diff --git a/actionpack/test/template/number_helper_test.rb b/actionpack/test/template/number_helper_test.rb
index d8fffe75edc33..854fabd21f8ad 100644
--- a/actionpack/test/template/number_helper_test.rb
+++ b/actionpack/test/template/number_helper_test.rb
@@ -179,13 +179,24 @@ def test_number_to_human_size_with_si_prefix
     assert_equal '3 Bytes',    number_to_human_size(3.14159265, :prefix => :si)
     assert_equal '123 Bytes',  number_to_human_size(123.0, :prefix => :si)
     assert_equal '123 Bytes',  number_to_human_size(123, :prefix => :si)
-    assert_equal '1.23 KB',    number_to_human_size(1234, :prefix => :si)
-    assert_equal '12.3 KB',    number_to_human_size(12345, :prefix => :si)
+    assert_equal '1.23 kB',    number_to_human_size(1234, prefix: :si)
+    assert_equal '12.3 kB',    number_to_human_size(12345, prefix: :si)
     assert_equal '1.23 MB',    number_to_human_size(1234567, :prefix => :si)
     assert_equal '1.23 GB',    number_to_human_size(1234567890, :prefix => :si)
     assert_equal '1.23 TB',    number_to_human_size(1234567890123, :prefix => :si)
   end
 
+  def test_number_to_human_size_with_iso_prefix
+    assert_equal '3 Bytes',    number_to_human_size(3.14159265, prefix: :iso)
+    assert_equal '123 Bytes',  number_to_human_size(123.0, prefix: :iso)
+    assert_equal '123 Bytes',  number_to_human_size(123, prefix: :iso)
+    assert_equal '1.21 KiB',   number_to_human_size(1234, prefix: :iso)
+    assert_equal '12.1 KiB',   number_to_human_size(12345, prefix: :iso)
+    assert_equal '1.18 MiB',   number_to_human_size(1234567, prefix: :iso)
+    assert_equal '1.15 GiB',   number_to_human_size(1234567890, prefix: :iso)
+    assert_equal '1.12 TiB',   number_to_human_size(1234567890123, prefix: :iso)
+  end
+
   def test_number_to_human_size_with_options_hash
     assert_equal '1.2 MB',   number_to_human_size(1234567, :precision => 2)
     assert_equal '3 Bytes',   number_to_human_size(3.14159265, :precision => 4)
diff --git a/activesupport/lib/active_support/locale/en.yml b/activesupport/lib/active_support/locale/en.yml
index f4900dc9359a1..979c55db07de4 100644
--- a/activesupport/lib/active_support/locale/en.yml
+++ b/activesupport/lib/active_support/locale/en.yml
@@ -106,6 +106,22 @@ en:
           mb: "MB"
           gb: "GB"
           tb: "TB"
+        si_units:
+          byte:
+            one:   "Byte"
+            other: "Bytes"
+          kb: "kB"
+          mb: "MB"
+          gb: "GB"
+          tb: "TB"
+        iso_units:
+          byte:
+            one:   "Byte"
+            other: "Bytes"
+          kb: "KiB"
+          mb: "MiB"
+          gb: "GiB"
+          tb: "TiB"
       # Used in NumberHelper.number_to_human()
       decimal_units:
         format: "%n %u"
diff --git a/activesupport/lib/active_support/number_helper.rb b/activesupport/lib/active_support/number_helper.rb
index 2191471daac23..afc4c76cc2af7 100644
--- a/activesupport/lib/active_support/number_helper.rb
+++ b/activesupport/lib/active_support/number_helper.rb
@@ -4,9 +4,19 @@
 require 'active_support/i18n'
 
 module ActiveSupport
+  class << self
+    delegate :default_size_prefix, :default_size_prefix=,
+    to: :'ActiveSupport::NumberHelper'
+  end
+
   module NumberHelper
     extend self
 
