diff --git a/scripts/notify_backup.rb b/scripts/notify_backup.rb
index 6f703dd..8e14e61 100644
--- a/scripts/notify_backup.rb
+++ b/scripts/notify_backup.rb
@@ -42,6 +42,17 @@ require 'json'
 require 'optparse'
 
 class ByteFormatter
+  UNITS = [['GB', 1_073_741_824], ['MB', 1_048_576], ['KB', 1_024]].freeze
+
+  def self.format(bytes)
+    return nil if bytes.nil?
+
+    bytes = bytes.to_i
+    unit, divisor = UNITS.find { |_, d| bytes >= d }
+    return "#{bytes} B" unless unit
+
+    '%.2f %s' % [bytes.to_f / divisor, unit]
+  end
 end
 
 class PayloadBuilder
diff --git a/scripts/notify_backup_test.rb b/scripts/notify_backup_test.rb
index 9bf08c2..1bf7a7b 100644
--- a/scripts/notify_backup_test.rb
+++ b/scripts/notify_backup_test.rb
@@ -3,3 +3,29 @@
 
 require 'minitest/autorun'
 require_relative 'notify_backup'
+
+class ByteFormatterTest < Minitest::Test
+  def test_gigabytes
+    assert_equal '4.50 GB', ByteFormatter.format(4_831_838_208)
+  end
+
+  def test_megabytes
+    assert_equal '823.00 MB', ByteFormatter.format(862_978_048)
+  end
+
+  def test_kilobytes
+    assert_equal '12.00 KB', ByteFormatter.format(12_288)
+  end
+
+  def test_bytes_below_one_kb
+    assert_equal '512 B', ByteFormatter.format(512)
+  end
+
+  def test_nil_returns_nil
+    assert_nil ByteFormatter.format(nil)
+  end
+
+  def test_string_integer_is_coerced
+    assert_equal '1.00 GB', ByteFormatter.format('1073741824')
+  end
+end