diff --git a/Makefile b/Makefile
index 2633bef..ea3b89d 100644
--- a/Makefile
+++ b/Makefile
@@ -15,6 +15,7 @@ CFLAGS += $(shell pkg-config --cflags cairo xcb-dpms xcb-xinerama xcb-atom xkbco
 LIBS += $(shell pkg-config --libs cairo xcb-dpms xcb-xinerama xcb-atom xcb-image xkbcommon xkbfile x11 x11-xcb)
 LIBS += -lpam
 LIBS += -lev
+LIBS += -lm
 
 FILES:=$(wildcard *.c)
 FILES:=$(FILES:.c=.o)
diff --git a/unlock_indicator.c b/unlock_indicator.c
index daaeeb1..ac7db09 100644
--- a/unlock_indicator.c
+++ b/unlock_indicator.c
@@ -1,19 +1,21 @@
 /*
  * vim:ts=4:sw=4:expandtab
  *
- * © 2010-2012 Michael Stapelberg
+ * © 2010-2014 Michael Stapelberg
  *
  * See LICENSE for licensing information
  *
  */
 #include <stdbool.h>
 #include <stdlib.h>
+#include <stdio.h>
 #include <math.h>
 #include <xcb/xcb.h>
 #include <ev.h>
 #include <cairo.h>
 #include <cairo/cairo-xcb.h>
 
+#include "i3lock.h"
 #include "xcb.h"
 #include "unlock_indicator.h"
 #include "xinerama.h"
@@ -27,6 +29,8 @@
  * Variables defined in i3lock.c.
  ******************************************************************************/
 
+extern bool debug_mode;
+
 /* The current position in the input buffer. Useful to determine if any
  * characters of the password have already been entered or not. */
 int input_position;
@@ -48,6 +52,13 @@ extern bool tile;
 /* The background color to use (in hex). */
 extern char color[7];
 
+/*******************************************************************************
+ * Variables defined in xcb.c.
+ ******************************************************************************/
+
+/* The root screen, to determine the DPI. */
+extern xcb_screen_t *screen;
+
 /*******************************************************************************
  * Local variables.
  ******************************************************************************/
@@ -60,6 +71,17 @@ static xcb_visualtype_t *vistype;
 unlock_state_t unlock_state;
 pam_state_t pam_state;
 
+/*
+ * Returns the scaling factor of the current screen. E.g., on a 227 DPI MacBook
+ * Pro 13" Retina screen, the scaling factor is 227/96 = 2.36.
+ *
+ */
+static double scaling_factor(void) {
+    const int dpi = (double)screen->height_in_pixels * 25.4 /
+                    (double)screen->height_in_millimeters;
+    return (dpi / 96.0);
+}
+
 /*
  * Draws global image with fill color onto a pixmap with the given
  * resolution and returns it.
@@ -67,6 +89,9 @@ pam_state_t pam_state;
  */
 xcb_pixmap_t draw_image(uint32_t *resolution) {
     xcb_pixmap_t bg_pixmap = XCB_NONE;
+    int button_diameter_physical = ceil(scaling_factor() * BUTTON_DIAMETER);
+    DEBUG("scaling_factor is %.f, physical diameter is %d px\n",
+          scaling_factor(), button_diameter_physical);
 
     if (!vistype)
         vistype = get_root_visual_type(screen);
@@ -74,7 +99,7 @@ xcb_pixmap_t draw_image(uint32_t *resolution) {
     /* Initialize cairo: Create one in-memory surface to render the unlock
      * indicator on, create one XCB surface to actually draw (one or more,
      * depending on the amount of screens) unlock indicators on. */
-    cairo_surface_t *output = cairo_image_surface_create(CAIRO_FORMAT_ARGB32, BUTTON_DIAMETER, BUTTON_DIAMETER);
+    cairo_surface_t *output = cairo_image_surface_create(CAIRO_FORMAT_ARGB32, button_diameter_physical, button_diameter_physical);
     cairo_t *ctx = cairo_create(output);
 
     cairo_surface_t *xcb_output = cairo_xcb_surface_create(conn, bg_pixmap, vistype, resolution[0], resolution[1]);
@@ -107,6 +132,7 @@ xcb_pixmap_t draw_image(uint32_t *resolution) {
     }
 
     if (unlock_state >= STATE_KEY_PRESSED && unlock_indicator) {
+        cairo_scale(ctx, scaling_factor(), scaling_factor());
         /* Draw a (centered) circle with transparent background. */
         cairo_set_line_width(ctx, 10.0);
         cairo_arc(ctx,
@@ -231,20 +257,20 @@ xcb_pixmap_t draw_image(uint32_t *resolution) {
     if (xr_screens > 0) {
         /* Composite the unlock indicator in the middle of each screen. */
         for (int screen = 0; screen < xr_screens; screen++) {
-            int x = (xr_resolutions[screen].x + ((xr_resolutions[screen].width / 2) - (BUTTON_DIAMETER / 2)));
-            int y = (xr_resolutions[screen].y + ((xr_resolutions[screen].height / 2) - (BUTTON_DIAMETER / 2)));
+            int x = (xr_resolutions[screen].x + ((xr_resolutions[screen].width / 2) - (button_diameter_physical / 2)));
+            int y = (xr_resolutions[screen].y + ((xr_resolutions[screen].height / 2) - (button_diameter_physical / 2)));
             cairo_set_source_surface(xcb_ctx, output, x, y);
-            cairo_rectangle(xcb_ctx, x, y, BUTTON_DIAMETER, BUTTON_DIAMETER);
+            cairo_rectangle(xcb_ctx, x, y, button_diameter_physical, button_diameter_physical);
             cairo_fill(xcb_ctx);
         }
     } else {
         /* We have no information about the screen sizes/positions, so we just
          * place the unlock indicator in the middle of the X root window and
          * hope for the best. */
-        int x = (last_resolution[0] / 2) - (BUTTON_DIAMETER / 2);
-        int y = (last_resolution[1] / 2) - (BUTTON_DIAMETER / 2);
+        int x = (last_resolution[0] / 2) - (button_diameter_physical / 2);
+        int y = (last_resolution[1] / 2) - (button_diameter_physical / 2);
         cairo_set_source_surface(xcb_ctx, output, x, y);
-        cairo_rectangle(xcb_ctx, x, y, BUTTON_DIAMETER, BUTTON_DIAMETER);
+        cairo_rectangle(xcb_ctx, x, y, button_diameter_physical, button_diameter_physical);
         cairo_fill(xcb_ctx);
     }