+    class << self
+      attr_accessor :default_size_prefix
+    end
+    self.default_size_prefix = :customary
+
     DEFAULTS = {
       # Used in number_to_delimited
       # These are also the defaults for 'currency', 'percentage', 'precision', and 'human'
@@ -74,6 +84,20 @@ module NumberHelper
             mb: "MB",
             gb: "GB",
             tb: "TB"
+          },
+          iso_units: {
+            byte: "Bytes",
+            kb: "KiB",
+            mb: "MiB",
+            gb: "GiB",
+            tb: "TiB"
+          },
+          si_units: {
+            byte: "Bytes",
+            kb: "kB",
+            mb: "MB",
+            gb: "GB",
+            tb: "TB"
           }
         },
         # Used in number_to_human
@@ -398,8 +422,12 @@ def number_to_rounded(number, options = {})
     # * <tt>:strip_insignificant_zeros</tt> - If +true+ removes
     #   insignificant zeros after the decimal separator (defaults to
     #   +true+)
-    # * <tt>:prefix</tt> - If +:si+ formats the number using the SI
-    #   prefix (defaults to :binary)
+    # * <tt>:prefix</tt> - If +:si+ formats the number using SI
+    #   prefixes. If +:iec+ or +:iso+ formats the numbers using
+    #   ISO/IEC 80000 binary prefixes. If +:customary+ formats the
+    #   number to customary binary prefixes. Defaults to
+    #   +ActiveSupport.default_size_prefix+, which is +:customary+,
+    #   but can be set to one of the others.
     #
     # ==== Examples
     #
@@ -432,10 +460,23 @@ def number_to_human_size(number, options = {})
 
       storage_units_format = translate_number_value_with_default('human.storage_units.format', :locale => options[:locale], :raise => true)
 
-      base = options[:prefix] == :si ? 1000 : 1024
+      options[:prefix] ||= ActiveSupport.default_size_prefix
+      case options[:prefix]
+      when :si
+        base = 1000
+        system_key = "si_units"
+      when :iec, :iso
+        base = 1024
+        system_key = "iso_units"
+      when :customary
+        base = 1024
+        system_key = "units"
+      else
+        raise ArgumentError, ":prefix must be :customary, :si, :iec, or :iso"
+      end
 
       if number.to_i < base
-        unit = translate_number_value_with_default('human.storage_units.units.byte', :locale => options[:locale], :count => number.to_i, :raise => true)
+        unit = translate_number_value_with_default("human.storage_units.#{system_key}.byte", locale: options[:locale], count: number.to_i, raise: true)
         storage_units_format.gsub(/%n/, number.to_i.to_s).gsub(/%u/, unit)
       else
         max_exp  = STORAGE_UNITS.size - 1
@@ -444,7 +485,7 @@ def number_to_human_size(number, options = {})
         number  /= base ** exponent
 
         unit_key = STORAGE_UNITS[exponent]
-        unit = translate_number_value_with_default("human.storage_units.units.#{unit_key}", :locale => options[:locale], :count => number, :raise => true)
+        unit = translate_number_value_with_default("human.storage_units.#{system_key}.#{unit_key}", locale: options[:locale], count: number, raise: true)
 
         formatted_number = self.number_to_rounded(number, options)
         storage_units_format.gsub(/%n/, formatted_number).gsub(/%u/, unit)
diff --git a/activesupport/test/core_ext/numeric_ext_test.rb b/activesupport/test/core_ext/numeric_ext_test.rb
index 435f4aa5a14b3..d5935a353dc44 100644
--- a/activesupport/test/core_ext/numeric_ext_test.rb
+++ b/activesupport/test/core_ext/numeric_ext_test.rb
@@ -344,13 +344,24 @@ def test_to_s__human_size_with_si_prefix
     assert_equal '3 Bytes',    3.14159265.to_s(:human_size, :prefix => :si)
     assert_equal '123 Bytes',  123.0.to_s(:human_size, :prefix => :si)
     assert_equal '123 Bytes',  123.to_s(:human_size, :prefix => :si)
-    assert_equal '1.23 KB',    1234.to_s(:human_size, :prefix => :si)
-    assert_equal '12.3 KB',    12345.to_s(:human_size, :prefix => :si)
+    assert_equal '1.23 kB',    1234.to_s(:human_size, prefix: :si)
+    assert_equal '12.3 kB',    12345.to_s(:human_size, prefix: :si)
     assert_equal '1.23 MB',    1234567.to_s(:human_size, :prefix => :si)
     assert_equal '1.23 GB',    1234567890.to_s(:human_size, :prefix => :si)
     assert_equal '1.23 TB',    1234567890123.to_s(:human_size, :prefix => :si)
   end
   
