prawn：Prawn::Table::Cell#text_area_width, #width

#!/usr/bin/env ruby

require "prawn"
require "prawn/layout"

pdf = Prawn::Document.new

y = 700
[50, 100].each do |w|
  c = Prawn::Table::Cell.new(
        :document   => pdf,
        :point      => [100, y],
        :text       => "sample text",
        :padding    => 2,
        :width      => w)
  c.draw
  p [c.text_area_width, c.width]
  y -= 50
end

pdf.render_file(File.basename($0, ".rb") + ".pdf")

で、

[46, 50]
[96, 100]