diff --git a/src/calibre/gui2/viewer/config.ui b/src/calibre/gui2/viewer/config.ui
index 6e37170154..fd43cd79ad 100644
--- a/src/calibre/gui2/viewer/config.ui
+++ b/src/calibre/gui2/viewer/config.ui
@@ -7,7 +7,7 @@
     <x>0</x>
     <y>0</y>
     <width>479</width>
-    <height>591</height>
+    <height>630</height>
    </rect>
   </property>
   <property name="windowTitle">
@@ -277,6 +277,27 @@
            </property>
           </widget>
          </item>
+         <item row="0" column="0">
+          <widget class="QLabel" name="label_12">
+           <property name="text">
+            <string>Font &amp;magnification step size:</string>
+           </property>
+           <property name="buddy">
+            <cstring>opt_font_mag_step</cstring>
+           </property>
+          </widget>
+         </item>
+         <item row="0" column="1">
+          <widget class="QSpinBox" name="opt_font_mag_step">
+           <property name="toolTip">
+            <string>The amount by which the font size is increased/decreased
+ when you click the font size larger/smaller buttons</string>
+           </property>
+           <property name="suffix">
+            <string>%</string>
+           </property>
+          </widget>
+         </item>
         </layout>
        </item>
       </layout>
diff --git a/src/calibre/gui2/viewer/documentview.py b/src/calibre/gui2/viewer/documentview.py
index 5903817149..4e8d59f61d 100644
--- a/src/calibre/gui2/viewer/documentview.py
+++ b/src/calibre/gui2/viewer/documentview.py
@@ -62,6 +62,10 @@ def config(defaults=None):
     c.add_opt('page_flip_duration', default=0.5,
             help=_('The time, in seconds, for the page flip animation. Default'
                 ' is half a second.'))
+    c.add_opt('font_magnification_step', default=0.2,
+            help=_('The amount by which to change the font size when clicking'
+                ' the font larger/smaller buttons. Should be a number between '
+                '0 and 1.'))
 
     fonts = c.add_group('FONTS', _('Font options'))
     fonts('serif_family', default='Times New Roman' if iswindows else 'Liberation Serif',
@@ -87,6 +91,10 @@ class ConfigDialog(QDialog, Ui_Dialog):
         self.opt_remember_current_page.setChecked(opts.remember_current_page)
         self.opt_wheel_flips_pages.setChecked(opts.wheel_flips_pages)
         self.opt_page_flip_duration.setValue(opts.page_flip_duration)
+        fms = opts.font_magnification_step
+        if fms < 0.01 or fms > 1:
+            fms = 0.2
+        self.opt_font_mag_step.setValue(int(fms*100))
         self.serif_family.setCurrentFont(QFont(opts.serif_family))
         self.sans_family.setCurrentFont(QFont(opts.sans_family))
         self.mono_family.setCurrentFont(QFont(opts.mono_family))
@@ -143,6 +151,8 @@ class ConfigDialog(QDialog, Ui_Dialog):
         c.set('remember_current_page', self.opt_remember_current_page.isChecked())
         c.set('wheel_flips_pages', self.opt_wheel_flips_pages.isChecked())
         c.set('page_flip_duration', self.opt_page_flip_duration.value())
+        c.set('font_magnification_step',
+                float(self.opt_font_mag_step.value())/100.)
         idx = self.hyphenate_default_lang.currentIndex()
         c.set('hyphenate_default_lang',
                 str(self.hyphenate_default_lang.itemData(idx).toString()))
@@ -223,6 +233,7 @@ class Document(QWebPage): # {{{
         self.do_fit_images = opts.fit_images
         self.page_flip_duration = opts.page_flip_duration
         self.enable_page_flip = self.page_flip_duration > 0.1
+        self.font_magnification_step = opts.font_magnification_step
         self.wheel_flips_pages = opts.wheel_flips_pages
 
     def fit_images(self):
@@ -932,13 +943,17 @@ class DocumentView(QWebView): # {{{
             self.magnification_changed.emit(val)
         return property(fget=fget, fset=fset)
 
-    def magnify_fonts(self):
-        self.multiplier += 0.2
+    def magnify_fonts(self, amount=None):
+        if amount is None:
+            amount = self.document.font_magnification_step
+        self.multiplier += amount
         return self.document.scroll_fraction
 
-    def shrink_fonts(self):
-        if self.multiplier >= 0.2:
-            self.multiplier -= 0.2
+    def shrink_fonts(self, amount=None):
+        if amount is None:
+            amount = self.document.font_magnification_step
+        if self.multiplier >= amount:
+            self.multiplier -= amount
         return self.document.scroll_fraction
 
     def changeEvent(self, event):