Dusko Pijetlovic

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X11 DPI Reference

22 Nov 2025 » x11, xorg, xterm, config, dotfiles, howto, font, freebsd, utf8, unicode, unix, cli, terminal, shell, tip, c, programming, tutorial

X11 DPI & Font Scaling: Essential Reference

My environment:

  • FreeBSD 14.3
  • Shell: csh
  • WM (window manager): FVWM3
  • No DE (Desktop Environment)

TL;DR

  • For my 14-inch laptop, set Xft.dpi: 120 in ~/.Xresources to match the external monitor scaling.
  • Physical DPI (from EDID): use xdpyinfo | grep dimensions -> compute with (pixels/mm) × 25.4.
  • Logical DPI (what apps use): check with xrdb -query | grep Xft.dpi.
  • xdpyinfo shows X server DPI (rarely used by modern apps).
  • Modern apps (GTK, Qt, Firefox, Thunderbird) need their own scaling settings.
  • Xft.dpi can be changed live (xrdb -merge). X server DPI cannot.
  • Scaling on X11 is per-toolkit; no universal scaling unless inside a Desktop Environment.
  • For Firefox/Thunderbird, use layout.css.devPixelsPerPx = 1.0 for 1× scaling.
  • xterm for bitmap fonts (XLFD) uses its own font sizes, independent of DPI.

1. Core Terms

Physical dimensions - The size of the display in millimeters, usually read from EDID.

Physical DPI - Pixels per inch computed from physical size; often inaccurate on laptops.

Logical DPI - The DPI value X uses for UI and font scaling; controlled by Xft.dpi or XSETTINGS.

Global Xft.dpi - A logical DPI value that affects all applications using Xft/Fontconfig (not XLFD).

X Server DPI (xdpyinfo DPI) - The DPI value determined by the X server at startup, based on EDID or defaulting to 96.

EDID - A small data block from the display that reports resolution, size, and capabilities.

2. What Each Command Shows

xdpyinfo - Shows what the X server believes: physical size, resolution, and the X server’s DPI.

xrandr - Shows physical size and resolution per output, as reported via EDID.

xrdb -query - Shows the current logical DPI (Xft.dpi) used by Xft/Fontconfig-based apps.

/var/log/Xorg.0.log - Shows EDID-reported millimeter size and the DPI computed by Xorg at startup.

xdpi (external tool) - Shows physical DPI per display using EDID values.

3. Commands

Show X server DPI

xdpyinfo | grep -i dots

Show Xft.dpi (logical DPI for modern font rendering)

xrdb -query | grep -i dpi

Show physical dimensions and resolution

xdpyinfo | grep dimensions
xrandr | grep -w connected

Show EDID-derived dimensions from Xorg

grep -i "dimension" /var/log/Xorg.0.log

4. DPI Formulas

Physical DPI:

(pixels / millimeters) × 25.4

Logical DPI - no formula; it’s whatever you set in:

  • ~/.Xresources (Xft.dpi)
  • xrdb -merge
  • A desktop environment’s XSETTINGS daemon

5. Examples

Example laptop:

Resolution: 1920 × 1200
Size:       301 × 188 mm

Physical DPI:

(1200 / 188) × 25.4 = 162.05
(1920 / 301) × 25.4 = 161.79
≈ 162 DPI

If Xft.dpi is set to 120 (in ~/.Xresources):

xrdb -query | grep Xft.dpi -> 120 
xdpyinfo -> still shows 162 (X server DPI)

This is normal because X server DPI does not change automatically when Xft.dpi changes.

6. Changing DPI

Change logical DPI (affects fonts/UI in modern apps)

printf "Xft.dpi: 120" | xrdb -merge

Effective immediately for most apps (you’ll probably need to restart them).

Change X server DPI (rare, requires restart)

startx -- -dpi 120

Or add to Xorg config.

Changing X server DPI live is generally not supported.

7. Q&A

Q: How do I check physical DPI?
A: xdpyinfo | grep dimensions or xrandr, then compute using the formula.

Q: How do I check logical DPI?
A: xrdb -query | grep Xft.dpi.

Q: Why do xdpyinfo DPI and Xft.dpi differ?
A: xdpyinfo shows X server DPI; Xft.dpi is a separate logical DPI for modern font systems.

Q: Is Xft.dpi global?
A: Yes - global for all Xft/Fontconfig-based applications; does not affect legacy XLFD apps.

Q: Can I change Xft.dpi live?
A: Yes, using xrdb -merge.

Q: Can I change the X server DPI live?
A: No, it requires restarting X.

Q: How can I verify whether EDID-reported millimeters are correct?
A: Compare xrandr or xdpyinfo millimeter values with the actual physical measurement of the screen.

8. Additional Answers for FreeBSD + FVWM3 Environment

What DPI should I set on my laptop?
To match a 94-DPI external monitor, set Xft.dpi: 120 so all Fontconfig/Xft applications scale similarly.

Can this be set globally?
Yes: put Xft.dpi: 120 in ~/.Xresources and load with xrdb -merge. Some toolkits (GTK, Qt, Firefox, Thunderbird) still need separate per-toolkit scaling.

Is today’s scaling situation per-application?
Yes. There is no universal scaling across X11; each toolkit may require overrides.

Font scaling vs. full scaling
Font scaling affects text size only. Full scaling enlarges entire UI elements (buttons, icons, padding, windows), typically via toolkit DPI multipliers.

Do I need fractional scaling?
No. Since you want ~120 DPI, set Xft.dpi: 120 and adjust toolkit settings; fractional scaling is mostly needed for DEs (Desktop Environments)

Do apps render at their own resolution?
Yes. Applications draw at native resolution unless toolkit scaling is applied.

Why is scaling difficult?
Because each toolkit (GTK, Qt, Xlib, Xft) uses its own environment variables and X settings.

Which tools give DPI?

  • Physical DPI: xdpyinfo, xrandr, Xorg logs, xdpi.
  • Logical DPI: xrdb -query (Xft.dpi).
  • Real DPI: same as physical DPI.
    xrandr shows physical size but does not compute DPI.

DPI vs PPI?
DPI originated from printing; PPI is correct for screens. In practice, DPI ~= PPI.

Can Xft.dpi be changed live?
Yes: printf "Xft.dpi: 120" | xrdb -merge.
Toolkit-specific DPI (Qt, GTK) can also be changed live via environment variables.

What do xdpi and qtdpi report?
xdpi reports EDID-based physical DPI. qtdpi reports logical DPI per screen (Qt’s internal interpretation).

Toolkits you need to configure:

  • GTK2: ~/.gtkrc-2.0 (font sizes).
  • GTK3: ~/.config/gtk-3.0/settings.ini (scaling + font sizes).
  • Qt5/Qt6: env vars (QT_SCALE_FACTOR, QT_FONT_DPI, etc.).
  • Firefox/Thunderbird: set layout.css.devPixelsPerPx = 1.0 for 1× scaling.

xterm scaling (for bitmap fonts)
Bitmap fonts: use XLFD names in ~/.Xresources.

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