diff --git a/CHANGELOG.md b/CHANGELOG.md
index e0ee6d4..66d15ce 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -3,6 +3,7 @@
 ## v0.3.0a
 * Masks! The `png` and `svg` commands can be used as if they are a mask, so you can color the icon with any color you like. Can be handy for switching to black-and-white, or for reusing the same image but different colors across cards.
 * Gradients! Can now specify linear or radial gradients anywhere you specify colors. See README and `samples/gradients.rb` for more details.
+* Number padding! `save_png` will now pad zeros on the filenames for friendlier sorting. You can also specify your own with `count_format` using the classical format string from Ruby's `Kernel::sprintf` (mostly just C-style format strings). Default: `'%02d'. The `prefix:` option is still there too.
 * Special thanks to Shalom Craimer for the idea and proof-of-concept on the above two features!
 * Added new sample table for color constants in `samples/colors.rb`
 
diff --git a/lib/squib/api/save.rb b/lib/squib/api/save.rb
index 145637c..e6b64b9 100644
--- a/lib/squib/api/save.rb
+++ b/lib/squib/api/save.rb
@@ -25,14 +25,15 @@ module Squib
     # @option opts [Enumerable] range (:all) the range of cards over which this will be rendered. See {file:README.md#Specifying_Ranges Specifying Ranges}
     # @option opts [String] dir (_output) the directory for the output to be sent to. Will be created if it doesn't exist.
     # @option opts [String] prefix (card_) the prefix of the file name to be printed.
+    # @option opts [String] count_format (%02d) the format string used for formatting the card count (e.g. padding zeros). Uses a Ruby format string (see the Ruby doc for Kernel::sprintf for specifics)
     # @option opts [Boolean, :clockwise, :counterclockwise] rotate (false) if true, the saved cards will be rotated 90 degrees clockwise. Or, rotate by the number of radians. Intended to rendering landscape instead of portrait.
     # @return [nil] Returns nothing
     # @api public
     def save_png(opts = {})
-      opts = needs(opts,[:range, :creatable_dir, :prefix, :rotate])
+      opts = needs(opts,[:range, :creatable_dir, :prefix, :count_format, :rotate])
       @progress_bar.start("Saving PNGs to #{opts[:dir]}/#{opts[:prefix]}*", @cards.size) do |bar|
         opts[:range].each do |i|
-          @cards[i].save_png(i, opts[:dir], opts[:prefix], opts[:rotate], opts[:angle])
+          @cards[i].save_png(i, opts[:dir], opts[:prefix], opts[:count_format], opts[:rotate], opts[:angle])
           bar.increment
         end
       end
diff --git a/lib/squib/constants.rb b/lib/squib/constants.rb
index 72dcaab..1514fc4 100644
--- a/lib/squib/constants.rb
+++ b/lib/squib/constants.rb
@@ -9,6 +9,7 @@ module Squib
       :blend => :none,
       :color => :black,
       :columns => 1,
+      :count_format => '%02d',
       :default_font => 'Arial 36',
       :dir => '_output',
       :ellipsize => :end,
diff --git a/lib/squib/graphics/save_doc.rb b/lib/squib/graphics/save_doc.rb
index d3d4b39..ef0b00f 100644
--- a/lib/squib/graphics/save_doc.rb
+++ b/lib/squib/graphics/save_doc.rb
@@ -49,6 +49,7 @@ module Squib
     # @option opts colulmns [Integer] (1) the number of columns in the grid
     # @option opts rows [Integer] (:infinite) the number of rows in the grid. When set to :infinite, the sheet scales to the rows needed. If there are more cards than rows*columns, new sheets are started.
     # @option opts [String] prefix (card_) the prefix of the file name(s)
+    # @option opts [String] count_format (%02d) the format string used for formatting the card count (e.g. padding zeros). Uses a Ruby format string (see the Ruby doc for Kernel::sprintf for specifics)