+  def test_to_s__human_size_with_iso_prefix
+    assert_equal '3 Bytes',    3.14159265.to_s(:human_size, prefix: :iso)
+    assert_equal '123 Bytes',  123.0.to_s(:human_size, prefix: :iso)
+    assert_equal '123 Bytes',  123.to_s(:human_size, prefix: :iso)
+    assert_equal '1.21 KiB',   1234.to_s(:human_size, prefix: :iso)
+    assert_equal '12.1 KiB',   12345.to_s(:human_size, prefix: :iso)
+    assert_equal '1.18 MiB',   1234567.to_s(:human_size, prefix: :iso)
+    assert_equal '1.15 GiB',   1234567890.to_s(:human_size, prefix: :iso)
+    assert_equal '1.12 TiB',   1234567890123.to_s(:human_size, prefix: :iso)
+  end
+  
   def test_to_s__human_size_with_options_hash
     assert_equal '1.2 MB',   1234567.to_s(:human_size, :precision => 2)
     assert_equal '3 Bytes',  3.14159265.to_s(:human_size, :precision => 4)
diff --git a/activesupport/test/number_helper_test.rb b/activesupport/test/number_helper_test.rb
index 5f54587f93ff6..f309dc1a81e9e 100644
--- a/activesupport/test/number_helper_test.rb
+++ b/activesupport/test/number_helper_test.rb
@@ -209,14 +209,51 @@ def test_number_to_human_size_with_si_prefix
           assert_equal '3 Bytes',    number_helper.number_to_human_size(3.14159265, :prefix => :si)
           assert_equal '123 Bytes',  number_helper.number_to_human_size(123.0, :prefix => :si)
           assert_equal '123 Bytes',  number_helper.number_to_human_size(123, :prefix => :si)
-          assert_equal '1.23 KB',    number_helper.number_to_human_size(1234, :prefix => :si)
-          assert_equal '12.3 KB',    number_helper.number_to_human_size(12345, :prefix => :si)
+          assert_equal '1.23 kB',    number_helper.number_to_human_size(1234, prefix: :si)
+          assert_equal '12.3 kB',    number_helper.number_to_human_size(12345, prefix: :si)
           assert_equal '1.23 MB',    number_helper.number_to_human_size(1234567, :prefix => :si)
           assert_equal '1.23 GB',    number_helper.number_to_human_size(1234567890, :prefix => :si)
           assert_equal '1.23 TB',    number_helper.number_to_human_size(1234567890123, :prefix => :si)
         end
       end
 
+      def test_number_to_human_size_with_iso_prefix
+        [:iec, :iso].each do |prefix|
+          [@instance_with_helpers, TestClassWithClassNumberHelpers, ActiveSupport::NumberHelper].each do |number_helper|
+            assert_equal '3 Bytes',    number_helper.number_to_human_size(3.14159265, prefix: prefix)
+            assert_equal '123 Bytes',  number_helper.number_to_human_size(123.0, prefix: prefix)
+            assert_equal '123 Bytes',  number_helper.number_to_human_size(123, prefix: prefix)
+            assert_equal '1.21 KiB',   number_helper.number_to_human_size(1234, prefix: prefix)
+            assert_equal '12.1 KiB',   number_helper.number_to_human_size(12345, prefix: prefix)
+            assert_equal '1.18 MiB',   number_helper.number_to_human_size(1234567, prefix: prefix)
+            assert_equal '1.15 GiB',   number_helper.number_to_human_size(1234567890, prefix: prefix)
+            assert_equal '1.12 TiB',   number_helper.number_to_human_size(1234567890123, prefix: prefix)
+          end
+        end
+      end
+
+      def test_number_to_human_size_using_active_support_default_size_prefix
+        begin
+          ActiveSupport.default_size_prefix = :iso
+          [@instance_with_helpers, TestClassWithClassNumberHelpers, ActiveSupport::NumberHelper].each do |number_helper|
+            assert_equal '1.12 TiB',   number_helper.number_to_human_size(1234567890123)
+          end
+          [@instance_with_helpers, TestClassWithClassNumberHelpers, ActiveSupport::NumberHelper].each do |number_helper|
+            assert_equal '1.12 TB',   number_helper.number_to_human_size(1234567890123, prefix: :customary)
+          end
+        ensure
+          ActiveSupport.default_size_prefix = :customary
+        end
+      end
+
+      def test_number_to_human_size_with_invalid_prefix
+        [@instance_with_helpers, TestClassWithClassNumberHelpers, ActiveSupport::NumberHelper].each do |number_helper|
+          assert_raise ArgumentError do
+            number_helper.number_to_human_size(1, prefix: :unknown_prefix)
+          end
+        end
+      end
+
       def test_number_to_human_size_with_options_hash
         [@instance_with_helpers, TestClassWithClassNumberHelpers, ActiveSupport::NumberHelper].each do |number_helper|
           assert_equal '1.2 MB',   number_helper.number_to_human_size(1234567, :precision => 2)