{csname one#1endcsname}}
             {csname one#3endcsname}%
}%
defconvertexactly@c #1#2#3Z #4%
{%
    xintdiv {xintMul {xintReverseOrder{#3}}{csname one#2#1endcsname}}
             {csname one#4endcsname}%
}%

makeatother

defbigstrut {vbox to 24pt{}vbox to 12pt{}}%

begin{document}

Testing:

72.27pt is exactly convertexactly 72.27ptto {bp}bp

1/2.54in is exactly convertexactly 1/2.54into {mm}mm

10pt is exactly (for this font) convertexactly 10ptto {ex}ex,
or approximately convertwithrounding{20}10ptto {ex}ex

10pt is exactly (for this font) convertexactly 10ptto {em}em,
or approximately convertwithrounding{20}10ptto {em}em

1em is exactly (for this font) convertexactly
1emto {pt}pt, or approximately
convertwithrounding{20}1emto {pt}pt.

And indeed
verb+thedimexpr 1emrelax+ gives thedimexpr 1emrelax{} and
verb+thefontdimen6font+ gives thefontdimen6font

1ex is exactly (for this font) convertexactly 1exto {pt}pt, or approximately
convertwithrounding{20}1exto {pt}pt.

And indeed
verb+thedimexpr 1exrelax+ gives thedimexpr 1exrelax.

deftableentry{$displaystylexintFrac{convertexactly 1colunittorowunit}$bigstrut}

begin{center}begin{tabular}
  {>{defcolunit{pt}}l<{tableentry}
   >{defcolunit{mm}}l<{tableentry}
   >{defcolunit{cm}}l<{tableentry}
   >{defcolunit{ex}}l<{tableentry}
   >{defcolunit{em}}l<{tableentry}
   >{defcolunit{bp}}l<{tableentry}
   >{defcolunit{dd}}l<{tableentry}
   >{defcolunit{pc}}l<{tableentry}
   >{defcolunit{in}}l<{tableentry}
   >{bfseries}l}
multicolumn{1}{l}{bfseries 1pt} & multicolumn{1}{l}{bfseries 1mm} &
multicolumn{1}{l}{bfseries 1cm} & multicolumn{1}{l}{bfseries 1ex} &
multicolumn{1}{l}{bfseries 1em} & multicolumn{1}{l}{bfseries 1bp} &
multicolumn{1}{l}{bfseries 1dd} & multicolumn{1}{l}{bfseries 1pc} &
multicolumn{1}{l}{bfseries 1in} & \
gdefrowunit{pt} & & & & & & & & & rowunit\
gdefrowunit{mm} & & & & & & & & & rowunit\
gdefrowunit{cm} & & & & & & & & & rowunit\
gdefrowunit{ex} & & & & & & & & & rowunit\
gdefrowunit{em} & & & & & & & & & rowunit\
gdefrowunit{bp} & & & & & & & & & rowunit\
gdefrowunit{dd} & & & & & & & & & rowunit\
gdefrowunit{pc} & & & & & & & & & rowunit\
gdefrowunit{in} & & & & & & & & & rowunit\
end{tabular}end{center}

clearpage

deftableentry{convertwithrounding {5}1colunittorowunit}

begin{center}begin{tabular}
  {>{defcolunit{pt}}l<{tableentry}
   >{defcolunit{mm}}l<{tableentry}
   >{defcolunit{cm}}l<{tableentry}
   >{defcolunit{ex}}l<{tableentry}
   >{defcolunit{em}}l<{tableentry}
   >{defcolunit{bp}}l<{tableentry}
   >{defcolunit{dd}}l<{tableentry}
   >{defcolunit{pc}}l<{tableentry}
   >{defcolunit{in}}l<{tableentry}
   >{bfseries}l}
multicolumn{1}{l}{bfseries 1pt} & multicolumn{1}{l}{bfseries 1mm} &
multicolumn{1}{l}{bfseries 1cm} & multicolumn{1}{l}{bfseries 1ex} &
multicolumn{1}{l}{bfseries 1em} & multicolumn{1}{l}{bfseries 1bp} &
multicolumn{1}{l}{bfseries 1dd} & multicolumn{1}{l}{bfseries 1pc} &
multicolumn{1}{l}{bfseries 1in} & \
gdefrowunit{pt} & & & & & & & & & rowunit\
gdefrowunit{mm} & & & & & & & & & rowunit\
gdefrowunit{cm} & & & & & & & & & rowunit\
gdefrowunit{ex} & & & & & & & & & rowunit\
gdefrowunit{em} & & & & & & & & & rowunit\
gdefrowunit{bp} & & & & & & & & & rowunit\
gdefrowunit{dd} & & & & & & & & & rowunit\
gdefrowunit{pc} & & & & & & & & & rowunit\
gdefrowunit{in} & & & & & & & & & rowunit\
end{tabular}end{center}

