diff --git a/src/Command/RunCommand.php b/src/Command/RunCommand.php
index 5ec9103d1..dc3311e43 100644
--- a/src/Command/RunCommand.php
+++ b/src/Command/RunCommand.php
@@ -57,9 +57,12 @@
 use Infection\Logger\GitHub\NoFilesInDiffToMutate;
 use Infection\Metrics\MinMsiCheckFailed;
 use Infection\Process\Runner\InitialTestsFailed;
+use Infection\Resource\Processor\CpuCoresCountProvider;
 use Infection\TestFramework\Coverage\XmlReport\NoLineExecutedInDiffLinesMode;
 use Infection\TestFramework\TestFrameworkTypes;
 use InvalidArgumentException;
+use function is_numeric;
+use function max;
 use const PHP_SAPI;
 use Psr\Log\LoggerInterface;
 use function sprintf;
@@ -67,6 +70,7 @@
 use Symfony\Component\Console\Input\InputInterface;
 use Symfony\Component\Console\Input\InputOption;
 use function trim;
+use Webmozart\Assert\Assert;
 
 /**
  * @internal
@@ -177,7 +181,7 @@ protected function configure(): void
                 self::OPTION_THREADS,
                 'j',
                 InputOption::VALUE_REQUIRED,
-                'Number of threads to use by the runner when executing the mutations',
+                'Number of threads to use by the runner when executing the mutations. Use "max" to auto calculate it.',
                 Container::DEFAULT_THREAD_COUNT
             )
             ->addOption(
@@ -478,8 +482,7 @@ private function createContainer(IO $io, LoggerInterface $logger): Container
                 ? Container::DEFAULT_TEST_FRAMEWORK_EXTRA_OPTIONS
                 : $testFrameworkExtraOptions,
             $filter,
-            // TODO: more validation here?
-            (int) $input->getOption(self::OPTION_THREADS),
+            $this->getThreadCount($input),
             // To keep in sync with Container::DEFAULT_DRY_RUN
             (bool) $input->getOption(self::OPTION_DRY_RUN),
             $gitDiffFilter,
@@ -643,4 +646,18 @@ private function getUseGitHubLogger(InputInterface $input): ?bool
             self::OPTION_LOGGER_GITHUB
         ));
     }
+
+    private function getThreadCount(InputInterface $input): int
+    {
+        $threads = $input->getOption(self::OPTION_THREADS);
+
+        if (is_numeric($threads)) {
+            return (int) $threads;
+        }
+
+        Assert::same($threads, 'max', sprintf('The value of option `--threads` must be of type integer or string "max". String "%s" provided.', $threads));
+
+        // we subtract 1 here to not use all the available cores by Infection
+        return max(1, CpuCoresCountProvider::provide() - 1);
+    }
 }
diff --git a/src/Resource/Processor/CpuCoresCountProvider.php b/src/Resource/Processor/CpuCoresCountProvider.php
new file mode 100644
index 000000000..9d8599c53
--- /dev/null
+++ b/src/Resource/Processor/CpuCoresCountProvider.php
@@ -0,0 +1,99 @@
+<?php
+/**
+ * This code is licensed under the BSD 3-Clause License.
+ *
+ * Copyright (c) 2017, Maks Rafalko
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * * Redistributions of source code must retain the above copyright notice, this
+ *   list of conditions and the following disclaimer.
+ *
+ * * Redistributions in binary form must reproduce the above copyright notice,
+ *   this list of conditions and the following disclaimer in the documentation
+ *   and/or other materials provided with the distribution.