     # @option opts dir [String] (_output) the directory to save to. Created if it doesn't exist.
     # @option opts margin [Integer] (0) the margin around the outside of the page
     # @option opts gap [Integer] (0) the space in pixels between the cards
@@ -57,7 +58,7 @@ module Squib
     # @api public
     def save_sheet(opts = {})
       opts = {margin: 0}.merge(opts) # overriding the non-system default
-      p = needs(opts, [:range, :prefix, :creatable_dir, :margin, :gap, :trim, :rows, :columns])
+      p = needs(opts, [:range, :prefix, :count_format, :creatable_dir, :margin, :gap, :trim, :rows, :columns])
       # EXTRACT METHOD HERE
       sheet_width = (p[:columns] * (@width + 2 * p[:gap] - 2 * p[:trim])) + (2 * p[:margin])
       sheet_height = (p[:rows] * (@height + 2 * p[:gap] - 2 * p[:trim])) + (2 * p[:margin])
@@ -68,7 +69,8 @@ module Squib
       @progress_bar.start("Saving PNG sheet to #{p[:dir]}/#{p[:prefix]}_*", @cards.size + 1) do |bar|
         p[:range].each do |i|
           if num_this_sheet >= (p[:columns] * p[:rows]) # new sheet
-            cc.target.write_to_png("#{p[:dir]}/#{p[:prefix]}#{sheet_num}.png")
+            filename = "#{p[:dir]}/#{p[:prefix]}#{p[:count_format] % sheet_num}.png"
+            cc.target.write_to_png(filename)
             new_sheet = false
             num_this_sheet = 0
             sheet_num += 1
@@ -87,7 +89,7 @@ module Squib
           end
           bar.increment
         end
-        cc.target.write_to_png("#{p[:dir]}/#{p[:prefix]}#{sheet_num}.png")
+        cc.target.write_to_png("#{p[:dir]}/#{p[:prefix]}#{p[:count_format] % sheet_num}.png")
       end
     end
 
diff --git a/lib/squib/graphics/save_images.rb b/lib/squib/graphics/save_images.rb
index 147eba6..b915045 100644
--- a/lib/squib/graphics/save_images.rb
+++ b/lib/squib/graphics/save_images.rb
@@ -3,13 +3,13 @@ module Squib
 
     # :nodoc:
     # @api private
-    def save_png(i, dir, prefix, do_rotate, angle)
+    def save_png(i, dir, prefix, count_format, do_rotate, angle)
       if [true, :clockwise, :counterclockwise].include?(do_rotate)
         surface = rotated_image(angle)
       else
         surface = @cairo_surface
       end
-      write_png(surface, i, dir, prefix)
+      write_png(surface, i, dir, prefix, count_format)
     end
 
     # :nodoc:
@@ -25,8 +25,8 @@ module Squib
     end
     # :nodoc:
     # @api private
-    def write_png(surface, i, dir, prefix)
-      surface.write_to_png("#{dir}/#{prefix}#{i}.png")
+    def write_png(surface, i, dir, prefix, count_format)
+      surface.write_to_png("#{dir}/#{prefix}#{count_format % i}.png")
     end
 
   end
diff --git a/spec/data/samples/autoscale_font.rb.txt b/spec/data/samples/autoscale_font.rb.txt
index 463ef79..f0ee6b0 100644
--- a/spec/data/samples/autoscale_font.rb.txt
+++ b/spec/data/samples/autoscale_font.rb.txt
@@ -76,6 +76,6 @@ cairo: set_source_color([:red])
 cairo: set_line_width([2.0])
 cairo: stroke([])
 cairo: restore([])
-surface: write_to_png(["_output/autoscale_0.png"])
-surface: write_to_png(["_output/autoscale_1.png"])
-surface: write_to_png(["_output/autoscale_2.png"])
+surface: write_to_png(["_output/autoscale_00.png"])
+surface: write_to_png(["_output/autoscale_01.png"])
+surface: write_to_png(["_output/autoscale_02.png"])
diff --git a/spec/data/samples/basic.rb.txt b/spec/data/samples/basic.rb.txt
index 0b08a81..9014703 100644
--- a/spec/data/samples/basic.rb.txt
+++ b/spec/data/samples/basic.rb.txt
@@ -201,6 +201,6 @@ cairo: translate([-620, -218])
