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+\newdimen\zdim \newdimen\tempdim \newdimen\tempdima
+
+% Conversion factors:
+%
+% According Scaled Rational Prime
+% Unit to TeX Points Form Factorization
+%-----------------------------------------------------------------
+% 1 sp 0.00002 pt 1 1/65536 pt 1 / 2^16
+% 1 mm 2.84526 pt 186467 7227/2540 pt 3*3*11*73 / 2*2*5*127
+% 1 cm 28.45274 pt 1864679 7227/254 pt 3*3*11*73 / 2*127
+% 1 pt 1.0 pt 65536 100/7227 in 2*2*5*5 / 3*3*11*73
+% 1 pc 12.0 pt 786432 12/1 pt
+% 1 dd 1.07 pt 70124 1238/1157 pt 2*619 / 13*89
+% 1 cc 12.8401 pt 841489 14856/1157 pt 2*2*2*3*619 / 13*89
+% 1 bp 1.00374 pt 65781 803/800 pt 11*73 / 2*2*2*2*2*5*5
+% [1/72 in]
+% 1 in 72.2699 pt 4736286 7227/100 pt 3*3*11*73 / 2*2*5*5
+% [2.54 cm] [254/100 cm]
+
+\def\points#1#2#3{\tempdim#2\relax
+ \edef#3{\csname cnvunits#1\expandafter\endcsname\the\tempdim}%
+}
+
+\def\inches#1#2#3{%
+ \tempdim=#2\relax
+ \tempdima=\ifdim\tempdim<\zdim -\fi\tempdim % absolute value
+ \roundup\tempdima{#1}{in}%
+% In the interest of maximum accuracy we push \tempdima as near
+% to \maxdimen as possible before dividing, using the prime
+% factorization of the fraction 7227/100 which is the
+% points/inches conversion factor.
+ \ifdim\tempdima<.01\maxdimen
+ \multiply\tempdima 100 \divide\tempdima 7227
+ \else
+ \ifdim\tempdima<.1\maxdimen
+ \multiply\tempdima 10 \divide\tempdima 11
+ \multiply\tempdima 10 \divide\tempdima 657
+ \else
+ \divide\tempdima 9 \multiply\tempdima 5
+ \divide\tempdima 803 \multiply\tempdima 20
+ \fi
+ \fi
+ \tempdim=\ifdim\tempdim<\zdim -\fi \tempdima
+ \edef#3{%
+ \csname cnvunits#1\expandafter\endcsname\the\tempdim}%
+}
+
+% Function \roundup for rounding upward. #1 must be a dimension
+% register. If it holds a negative value it will be rounded
+% `outward' away from zero rather than `upward' toward zero. #3
+% is a TeX units string such as "pt" or "in". If #2 = 0 then
+% this will round up to the nearest tenth; if #2 = 00, nearest
+% hundredth; and so forth (up to 5 zeros). If #2 is empty then
+% full accuracy up to TeX's limits will be used.
+%
+% The rounded result will be returned in the dimension register
+% #1.
+
+\def\roundup#1#2#3{%
+ \if .#2.\else
+ \begingroup
+ \ifdim#1>\zdim
+ \advance#1-\maxdimen \advance#1.#25#3\relax
+ \fi
+ \ifdim#1<\zdim
+ \endgroup
+ \advance#1.#25#3
+ \else
+ \endgroup
+ \fi
+ \fi
+}
+
+\begingroup
+\catcode`\P=12 \catcode`\T=12
+\lowercase{%
+\expandafter\gdef\csname cnvunits\endcsname#1PT{#1}
+\expandafter\gdef\csname cnvunits0\endcsname#1.#2PT{%
+ #1.\takeone#20\takeone}
+\expandafter\gdef\csname cnvunits00\endcsname#1.#2PT{%
+ #1.\taketwo#200\taketwo}
+\expandafter\gdef\csname cnvunits000\endcsname#1.#2PT{%
+ #1.\takethree#2000\takethree}
+\expandafter\gdef\csname cnvunits0000\endcsname#1.#2PT{%
+ #1.\takefour#20000\takefour}
+\expandafter\gdef\csname cnvunits00000\endcsname#1.#2PT{%
+ #1.\takefive#200000\takefive}
+}%
+\endgroup
+
+\def\takeone#1#2\takeone{#1}
+\def\taketwo#1#2#3\taketwo{#1#2}
+\def\takethree#1#2#3#4\takethree{#1#2#3}
+\def\takefour#1#2#3#4#5\takefour{#1#2#3#4}
+\def\takefive#1#2#3#4#5#6\takefive{#1#2#3#4#5}
+
+\def\showinches#1{\inches{00}{#1}\converted
+ \immediate\write16{%
+ #1 = (after conversion) \converted\space inches}}
+
+\showinches{0in} \showinches{1in} \showinches{2.0in}
+\showinches{2.2in} \showinches{8.5in} \showinches{1pc}
+\showinches{6pc} \showinches{1cm} \showinches{1mm}
+\showinches{1bp} \showinches{72bp} \showinches{1cc}
+\showinches{1dd} \showinches{72dd} \showinches{5000pt}
+\showinches{-5000pt} \showinches{\maxdimen}
+\showinches{-\maxdimen} \showinches{.999\maxdimen}
+\showinches{1pt} \showinches{.01pt}
+
+\endinput
+
+% From the TeX log:
+
+0in = (after conversion) 0.00 inches
+1in = (after conversion) 1.00 inches
+2.0in = (after conversion) 2.00 inches
+2.2in = (after conversion) 2.20 inches
+8.5in = (after conversion) 8.50 inches
+1pc = (after conversion) 0.17 inches
+6pc = (after conversion) 1.00 inches
+1cm = (after conversion) 0.39 inches
+1mm = (after conversion) 0.04 inches
+1bp = (after conversion) 0.01 inches
+72bp = (after conversion) 1.00 inches
+1cc = (after conversion) 0.18 inches
+1dd = (after conversion) 0.01 inches
+72dd = (after conversion) 1.07 inches
+5000pt = (after conversion) 69.18 inches
+-5000pt = (after conversion) -69.18 inches
+\maxdimen = (after conversion) 226.70 inches
+-\maxdimen = (after conversion) -226.70 inches
+.999\maxdimen = (after conversion) 226.48 inches
+1pt = (after conversion) 0.01 inches
+.01pt = (after conversion) 0.00 inches