diff --git a/spec/std/sprintf_spec.cr b/spec/std/sprintf_spec.cr
index 674a32c2ab30..520cb642bb50 100644
--- a/spec/std/sprintf_spec.cr
+++ b/spec/std/sprintf_spec.cr
@@ -372,6 +372,18 @@ describe "::sprintf" do
       assert_sprintf "%300.250d", 10.to_big_i ** 200, "#{" " * 50}#{"0" * 49}1#{"0" * 200}"
       assert_sprintf "%- #300.250X", 16.to_big_i ** 200 - 1, " 0X#{"0" * 50}#{"F" * 200}#{" " * 47}"
     end
+
+    it "works with BigFloat" do
+      assert_sprintf "%d", 123.to_big_f, "123"
+      assert_sprintf "%80.70d", 2.to_big_i ** 200, "          0000000001606938044258990275541962092341162602522202993782792835301376"
+      assert_sprintf "%- #70.60X", 2.to_big_f ** 200 - 2.to_big_f ** 120, " 0X0000000000FFFFFFFFFFFFFFFFFFFF000000000000000000000000000000       "
+    end
+
+    it "works with BigDecimal" do
+      assert_sprintf "%d", 123.to_big_d, "123"
+      assert_sprintf "%300.250d", 10.to_big_d ** 200, "#{" " * 50}#{"0" * 49}1#{"0" * 200}"
+      assert_sprintf "%- #300.250X", 16.to_big_d ** 200 - 1, " 0X#{"0" * 50}#{"F" * 200}#{" " * 47}"
+    end
   end
 
   it "doesn't stop at null character when doing '%'" do
diff --git a/src/big/big_decimal.cr b/src/big/big_decimal.cr
index a2c95fac2fff..bc33c9eaf617 100644
--- a/src/big/big_decimal.cr
+++ b/src/big/big_decimal.cr
@@ -894,3 +894,10 @@ struct Crystal::Hasher
     v &* value.sign
   end
 end
+
+# :nodoc:
+struct String::Formatter(A)
+  def int(flags, arg : BigDecimal) : Nil
+    int(flags, arg.to_big_i)
+  end
+end
diff --git a/src/big/big_float.cr b/src/big/big_float.cr
index 5a57500fbdd7..4f0feeb7e50d 100644
--- a/src/big/big_float.cr
+++ b/src/big/big_float.cr
@@ -671,3 +671,10 @@ module Math
     BigFloat.new { |mpf| LibGMP.mpf_sqrt(mpf, value) }
   end
 end
+
+# :nodoc:
+struct String::Formatter(A)
+  def int(flags, arg : BigFloat) : Nil
+    int(flags, arg.to_big_i)
+  end
+end
diff --git a/src/string/formatter.cr b/src/string/formatter.cr
index 347d65bcb340..48d99925f0bd 100644
--- a/src/string/formatter.cr
+++ b/src/string/formatter.cr
@@ -269,33 +269,35 @@ struct String::Formatter(A)
     pad arg.to_s.size, flags if flags.right_padding?
   end
 
-  def int(flags, arg) : Nil
-    raise ArgumentError.new("Expected an integer, not #{arg.inspect}") unless arg.responds_to?(:to_i)
-    int = arg.is_a?(Int) ? arg : arg.to_i
-
-    precision = int_precision(int, flags)
-    base_str = int.to_s(flags.base, precision: precision, upcase: flags.uppercase?)
+  def int(flags, arg : Int) : Nil
+    precision = int_precision(arg, flags)
+    base_str = arg.to_s(flags.base, precision: precision, upcase: flags.uppercase?)
     str_size = base_str.bytesize
-    str_size += 1 if int >= 0 && (flags.plus || flags.space)
-    str_size += 2 if flags.sharp && flags.base != 10 && int != 0
+    str_size += 1 if arg >= 0 && (flags.plus || flags.space)
+    str_size += 2 if flags.sharp && flags.base != 10 && arg != 0
 
-    # If `int` is zero-padded, we let the precision argument do the right-justification
+    # If `arg` is zero-padded, we let the precision argument do the right-justification
     pad(str_size, flags) if flags.left_padding? && flags.padding_char != '0'
 
-    write_plus_or_space(int, flags)
+    write_plus_or_space(arg, flags)
 
-    if flags.sharp && int < 0
+    if flags.sharp && arg < 0
       @io << '-'
       write_base_prefix(flags)
       @io.write_string base_str.unsafe_byte_slice(1)
     else
-      write_base_prefix(flags) if flags.sharp && int != 0
+      write_base_prefix(flags) if flags.sharp && arg != 0
       @io << base_str
     end
 
     pad(str_size, flags) if flags.right_padding?
   end
 
+  def int(flags, arg) : Nil
+    raise ArgumentError.new("Expected an integer, not #{arg.inspect}") unless arg.responds_to?(:to_i)
+    int(flags, arg.to_i)
+  end
+
   private def write_plus_or_space(arg, flags)
     if arg >= 0
       if flags.plus