 cairo: set_source([MockDouble, 620, 218])
 cairo: paint([1.0])
 cairo: restore([])
-surface: write_to_png(["_output/basic_0.png"])
-surface: write_to_png(["_output/basic_1.png"])
-surface: write_to_png(["_output/basic_2.png"])
+surface: write_to_png(["_output/basic_00.png"])
+surface: write_to_png(["_output/basic_01.png"])
+surface: write_to_png(["_output/basic_02.png"])
diff --git a/spec/data/samples/cairo_access.rb.txt b/spec/data/samples/cairo_access.rb.txt
index b626073..21cd171 100644
--- a/spec/data/samples/cairo_access.rb.txt
+++ b/spec/data/samples/cairo_access.rb.txt
@@ -21,5 +21,5 @@ cairo: circle([50, 50, 50])
 cairo: set_source_color([:red])
 cairo: fill([])
 cairo: restore([])
-surface: write_to_png(["_output/cairo_access_0.png"])
-surface: write_to_png(["_output/cairo_access_1.png"])
+surface: write_to_png(["_output/cairo_access_00.png"])
+surface: write_to_png(["_output/cairo_access_01.png"])
diff --git a/spec/data/samples/csv_import.rb.txt b/spec/data/samples/csv_import.rb.txt
index 8644d38..4d01c23 100644
--- a/spec/data/samples/csv_import.rb.txt
+++ b/spec/data/samples/csv_import.rb.txt
@@ -108,5 +108,5 @@ cairo: update_pango_layout([MockDouble])
 cairo: update_pango_layout([MockDouble])
 cairo: show_pango_layout([MockDouble])
 cairo: restore([])
-surface: write_to_png(["_output/sample_csv_0.png"])
-surface: write_to_png(["_output/sample_csv_1.png"])
+surface: write_to_png(["_output/sample_csv_00.png"])
+surface: write_to_png(["_output/sample_csv_01.png"])
diff --git a/spec/data/samples/custom_config.rb.txt b/spec/data/samples/custom_config.rb.txt
index 64a109f..43ef6ad 100644
--- a/spec/data/samples/custom_config.rb.txt
+++ b/spec/data/samples/custom_config.rb.txt
@@ -40,6 +40,6 @@ cairo: translate([-620, -218])
 cairo: set_source([MockDouble, 620, 218])
 cairo: paint([1.0])
 cairo: restore([])
-surface: write_to_png(["_output/custom-config_0.png"])
+surface: write_to_png(["_output/custom-config_00.png"])
 cairo: set_source([MockDouble, 75, 75])
 cairo: paint([])
diff --git a/spec/data/samples/draw_shapes.rb.txt b/spec/data/samples/draw_shapes.rb.txt
index 8a1c935..6110ffa 100644
--- a/spec/data/samples/draw_shapes.rb.txt
+++ b/spec/data/samples/draw_shapes.rb.txt
@@ -32,4 +32,4 @@ cairo: set_source_color([:black])
 cairo: set_line_width([25.0])
 cairo: stroke([])
 cairo: restore([])
-surface: write_to_png(["_output/shape_0.png"])
+surface: write_to_png(["_output/shape_00.png"])
diff --git a/spec/data/samples/excel.rb.txt b/spec/data/samples/excel.rb.txt
index 90185cb..014c29e 100644
--- a/spec/data/samples/excel.rb.txt
+++ b/spec/data/samples/excel.rb.txt
@@ -163,6 +163,6 @@ cairo: update_pango_layout([MockDouble])
 cairo: update_pango_layout([MockDouble])
 cairo: show_pango_layout([MockDouble])
 cairo: restore([])
-surface: write_to_png(["_output/sample_excel_0.png"])
-surface: write_to_png(["_output/sample_excel_1.png"])
-surface: write_to_png(["_output/sample_excel_2.png"])
+surface: write_to_png(["_output/sample_excel_00.png"])
+surface: write_to_png(["_output/sample_excel_01.png"])
+surface: write_to_png(["_output/sample_excel_02.png"])
diff --git a/spec/data/samples/gradients.rb.txt b/spec/data/samples/gradients.rb.txt
index 7a5c909..54e154a 100644
--- a/spec/data/samples/gradients.rb.txt
+++ b/spec/data/samples/gradients.rb.txt
@@ -62,4 +62,4 @@ cairo: update_pango_layout([MockDouble])
 cairo: update_pango_layout([MockDouble])
 cairo: show_pango_layout([MockDouble])
 cairo: restore([])
-surface: write_to_png(["_output/gradient_0.png"])