end{document}

convertto{1in}{pt}

documentclass[a4paper]{article}

usepackage{array}
usepackage[hmargin=2cm]{geometry}

usepackage{xparse}
ExplSyntaxOn
NewExpandableDocumentCommand{convertto}{smm}
 {
  egreg_convertto:nn {#2}{#3}
  IfBooleanT{#1}{#2}
 }

cs_new:Npn egreg_convertto:nn #1 #2
 {
  fp_eval:n { round( dim_to_decimal:n { #1 } / dim_to_decimal:n {1#2} , 5 ) }
 }
ExplSyntaxOff

begin{document}

begin{center}begin{tabular}
  {>{defcolunit{pt}}l<{convertto{1colunit}{rowunit}}
   >{defcolunit{mm}}l<{convertto{1colunit}{rowunit}}
   >{defcolunit{cm}}l<{convertto{1colunit}{rowunit}}
   >{defcolunit{ex}}l<{convertto{1colunit}{rowunit}}
   >{defcolunit{em}}l<{convertto{1colunit}{rowunit}}
   >{defcolunit{bp}}l<{convertto{1colunit}{rowunit}}
   >{defcolunit{dd}}l<{convertto{1colunit}{rowunit}}
   >{defcolunit{pc}}l<{convertto{1colunit}{rowunit}}
   >{defcolunit{in}}l<{convertto{1colunit}{rowunit}}
   >{bfseries}l}
multicolumn{1}{l}{bfseries 1pt} & multicolumn{1}{l}{bfseries 1mm} &
multicolumn{1}{l}{bfseries 1cm} & multicolumn{1}{l}{bfseries 1ex} &
multicolumn{1}{l}{bfseries 1em} & multicolumn{1}{l}{bfseries 1bp} &
multicolumn{1}{l}{bfseries 1dd} & multicolumn{1}{l}{bfseries 1pc} &
multicolumn{1}{l}{bfseries 1in} & \
gdefrowunit{pt} & & & & & & & & & rowunit\
gdefrowunit{mm} & & & & & & & & & rowunit\
gdefrowunit{cm} & & & & & & & & & rowunit\
gdefrowunit{ex} & & & & & & & & & rowunit\
gdefrowunit{em} & & & & & & & & & rowunit\
gdefrowunit{bp} & & & & & & & & & rowunit\
gdefrowunit{dd} & & & & & & & & & rowunit\
gdefrowunit{pc} & & & & & & & & & rowunit\
gdefrowunit{in} & & & & & & & & & rowunit\
end{tabular}end{center}

end{document}

documentclass[a4paper]{article}

usepackage{siunitx,array}
usepackage[hmargin=1cm]{geometry}
usepackage{xparse}

ExplSyntaxOn

seq_new:N g_egreg_convertto_units_seq
seq_new:N l_egreg_convertto_temp_seq
tl_new:N l_egreg_convertto_body_tl

seq_gset_from_clist:Nn g_egreg_convertto_units_seq
 {
  pt, mm, cm, ex, em,  bp, dd, pc, in
 }

cs_new_protected:Nn __egreg_convertto_maketable:
 {
  seq_map_inline:Nn g_egreg_convertto_units_seq
   {
    seq_clear:N l_egreg_convertto_temp_seq
    seq_map_inline:Nn g_egreg_convertto_units_seq
     {
      seq_put_right:Nx l_egreg_convertto_temp_seq { egreg_convertto:nn { 1####1 } { ##1 } }
     }
    tl_put_right:Nx l_egreg_convertto_body_tl
     {
      seq_use:Nn l_egreg_convertto_temp_seq { & } & exp_not:n { textbf{##1} \ }
     }
   }
  seq_set_map:NNn l_egreg_convertto_temp_seq g_egreg_convertto_units_seq { {exp_not:N textbf{1,##1}} }
 }

