diff --git a/src/main/java/com/google/devtools/build/lib/BUILD b/src/main/java/com/google/devtools/build/lib/BUILD
index 54714bc7bbb8ce..a5fb612693b8f8 100644
--- a/src/main/java/com/google/devtools/build/lib/BUILD
+++ b/src/main/java/com/google/devtools/build/lib/BUILD
@@ -1284,7 +1284,10 @@ java_library(
 java_library(
     name = "loading-phase-threads-option",
     srcs = ["runtime/LoadingPhaseThreadsOption.java"],
-    deps = ["//src/main/java/com/google/devtools/common/options"],
+    deps = [
+        "//src/main/java/com/google/devtools/build/lib/actions:localhost_capacity",
+        "//src/main/java/com/google/devtools/common/options",
+    ],
 )
 
 java_library(
diff --git a/src/main/java/com/google/devtools/build/lib/runtime/LoadingPhaseThreadsOption.java b/src/main/java/com/google/devtools/build/lib/runtime/LoadingPhaseThreadsOption.java
index 75f10006f40343..812f571e1f70a9 100644
--- a/src/main/java/com/google/devtools/build/lib/runtime/LoadingPhaseThreadsOption.java
+++ b/src/main/java/com/google/devtools/build/lib/runtime/LoadingPhaseThreadsOption.java
@@ -13,22 +13,29 @@
 // limitations under the License.
 package com.google.devtools.build.lib.runtime;
 
+import com.google.devtools.build.lib.actions.LocalHostCapacity;
 import com.google.devtools.common.options.Converter;
 import com.google.devtools.common.options.Option;
 import com.google.devtools.common.options.OptionDocumentationCategory;
 import com.google.devtools.common.options.OptionEffectTag;
 import com.google.devtools.common.options.OptionsBase;
 import com.google.devtools.common.options.OptionsParsingException;
+import java.util.logging.Logger;
 
 /** Defines the --loading_phase_threads option which is used by multiple commands. */
 public class LoadingPhaseThreadsOption extends OptionsBase {
+
+  private static final Logger logger = Logger.getLogger(LoadingPhaseThreadsOption.class.getName());
+
   @Option(
     name = "loading_phase_threads",
-    defaultValue = LoadingPhaseThreadCountConverter.DEFAULT_VALUE,
+    defaultValue = "auto",
     documentationCategory = OptionDocumentationCategory.EXECUTION_STRATEGY,
     effectTags = {OptionEffectTag.BAZEL_INTERNAL_CONFIGURATION},
     converter = LoadingPhaseThreadCountConverter.class,
     help = "Number of parallel threads to use for the loading/analysis phase."
+        + " \"auto\" means to use a reasonable value derived from the machine's hardware profile"
+        + " (e.g. the number of processors)."
   )
   public int threads;
 
@@ -38,33 +45,44 @@ public class LoadingPhaseThreadsOption extends OptionsBase {
    */
   public static final class LoadingPhaseThreadCountConverter implements Converter<Integer> {
 
-    private static final String DEFAULT_VALUE = "default";
-    // Reduce thread count while running tests. Test cases are typically small, and large thread
-    // pools vying for a relatively small number of CPU cores may induce non-optimal
-    // performance.
-    private static final int NON_TEST_THREADS = 200;
-    private static final int TEST_THREADS = 20;
-
     @Override
     public Integer convert(String input) throws OptionsParsingException {
-      if (DEFAULT_VALUE.equals(input)) {
-        return System.getenv("TEST_TMPDIR") == null ? NON_TEST_THREADS : TEST_THREADS;
+      int threads;
+      if (input.equals("auto")) {
+        threads = (int) Math.ceil(LocalHostCapacity.getLocalHostCapacity().getCpuUsage());
+        logger.info("Flag \"loading_phase_threads\" was set to \"auto\"; using " + threads
+            + " threads");
+      } else {
+        try {
+          threads = Integer.decode(input);
+          if (threads < 1) {
+            throw new OptionsParsingException("'" + input + "' must be at least 1");
+          }
+        } catch (NumberFormatException e) {
+          throw new OptionsParsingException("'" + input + "' is not an int");
+        }
       }
 
-      try {
-        int result = Integer.decode(input);
-        if (result < 1) {
-          throw new OptionsParsingException("'" + input + "' must be at least 1");
-        }
-        return result;
-      } catch (NumberFormatException e) {
-        throw new OptionsParsingException("'" + input + "' is not an int");
+      if (System.getenv("TEST_TMPDIR") != null) {
+        // Cap thread count while running tests. Test cases are typically small and large thread
+        // pools vying for a relatively small number of CPU cores may induce non-optimal
+        // performance.
+        //
+        // TODO(jmmv): If tests care about this, it's them who should be setting a cap.
+        threads = Math.min(20, threads);
+        logger.info("Running under a test; loading_phase_threads capped at " + threads);
       }
+
+      // TODO(jmmv): Using the number of cores has proven to yield reasonable analysis times on
+      // Mac Pros and MacBook Pros but we should probably do better than this. (We haven't made
+      // any guarantees that "auto" means number of cores precisely to leave us room to tune this
+      // further in the future.)
+      return threads;
     }
 
     @Override
     public String getTypeDescription() {
-      return "an integer";
+      return "\"auto\" or an integer";
     }
   }
 }
diff --git a/src/test/shell/integration/execution_phase_tests.sh b/src/test/shell/integration/execution_phase_tests.sh
index 71951a21f55f21..fe5e3bf6087920 100755
--- a/src/test/shell/integration/execution_phase_tests.sh
+++ b/src/test/shell/integration/execution_phase_tests.sh
@@ -249,24 +249,39 @@ function test_cache_computed_file_digests_ui() {
       "Digests cache not reenabled"
 }
 
-function test_jobs_default_auto() {
+function do_threading_default_auto_test() {
+  local context="${1}"; shift
+  local flag_name="${1}"; shift
+
   local -r pkg="${FUNCNAME}"
   mkdir -p "$pkg" || fail "could not create \"$pkg\""
 
-  # The default flag value is only read if --jobs is not set explicitly.
-  # Do not use a bazelrc here, this would break the test.
   mkdir -p $pkg/package || fail "mkdir failed"
   echo "cc_library(name = 'foo', srcs = ['foo.cc'])" >$pkg/package/BUILD
   echo "int foo(void) { return 0; }" >$pkg/package/foo.cc
 
+  # The default value of the flag under test is only read if it is not set
+  # explicitly.  Do not use a bazelrc here as it would break the test.
   local output_base="$(bazel --nomaster_bazelrc --bazelrc=/dev/null info \
       output_base 2>/dev/null)" || fail "bazel info should work"
   local java_log="${output_base}/java.log"
   bazel --nomaster_bazelrc --bazelrc=/dev/null build $pkg/package:foo \
       >>"${TEST_log}" 2>&1 || fail "Should build"
 
-  assert_last_log "BuildRequest" 'Flag "jobs" was set to "auto"' "${java_log}" \
-      "--jobs was not set to auto by default"
+  assert_last_log \
+      "${context}" \
+      "Flag \"${flag_name}\" was set to \"auto\"" \
+      "${java_log}" \
+      "--${flag_name} was not set to auto by default"
+}
+
+function test_jobs_default_auto() {
+  do_threading_default_auto_test "BuildRequest" "jobs"
+}
+
+function test_loading_phase_threads_default_auto() {
+  do_threading_default_auto_test "LoadingPhaseThreadsOption" \
+      "loading_phase_threads"
 }
 
 function test_analysis_warning_cached() {