+surface: write_to_png(["_output/gradient_00.png"])
diff --git a/spec/data/samples/hello_world.rb.txt b/spec/data/samples/hello_world.rb.txt
index 9ed3528..d588f5e 100644
--- a/spec/data/samples/hello_world.rb.txt
+++ b/spec/data/samples/hello_world.rb.txt
@@ -32,5 +32,5 @@ cairo: update_pango_layout([MockDouble])
 cairo: update_pango_layout([MockDouble])
 cairo: show_pango_layout([MockDouble])
 cairo: restore([])
-surface: write_to_png(["_output/card_0.png"])
-surface: write_to_png(["_output/card_1.png"])
+surface: write_to_png(["_output/card_00.png"])
+surface: write_to_png(["_output/card_01.png"])
diff --git a/spec/data/samples/load_images.rb.txt b/spec/data/samples/load_images.rb.txt
index 6002c38..9aecf26 100644
--- a/spec/data/samples/load_images.rb.txt
+++ b/spec/data/samples/load_images.rb.txt
@@ -135,4 +135,4 @@ cairo: set_source([ImageSurface, 650, 950])
 cairo: set_source_color([:magenta])
 cairo: mask([ImageSurface, 650, 950])
 cairo: restore([])
-surface: write_to_png(["_output/load_images_0.png"])
+surface: write_to_png(["_output/load_images_00.png"])
diff --git a/spec/data/samples/portrait-landscape.rb.txt b/spec/data/samples/portrait-landscape.rb.txt
index 2f27837..8711305 100644
--- a/spec/data/samples/portrait-landscape.rb.txt
+++ b/spec/data/samples/portrait-landscape.rb.txt
@@ -19,7 +19,7 @@ cairo: update_pango_layout([MockDouble])
 cairo: update_pango_layout([MockDouble])
 cairo: show_pango_layout([MockDouble])
 cairo: restore([])
-surface: write_to_png(["_output/portrait_0.png"])
+surface: write_to_png(["_output/portrait_00.png"])
 cairo: save([])
 cairo: set_source_color(["#aaa"])
 cairo: paint([])
@@ -46,4 +46,4 @@ cairo: rotate([1.5707963267948966])
 cairo: translate([-562.5, -412.5])
 cairo: set_source([MockDouble])
 cairo: paint([])
-surface: write_to_png(["_output/landscape_0.png"])
+surface: write_to_png(["_output/landscape_00.png"])
diff --git a/spec/data/samples/ranges.rb.txt b/spec/data/samples/ranges.rb.txt
index 02f3e46..561f7f5 100644
--- a/spec/data/samples/ranges.rb.txt
+++ b/spec/data/samples/ranges.rb.txt
@@ -421,6 +421,6 @@ cairo: update_pango_layout([MockDouble])
 cairo: update_pango_layout([MockDouble])
 cairo: show_pango_layout([MockDouble])
 cairo: restore([])
-surface: write_to_png(["_output/ranges_0.png"])
-surface: write_to_png(["_output/ranges_1.png"])
-surface: write_to_png(["_output/ranges_2.png"])
+surface: write_to_png(["_output/ranges_00.png"])
+surface: write_to_png(["_output/ranges_01.png"])
+surface: write_to_png(["_output/ranges_02.png"])
diff --git a/spec/data/samples/saves.rb.txt b/spec/data/samples/saves.rb.txt
index 9ed1b68..6dfd00b 100644
--- a/spec/data/samples/saves.rb.txt
+++ b/spec/data/samples/saves.rb.txt
@@ -542,7 +542,7 @@ cairo: set_source([MockDouble, -37, -37])
 cairo: paint([])
 cairo: set_source([MockDouble, 1650, 75])
 cairo: paint([])
-surface: write_to_png(["_output/saves_notrim_1.png"])
+surface: write_to_png(["_output/saves_notrim_01.png"])
 cairo: set_source([MockDouble, -37, -37])
 cairo: paint([])
 cairo: set_source([MockDouble, 75, 75])
@@ -607,7 +607,7 @@ cairo: set_source([MockDouble, -37, -37])
 cairo: paint([])
 cairo: set_source([MockDouble, 180, 817])
 cairo: paint([])
-surface: write_to_png(["_output/save_single_sheet_0.png"])
+surface: write_to_png(["_output/save_single_sheet_00.png"])
 cairo: set_source([MockDouble, -37, -37])
 cairo: paint([])
 cairo: set_source([MockDouble, 75, 75])
@@ -640,7 +640,7 @@ cairo: set_source([MockDouble, -37, -37])
 cairo: paint([])
 cairo: set_source([MockDouble, 390, 181])
