diff --git a/pre_commit/xargs.py b/pre_commit/xargs.py
index bd9205b7..a382759c 100644
--- a/pre_commit/xargs.py
+++ b/pre_commit/xargs.py
@@ -5,6 +5,7 @@ from __future__ import unicode_literals
 import concurrent.futures
 import contextlib
 import math
+import os
 import sys
 
 import six
@@ -13,10 +14,24 @@ from pre_commit import parse_shebang
 from pre_commit.util import cmd_output
 
 
-# TODO: properly compute max_length value
-def _get_platform_max_length():
-    # posix minimum
-    return 4 * 1024
+def _environ_size(_env=None):
+    environ = _env if _env is not None else getattr(os, 'environb', os.environ)
+    size = 8 * len(environ)  # number of pointers in `envp`
+    for k, v in environ.items():
+        size += len(k) + len(v) + 2  # c strings in `envp`
+    return size
+
+
+def _get_platform_max_length():  # pragma: no cover (platform specific)
+    if os.name == 'posix':
+        maximum = os.sysconf(str('SC_ARG_MAX')) - 2048 - _environ_size()
+        maximum = min(maximum, 2 ** 17)
+        return maximum
+    elif os.name == 'nt':
+        return 2 ** 15 - 2048  # UNICODE_STRING max - headroom
+    else:
+        # posix minimum
+        return 2 ** 12
 
 
 def _command_length(*cmd):
@@ -52,7 +67,7 @@ def partition(cmd, varargs, target_concurrency, _max_length=None):
     # Reversed so arguments are in order
     varargs = list(reversed(varargs))
 
-    total_length = _command_length(*cmd)
+    total_length = _command_length(*cmd) + 1
     while varargs:
         arg = varargs.pop()
 
@@ -69,7 +84,7 @@ def partition(cmd, varargs, target_concurrency, _max_length=None):
             # We've exceeded the length, yield a command
             ret.append(cmd + tuple(ret_cmd))
             ret_cmd = []
-            total_length = _command_length(*cmd)
+            total_length = _command_length(*cmd) + 1
             varargs.append(arg)
 
     ret.append(cmd + tuple(ret_cmd))
@@ -99,7 +114,7 @@ def xargs(cmd, varargs, **kwargs):
     stderr = b''
 
     try:
-        parse_shebang.normexe(cmd[0])
+        cmd = parse_shebang.normalize_cmd(cmd)
     except parse_shebang.ExecutableNotFoundError as e:
         return e.to_output()
 
diff --git a/tests/xargs_test.py b/tests/xargs_test.py
index 0e91f9be..a6cffd72 100644
--- a/tests/xargs_test.py
+++ b/tests/xargs_test.py
@@ -10,9 +10,24 @@ import mock
 import pytest
 import six
 
+from pre_commit import parse_shebang
 from pre_commit import xargs
 
 
+@pytest.mark.parametrize(
+    ('env', 'expected'),
+    (
+        ({}, 0),
+        ({b'x': b'1'}, 12),
+        ({b'x': b'12'}, 13),
+        ({b'x': b'1', b'y': b'2'}, 24),
+    ),
+)
+def test_environ_size(env, expected):
+    # normalize integer sizing
+    assert xargs._environ_size(_env=env) == expected
+
+
 @pytest.fixture
 def win32_py2_mock():
     with mock.patch.object(sys, 'getfilesystemencoding', return_value='utf-8'):
@@ -56,7 +71,7 @@ def test_partition_limits():
             '.' * 6,
         ),
         1,
-        _max_length=20,
+        _max_length=21,
     )
     assert ret == (
         ('ninechars', '.' * 5, '.' * 4),
@@ -70,21 +85,21 @@ def test_partition_limit_win32_py3(win32_py3_mock):
     cmd = ('ninechars',)
     # counted as half because of utf-16 encode
     varargs = ('😑' * 5,)
-    ret = xargs.partition(cmd, varargs, 1, _max_length=20)
+    ret = xargs.partition(cmd, varargs, 1, _max_length=21)
     assert ret == (cmd + varargs,)
 
 
 def test_partition_limit_win32_py2(win32_py2_mock):
     cmd = ('ninechars',)
     varargs = ('😑' * 5,)  # 4 bytes * 5
-    ret = xargs.partition(cmd, varargs, 1, _max_length=30)
+    ret = xargs.partition(cmd, varargs, 1, _max_length=31)
     assert ret == (cmd + varargs,)
 
 
 def test_partition_limit_linux(linux_mock):
     cmd = ('ninechars',)
     varargs = ('😑' * 5,)
-    ret = xargs.partition(cmd, varargs, 1, _max_length=30)
+    ret = xargs.partition(cmd, varargs, 1, _max_length=31)
     assert ret == (cmd + varargs,)
 
 
@@ -134,9 +149,9 @@ def test_xargs_smoke():
     assert err == b''
 
 
-exit_cmd = ('bash', '-c', 'exit $1', '--')
+exit_cmd = parse_shebang.normalize_cmd(('bash', '-c', 'exit $1', '--'))
 # Abuse max_length to control the exit code
-max_length = len(' '.join(exit_cmd)) + 2
+max_length = len(' '.join(exit_cmd)) + 3
 
 
 def test_xargs_negate():
@@ -165,14 +180,14 @@ def test_xargs_retcode_normal():
 
 
 def test_xargs_concurrency():
-    bash_cmd = ('bash', '-c')
+    bash_cmd = parse_shebang.normalize_cmd(('bash', '-c'))
     print_pid = ('sleep 0.5 && echo $$',)
 
     start = time.time()
     ret, stdout, _ = xargs.xargs(
         bash_cmd, print_pid * 5,
         target_concurrency=5,
-        _max_length=len(' '.join(bash_cmd + print_pid)),
+        _max_length=len(' '.join(bash_cmd + print_pid)) + 1,
     )
     elapsed = time.time() - start
     assert ret == 0