NewDocumentCommand{maketable}{}
 {
  __egreg_convertto_maketable:
  begin{tabular}{*{seq_count:N g_egreg_convertto_units_seq}{S[table-format=2.5]}l}
  seq_use:Nn l_egreg_convertto_temp_seq { & } \
  l_egreg_convertto_body_tl
  end{tabular}
 }

NewExpandableDocumentCommand{convertto}{smm}
 {
  egreg_convertto:nn {#2}{#3}
  IfBooleanT{#1}{#2}
 }

cs_new:Npn egreg_convertto:nn #1 #2
 {
  fp_eval:n { round( dim_to_decimal:n { #1 } / dim_to_decimal:n {1#2} , 5 ) }
 }
ExplSyntaxOff

begin{document}

begin{center}
sisetup{group-digits=false}
maketable
end{center}

begin{center}
begin{tabular}
  {>{defcolunit{pt}}l<{convertto{1colunit}{rowunit}}
   >{defcolunit{mm}}l<{convertto{1colunit}{rowunit}}
   >{defcolunit{cm}}l<{convertto{1colunit}{rowunit}}
   >{defcolunit{ex}}l<{convertto{1colunit}{rowunit}}
   >{defcolunit{em}}l<{convertto{1colunit}{rowunit}}
   >{defcolunit{bp}}l<{convertto{1colunit}{rowunit}}
   >{defcolunit{dd}}l<{convertto{1colunit}{rowunit}}
   >{defcolunit{pc}}l<{convertto{1colunit}{rowunit}}
   >{defcolunit{in}}l<{convertto{1colunit}{rowunit}}
   >{bfseries}l}
multicolumn{1}{l}{bfseries 1pt} & multicolumn{1}{l}{bfseries 1mm} &
multicolumn{1}{l}{bfseries 1cm} & multicolumn{1}{l}{bfseries 1ex} &
multicolumn{1}{l}{bfseries 1em} & multicolumn{1}{l}{bfseries 1bp} &
multicolumn{1}{l}{bfseries 1dd} & multicolumn{1}{l}{bfseries 1pc} &
multicolumn{1}{l}{bfseries 1in} & \
gdefrowunit{pt} & & & & & & & & & rowunit\
gdefrowunit{mm} & & & & & & & & & rowunit\
gdefrowunit{cm} & & & & & & & & & rowunit\
gdefrowunit{ex} & & & & & & & & & rowunit\
gdefrowunit{em} & & & & & & & & & rowunit\
gdefrowunit{bp} & & & & & & & & & rowunit\
gdefrowunit{dd} & & & & & & & & & rowunit\
gdefrowunit{pc} & & & & & & & & & rowunit\
gdefrowunit{in} & & & & & & & & & rowunit\
end{tabular}end{center}

end{document}

documentclass[11pt]{article} % use larger type; default would be 10pt
usepackage{xcolor}
begin{document}

newdimentemp

defalength#1#2{
temp#1
nointerlineskip baselineskip=2pt
vbox{hbox{hskip-29pttexttt{textcolor{#2}{#1=thetemp}}}
hbox{vtop{color{#2}hrule width 130ptvspace{#1}hrule width 130pt}}%
vspace{0.9cm}}
}

alength{1cm}{blue}
alength{1pc}{red}
alength{1mm}{orange}
alength{1cc}{red}
alength{1dd}{red}
alength{100000sp}{purple}
alength{1in}{black}
alength{1bp}{brown}
alength{1em}{black}
alength{1ex}{black}
end{document}

documentclass{article}
usepackage[T1]{fontenc}
usepackage{mathpazo}
newsaveboxCBox
newlengthLength Length=1em
begin{document}

sboxCBox{M}
thewdCBox : theLength

tinyLength=1em
sboxCBox{M}
thewdCBox : theLength

HugeLength=1em
sboxCBox{M}
thewdCBox : theLength

end{document}

1bp=numberdimexpr 1bprelax sp

10bp=numberdimexpr 10bprelax sp

100bp=numberdimexpr 100bprelax sp

1000bp=numberdimexpr 1000bprelax sp

10000bp=numberdimexpr 10000bprelax sp


1pt=numberdimexpr 1ptrelax sp

10pt=numberdimexpr 10ptrelax sp

100pt=numberdimexpr 100ptrelax sp

1000pt=numberdimexpr 1000ptrelax sp

10000pt=numberdimexpr 10000ptrelax sp

bye

1bp/1pt=xintIrr{numberdimexpr 100bprelax/numberdimexpr 100ptrelax }

72.27/72=xintIrr{72.27/72}

bye