 cairo: paint([])
-surface: write_to_png(["_output/save_sheet_0.png"])
+surface: write_to_png(["_output/save_sheet_00.png"])
 cairo: set_source([MockDouble, -37, -37])
 cairo: paint([])
 cairo: set_source([MockDouble, 75, 75])
@@ -673,7 +673,7 @@ cairo: set_source([MockDouble, -37, -37])
 cairo: paint([])
 cairo: set_source([MockDouble, 390, 181])
 cairo: paint([])
-surface: write_to_png(["_output/save_sheet_1.png"])
+surface: write_to_png(["_output/save_sheet_01.png"])
 cairo: set_source([MockDouble, -37, -37])
 cairo: paint([])
 cairo: set_source([MockDouble, 75, 75])
@@ -690,7 +690,7 @@ cairo: set_source([MockDouble, -37, -37])
 cairo: paint([])
 cairo: set_source([MockDouble, 180, 181])
 cairo: paint([])
-surface: write_to_png(["_output/save_sheet_range_0.png"])
+surface: write_to_png(["_output/save_sheet_range_00.png"])
 cairo: set_source([MockDouble, -37, -37])
 cairo: paint([])
 cairo: set_source([MockDouble, 75, 75])
@@ -699,4 +699,4 @@ cairo: set_source([MockDouble, -37, -37])
 cairo: paint([])
 cairo: set_source([MockDouble, 180, 75])
 cairo: paint([])
-surface: write_to_png(["_output/save_sheet_range_1.png"])
+surface: write_to_png(["_output/save_sheet_range_01.png"])
diff --git a/spec/data/samples/showcase.rb.txt b/spec/data/samples/showcase.rb.txt
index b5929c7..64bdae3 100644
--- a/spec/data/samples/showcase.rb.txt
+++ b/spec/data/samples/showcase.rb.txt
@@ -5908,7 +5908,7 @@ cairo: restore([])
 cairo: set_source([MockDouble, 385.0, 100])
 cairo: paint([])
 surface: write_to_png(["_output/showcase2.png"])
-surface: write_to_png(["_output/showcase_individual_0.png"])
-surface: write_to_png(["_output/showcase_individual_1.png"])
-surface: write_to_png(["_output/showcase_individual_2.png"])
-surface: write_to_png(["_output/showcase_individual_3.png"])
+surface: write_to_png(["_output/showcase_individual_00.png"])
+surface: write_to_png(["_output/showcase_individual_01.png"])
+surface: write_to_png(["_output/showcase_individual_02.png"])
+surface: write_to_png(["_output/showcase_individual_03.png"])
diff --git a/spec/data/samples/text_options.rb.txt b/spec/data/samples/text_options.rb.txt
index 6af419e..0a84c45 100644
--- a/spec/data/samples/text_options.rb.txt
+++ b/spec/data/samples/text_options.rb.txt
@@ -844,6 +844,6 @@ cairo: set_source_color([:cyan])
 cairo: set_line_width([2.0])
 cairo: stroke([])
 cairo: restore([])
-surface: write_to_png(["_output/text_0.png"])
-surface: write_to_png(["_output/text_1.png"])
-surface: write_to_png(["_output/text_2.png"])
+surface: write_to_png(["_output/text_00.png"])
+surface: write_to_png(["_output/text_01.png"])
+surface: write_to_png(["_output/text_02.png"])
diff --git a/spec/data/samples/tgc_proofs.rb.txt b/spec/data/samples/tgc_proofs.rb.txt
index c62f652..6db9628 100644
--- a/spec/data/samples/tgc_proofs.rb.txt
+++ b/spec/data/samples/tgc_proofs.rb.txt
@@ -78,4 +78,4 @@ cairo: translate([0, 0])
 cairo: set_source([ImageSurface, 0, 0])
 cairo: paint([0.5])
 cairo: restore([])
-surface: write_to_png(["_output/tgc_sample_0.png"])
+surface: write_to_png(["_output/tgc_sample_00.png"])
diff --git a/spec/data/samples/units.rb.txt b/spec/data/samples/units.rb.txt
index 8d67438..1c166d7 100644
--- a/spec/data/samples/units.rb.txt
+++ b/spec/data/samples/units.rb.txt
@@ -29,4 +29,4 @@ cairo: rounded_rectangle([472.4409444, 472.4409444, 100, 100, 0, 0])
 cairo: set_source_color(["#0000"])
 cairo: fill([])
 cairo: restore([])
-surface: write_to_png(["_output/units_0.png"])
+surface: write_to_png(["_output/units_00.png"])