+ *
+ * * Neither the name of the copyright holder nor the names of its
+ *   contributors may be used to endorse or promote products derived from
+ *   this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+ * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+ * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+declare(strict_types=1);
+
+namespace Infection\Resource\Processor;
+
+use function defined;
+use function extension_loaded;
+use const FILTER_VALIDATE_INT;
+use function filter_var;
+use function function_exists;
+use function is_int;
+use function is_readable;
+use function Safe\file_get_contents;
+use function Safe\shell_exec;
+use function substr_count;
+use function trim;
+
+/**
+ * @internal
+ */
+final class CpuCoresCountProvider
+{
+    /**
+     * Copied and adapter from Psalm project: https://github.com/vimeo/psalm/blob/4.x/src/Psalm/Internal/Analyzer/ProjectAnalyzer.php#L1454
+     */
+    public static function provide(): int
+    {
+        if (defined('PHP_WINDOWS_VERSION_MAJOR')) {
+            return 1;
+        }
+
+        if (!extension_loaded('pcntl') || !function_exists('shell_exec')) {
+            return 1;
+        }
+
+        // for Linux
+        $hasNproc = trim(@shell_exec('command -v nproc'));
+
+        if ($hasNproc !== '') {
+            $nproc = trim(shell_exec('nproc'));
+            $cpuCount = filter_var($nproc, FILTER_VALIDATE_INT);
+
+            if (is_int($cpuCount)) {
+                return $cpuCount;
+            }
+        }
+
+        // for MacOS
+        $ncpu = trim(shell_exec('sysctl -n hw.ncpu'));
+        $cpuCount = filter_var($ncpu, FILTER_VALIDATE_INT);
+
+        if (is_int($cpuCount)) {
+            return $cpuCount;
+        }
+
+        if (is_readable('/proc/cpuinfo')) {
+            $cpuInfo = file_get_contents('/proc/cpuinfo');
+            $cpuCount = substr_count($cpuInfo, 'processor');
+
+            if ($cpuCount > 0) {
+                return $cpuCount;
+            }
+        }
+
+        return 1;
+    }
+}
diff --git a/tests/phpunit/AutoReview/ProjectCode/ProjectCodeProvider.php b/tests/phpunit/AutoReview/ProjectCode/ProjectCodeProvider.php
index 62b91da45..3f0fc006e 100644
--- a/tests/phpunit/AutoReview/ProjectCode/ProjectCodeProvider.php
+++ b/tests/phpunit/AutoReview/ProjectCode/ProjectCodeProvider.php
@@ -41,7 +41,6 @@
 use function in_array;
 use Infection\CannotBeInstantiated;
 use Infection\Command\ConfigureCommand;
-use Infection\Command\RunCommand;
 use Infection\Config\ConsoleHelper;
 use Infection\Config\Guesser\SourceDirGuesser;
 use Infection\Configuration\Schema\SchemaConfigurationFactory;
@@ -66,6 +65,7 @@
 use Infection\Mutator\NodeMutationGenerator;
 use Infection\Process\Runner\IndexedProcessBearer;
 use Infection\Process\ShellCommandLineExecutor;
+use Infection\Resource\Processor\CpuCoresCountProvider;
 use Infection\TestFramework\AdapterInstaller;
 use Infection\TestFramework\Coverage\JUnit\TestFileTimeData;
 use Infection\TestFramework\Coverage\NodeLineRangeData;
@@ -96,7 +96,6 @@ final class ProjectCodeProvider
      */
     public const NON_TESTED_CONCRETE_CLASSES = [
         ConfigureCommand::class,
-        RunCommand::class,
         Application::class,
         ProgressFormatter::class,
         ComposerExecutableFinder::class,
@@ -113,6 +112,7 @@ final class ProjectCodeProvider
         NullSubscriber::class,
         FormatterName::class,
         ShellCommandLineExecutor::class,
+        CpuCoresCountProvider::class,
     ];
 
     /**
diff --git a/tests/phpunit/Command/RunCommandTest.php b/tests/phpunit/Command/RunCommandTest.php
new file mode 100644
index 000000000..c05cbc2e9
--- /dev/null
+++ b/tests/phpunit/Command/RunCommandTest.php
@@ -0,0 +1,61 @@
+<?php
+/**
+ * This code is licensed under the BSD 3-Clause License.
+ *
+ * Copyright (c) 2017, Maks Rafalko
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * * Redistributions of source code must retain the above copyright notice, this
+ *   list of conditions and the following disclaimer.
+ *
+ * * Redistributions in binary form must reproduce the above copyright notice,
+ *   this list of conditions and the following disclaimer in the documentation
+ *   and/or other materials provided with the distribution.
+ *
+ * * Neither the name of the copyright holder nor the names of its
+ *   contributors may be used to endorse or promote products derived from
+ *   this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+ * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+ * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+declare(strict_types=1);
+
+namespace Infection\Tests\Command;
+
+use Infection\Console\Application;
+use Infection\Tests\SingletonContainer;
+use InvalidArgumentException;
+use PHPUnit\Framework\TestCase;
+use Symfony\Component\Console\Tester\CommandTester;
+
+/**
+ * @group integration
+ */
+final class RunCommandTest extends TestCase
+{
+    public function test_it_fails_when_threads_value_is_string_but_not_max(): void
+    {
+        $this->expectException(InvalidArgumentException::class);
+        $this->expectExceptionMessage('The value of option `--threads` must be of type integer or string "max". String "abc" provided.');
+
+        $app = new Application(SingletonContainer::getContainer());
+
+        $tester = new CommandTester($app->find('run'));
+
+        $result = $tester->execute(['--threads' => 'abc']);
+        $this->assertSame(1, $result);
+    }
+}