Using the siunitx package's table-column-width option as a workaround for long multicolumn heading problem

Question

I'm using the booktabs and siunitx packages to typeset scientific quantities in a table.

It is known that for long column headings -- created using the multicolumn command in the booktabs package -- the extra column width is assigned to the last column of the multicolumn. Table 1 below shows an example (vertical grids are shown to emphasize the column widths).

I noticed that the siunitx package has an option table-column-width to set fixed column widths. I decided to try to use this option to force each of my three columns labeled by multicolumn to have one-third the width of the multicolumn heading.

So in the MWE below, I used \newlength and \settowidth to store the length, in pt, of the multicolumn heading in myl. The use of \the\myl indicates that the multicolumn heading is 128.6415pt. 128.6415 divided by 3 is approximately 42.88, so in Table 2 below I set table-column-width to 42.88pt for each of the multicolumn columns.

Table 2 looks better than Table 1, but why is the total width of the multicolumn significantly larger than the apparent width of Long parameter, $\alpha$ (\si{\kilo\newton\per\kilo\gram})?

\documentclass{article}
\usepackage{amsmath,siunitx,booktabs}
\sisetup{%
  detect-family, detect-shape, detect-weight, detect-mode,
  product-units = power,
  list-final-separator = {, and },
  retain-explicit-plus,
  input-comparators = {<=>\approx\ge\geq\gg\le\leq\ll\sim\lesssim\gtrsim}
}
\begin{document}
% Table 1
\begin{table}[!h]
  \centering
  \begin{tabular}{l|
S[table-format=1.1,table-number-alignment=left]|
S[table-format=1.1,table-number-alignment=left]|
S[table-format=1.1,table-number-alignment=left]|}
    \toprule
    & \multicolumn{3}{c}{Long parameter, $\alpha$ (\si{\kilo\newton\per\kilo\gram})} \
    \cmidrule(lr){2-4}
    & {$\alpha_1$} & {$\alpha_2$} & {$\alpha_3$} \
    Trial A & 1.1 & 2.2 & 3.3 \
    \bottomrule
  \end{tabular}\caption{Problem (MWE)}
\end{table}
% Store the length of the multicolumn header in \myl
\newlength{\myl}
\settowidth{\myl}{Long parameter, $\alpha$ (\si{\kilo\newton\per\kilo\gram})}
\the\myl
% Table 2
\begin{table}[!h]
  \centering
  \begin{tabular}{l|
S[table-format=1.1,table-number-alignment=left,table-column-width=42.88pt]|
S[table-format=1.1,table-number-alignment=left,table-column-width=42.88pt]|
S[table-format=1.1,table-number-alignment=left,table-column-width=42.88pt]|}
    \toprule
    & \multicolumn{3}{c}{Long parameter, $\alpha$ (\si{\kilo\newton\per\kilo\gram})} \
    \cmidrule(lr){2-4}
    & {$\alpha_1$} & {$\alpha_2$} & {$\alpha_3$} \
    Trial A & 1.1 & 2.2 & 3.3 \
    \bottomrule
  \end{tabular}\caption{Attempt at solution}
\end{table}
\end{document}

Answer 1

Like this?

Last three columns should have equal, long enough width. You my consider X columns type (defined in tabularx and also in tabularray package and then sett table width accordingly (by trial). Wit latter option, the MWE is:

\documentclass{article}
\usepackage{amsmath}
\usepackage{tabularray}
\UseTblrLibrary{booktabs, siunitx, varwidth}
\begin{document}
    \begin{table}[ht]
    \centering
\sisetup{table-format=1.1, table-number-alignment=left}
\begin{tblr}{width=0.6\linewidth,
             vlines,
             colspec = {l *{3}{X[c, si]} },
              row{2} = {mode=math},
             measure = vbox
             }
    \toprule
    & \SetCell[c=3]{c}    {{{Long parameter, $\alpha$ (\si{\kilo\newton\per\kilo\gram})}}}
                            &                   &                   \
    \cmidrule{2-4}
        & {{{\alpha_1}}}    & {{{\alpha_2}}}    & {{{\alpha_3}}}    \
    \midrule
Trial A & 1.1               & 2.2               & 3.3               \
    \bottomrule
  \end{tblr}
\caption{Problem (MWE)}
    \end{table}
\end{document}

Edit: or by calculating of columns widths from width of multi column cell In this case you need to define new lenght, for example \colwidth:

\documentclass{article}
\usepackage{amsmath}
\usepackage{tabularray}
\UseTblrLibrary{booktabs, siunitx, varwidth}
\newlength{\colwidth}
\begin{document}
    \begin{table}[ht]
    \centering
\sisetup{table-format=1.1, table-number-alignment=left}
\settowidth\colwidth{Long parameter, $\alpha$ (\si{\kilo\newton\per\kilo\gram})}
\begin{tblr}{vlines,
             colspec = {l *{3}{Q[c, si, wd=\colwidth/3]} },
              row{2} = {mode=math},
             measure = vbox
             }
    \toprule
    & \SetCell[c=3]{c}    {{{Long parameter, $\alpha$ (\si{\kilo\newton\per\kilo\gram})}}}
                            &                   &                   \
    \cmidrule{2-4}
        & {{{\alpha_1}}}    & {{{\alpha_2}}}    & {{{\alpha_3}}}    \
    \midrule
Trial A & 1.1               & 2.2               & 3.3               \
    \bottomrule
  \end{tblr}
\caption{Problem (MWE)}
    \end{table}
\end{document}

BTW, formatting of cells with numbers is a bit unusual. People prefer to have centered numbers as well column header:

If you liked this, then remove option table-number-alignment=left from \sisetup.

Edt(2): and finally solution with standard table and defining S column width. Here observe of order of settings:

\documentclass{article}
\usepackage{amsmath}
\usepackage{tabularx}
\usepackage{makecell, siunitx}
\newlength{\colwidth}
\begin{document}
    \begin{table}[ht]
    \centering
\renewcommand\arraystretch{1.2}
\settowidth\colwidth{Long parameter, $\alpha$ (\si{\kilo\newton\per\kilo\gram})}
\sisetup{table-format=1.1,
         table-column-width=\colwidth/3}
\begin{tabular}{ | l  |*{3}{S|} }
    \Xhline{1pt}
    & \multicolumn{3}{c|}{Long parameter, $\alpha$ (\si{\kilo\newton\per\kilo\gram})}
                                                    \
    \Xcline{2-4}{0.5pt}
    & {$\alpha_1$}  & {$\alpha_2$}  & {$\alpha_3$}  \
    \Xhline{0.5pt}
Trial A & 1.1       & 2.2           & 3.3           \
    \Xhline{1pt}
  \end{tabular}
\caption{Problem (MWE)}
    \end{table}
\end{document}

Result is equal as before.