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A quick question that I can't find answer on the web (or Wickham's book):

What is the unit of the size argument in ggplot2? For example, geom_text(size = 10) -- 10 in what units?

The same question applies to default unit in ggsave(height = 10, width = 10).

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  • Until now, I believed the text size was in points (as in font sizes). Is this still true, albeit a simplified explanation? In pdf(), postscript() etc., the height and width are given in inches, so would it be wrong to assume that ggsave also uses inches? Commented Jun 26, 2013 at 7:31
  • Unfortunately, there is not a “unit of the size argument in ggplot2” because there is more than one size argument. The size argument to geom_text() does something very different from the size argument to element_text(), and I have no idea how to reconcile the two or why they would be different. Commented May 12, 2021 at 3:25

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Edit 3/13/2025: The text size in ggplot for geom_text is specified in mm, not points. To convert the unit to points, you can use the shortcut of font size/.pt, for example 10 pt font would be size = 10/.pt. Note that in theme arguments (using element_text) the size is default in points.

The answer is : The unit is the points. It is the unit of fontsize in the grid package. In ?unit, we find the following definition

"points" Points. There are 72.27 points per inch.

(but note the closely related "bigpts" Big Points. 72 bp = 1 in.)

Internally ggplot2 will multiply the font size by a magic number ggplot2:::.pt, defined as 1/0.352777778.

Here a demonstration, I create a letter using grid and ggplot2 with same size:

library(grid)
library(ggplot2)
ggplot(data=data.frame(x=1,y=1,label=c('A'))) +
  geom_text(aes(x,y,label=label),size=100)
## I divide by the magic number to get the same size.
grid.text('A',gp=gpar(fontsize=100/0.352777778,col='red'))

enter image description here

Addendum Thanks to @baptiste

The "magic number"(defined in aaa-constants.r as .pt <- 1 / 0.352777778) is really just the conversion factor between "points" and "mm", that is 1/72 * 25.4 = 0.352777778. Unfortunately, grid makes the subtle distinction between "pts" and "bigpts", which explains why convertUnit(unit(1, "pt"), "mm", valueOnly=TRUE) gives the slightly different value of 0.3514598.

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16 Comments

@thelatemail thanks. hackish do you talk about the division of the magic number? it looks like a dirty implementation but I don't think so it is a compromise.Note that :1-) There isn't a lot of constants in ggplot2, 2-)when you deal with text you are obliged to do something like this , (see also strheight() and strWidth() in basic plot)... But I think we will get better explanations from ggplot2 proficients...
The magic number used throughout ggplot2 is fontsize = size * ggplot2:::.pt, which is defined in aaa-constants.r as .pt <- 1 / 0.352777778
and for the sake of clarity, one may note that 1/72 * 25.4 = 0.352777778, that is point -> inch -> mm. Grid converts points to mm slightly differently, presumably because the definition of points is sometimes argued to be device-specific.
A rewrite of this answer would be appreciated. The unit of size in ggplot2 is mm, not pt, and I've just redefined .pt as 72 / 25.4 to hopefully make that more clear in the code
And I've now changed it to 72.27 / 25.4 since as pointed out in the comments, grid uses printers' pointers, not Adobe/big points.
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The 'ggplot2' package, like 'lattice' before it, is built on the grid package. You can get the available units at:

?grid::unit
?grid::convertX
?grid::convertY

grid::convertX(grid::unit(72.27, "points"), "inches")

(I use the formalism pkg::func because in most cases grid is loaded a a NAMESPACE but not attached when either lattice or `ggplot2 are loaded.)

I earlier posted a comment that I later deleted saying that size was in points. I did so after seeing that the size of the text with size=10 was roughly 10 mm. The "magic" number mentioned by agstudy is in fact within 1% of:

as.numeric(grid::convertX(grid::unit(1, "points"), "mm"))
#[1] 0.3514598
0.352777778/.Last.value
#[1] 1.00375
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4 Comments

I had thought units of size were in mm groups.google.com/group/ggplot2-dev/tree/browse_frm/thread/… I wonder why the magic number is not 0.3514598.
Perhaps it's that size ratio on hadley's device. I'm working on a six year-old Mac.
I'd repeat my question for the poster above: so given this information, do you recommend any good practice regarding specifying size in points? I often find myself wasting time with trial and error.
I guess if you want to specify points multiply the points you want by the magic number.
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From ?aes_linetype_size_shape 

# Size examples
# Should be specified with a numerical value (in millimetres),
# or from a variable source

height and width in ggsave relate to par("din") from ?par

din

  R.O.; the device dimensions, (width, height), in inches. See also dev.size,
  which is updated immediately when an on-screen device windows is re-sized.

So I guess size in aes is in millimetres and ggsave height and width are in inches.

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1 Comment

This should be the accepted answer. Also make note that this is true of geom_text BUT not true of element_text, which is by default in pt units.

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