diff --git a/mockwebserver/src/main/java/okhttp3/mockwebserver/MockWebServer.java b/mockwebserver/src/main/java/okhttp3/mockwebserver/MockWebServer.java
index 8ff4708d723a..bfddfa89e610 100644
--- a/mockwebserver/src/main/java/okhttp3/mockwebserver/MockWebServer.java
+++ b/mockwebserver/src/main/java/okhttp3/mockwebserver/MockWebServer.java
@@ -649,7 +649,7 @@ private void handleWebSocketUpgrade(Socket socket, BufferedSource source, Buffer
 
     ThreadPoolExecutor replyExecutor =
         new ThreadPoolExecutor(1, 1, 1, SECONDS, new LinkedBlockingDeque<Runnable>(),
-            Util.threadFactory(String.format("MockWebServer %s WebSocket", request.getPath()),
+            Util.threadFactory(Util.format("MockWebServer %s WebSocket", request.getPath()),
                 true));
     replyExecutor.allowCoreThreadTimeOut(true);
     final RealWebSocket webSocket =
diff --git a/mockwebserver/src/test/java/okhttp3/mockwebserver/MockWebServerTest.java b/mockwebserver/src/test/java/okhttp3/mockwebserver/MockWebServerTest.java
index 9a1b0daf8f96..b951635a8a28 100644
--- a/mockwebserver/src/test/java/okhttp3/mockwebserver/MockWebServerTest.java
+++ b/mockwebserver/src/test/java/okhttp3/mockwebserver/MockWebServerTest.java
@@ -32,6 +32,7 @@
 import java.util.concurrent.TimeUnit;
 import java.util.concurrent.atomic.AtomicBoolean;
 import okhttp3.Headers;
+import okhttp3.internal.Util;
 import org.junit.After;
 import org.junit.Rule;
 import org.junit.Test;
@@ -231,8 +232,8 @@ public final class MockWebServerTest {
     long elapsedNanos = System.nanoTime() - startNanos;
     long elapsedMillis = NANOSECONDS.toMillis(elapsedNanos);
 
-    assertTrue(String.format("Request + Response: %sms", elapsedMillis), elapsedMillis >= 500);
-    assertTrue(String.format("Request + Response: %sms", elapsedMillis), elapsedMillis < 1000);
+    assertTrue(Util.format("Request + Response: %sms", elapsedMillis), elapsedMillis >= 500);
+    assertTrue(Util.format("Request + Response: %sms", elapsedMillis), elapsedMillis < 1000);
   }
 
   /**
@@ -257,8 +258,8 @@ public final class MockWebServerTest {
     long elapsedNanos = System.nanoTime() - startNanos;
     long elapsedMillis = NANOSECONDS.toMillis(elapsedNanos);
 
-    assertTrue(String.format("Request + Response: %sms", elapsedMillis), elapsedMillis >= 500);
-    assertTrue(String.format("Request + Response: %sms", elapsedMillis), elapsedMillis < 1000);
+    assertTrue(Util.format("Request + Response: %sms", elapsedMillis), elapsedMillis >= 500);
+    assertTrue(Util.format("Request + Response: %sms", elapsedMillis), elapsedMillis < 1000);
   }
 
   /** Delay the response body by sleeping 1s. */
@@ -273,7 +274,7 @@ public final class MockWebServerTest {
     assertEquals('A', in.read());
     long elapsedNanos = System.nanoTime() - startNanos;
     long elapsedMillis = NANOSECONDS.toMillis(elapsedNanos);
-    assertTrue(String.format("Request + Response: %sms", elapsedMillis), elapsedMillis >= 1000);
+    assertTrue(Util.format("Request + Response: %sms", elapsedMillis), elapsedMillis >= 1000);
 
     in.close();
   }
diff --git a/okcurl/src/main/java/okhttp3/curl/Main.java b/okcurl/src/main/java/okhttp3/curl/Main.java
index 5b2f371b0eba..15e4f030d198 100644
--- a/okcurl/src/main/java/okhttp3/curl/Main.java
+++ b/okcurl/src/main/java/okhttp3/curl/Main.java
@@ -45,6 +45,7 @@
 import okhttp3.Request;
 import okhttp3.RequestBody;
 import okhttp3.Response;
+import okhttp3.internal.Util;
 import okhttp3.internal.framed.Http2;
 import okhttp3.internal.http.StatusLine;
 import okio.BufferedSource;
@@ -277,7 +278,7 @@ private static void enableHttp2FrameLogging() {
     handler.setLevel(Level.FINE);
     handler.setFormatter(new SimpleFormatter() {
       @Override public String format(LogRecord record) {
-        return String.format("%s%n", record.getMessage());
+        return Util.format("%s%n", record.getMessage());
       }
     });
     frameLogger.addHandler(handler);
diff --git a/okhttp-tests/src/test/java/okhttp3/CallTest.java b/okhttp-tests/src/test/java/okhttp3/CallTest.java
index c097fb6bbee1..ac0c106a3951 100644
--- a/okhttp-tests/src/test/java/okhttp3/CallTest.java
+++ b/okhttp-tests/src/test/java/okhttp3/CallTest.java
@@ -734,7 +734,7 @@ private void postBodyRetransmittedAfterAuthorizationFail(String body) throws Exc
       // Timed out as expected.
       long elapsedNanos = System.nanoTime() - startNanos;
       long elapsedMillis = TimeUnit.NANOSECONDS.toMillis(elapsedNanos);
-      assertTrue(String.format("Timed out: %sms", elapsedMillis), elapsedMillis < 500);
+      assertTrue(Util.format("Timed out: %sms", elapsedMillis), elapsedMillis < 500);
     } finally {
       bodySource.close();
     }
diff --git a/okhttp-tests/src/test/java/okhttp3/URLConnectionTest.java b/okhttp-tests/src/test/java/okhttp3/URLConnectionTest.java
index c50838c2fac9..f3c69dc51e71 100644
--- a/okhttp-tests/src/test/java/okhttp3/URLConnectionTest.java
+++ b/okhttp-tests/src/test/java/okhttp3/URLConnectionTest.java
@@ -1412,7 +1412,7 @@ private void testEarlyDisconnectDoesntHarmPooling(TransferKind transferKind) thr
     // If we're working correctly, this should be greater than 100ms, but less than double that.
     // Previously we had a bug where we would download the entire response body as long as no
     // individual read took longer than 100ms.
-    assertTrue(String.format("Time to close: %sms", elapsedMillis), elapsedMillis < 500);
+    assertTrue(Util.format("Time to close: %sms", elapsedMillis), elapsedMillis < 500);
 
     // Do another request to confirm that the discarded connection was not pooled.
     assertContent("A", urlFactory.open(server.url("/").url()));
diff --git a/okhttp-tests/src/test/java/okhttp3/UrlComponentEncodingTester.java b/okhttp-tests/src/test/java/okhttp3/UrlComponentEncodingTester.java
index 1645acc90718..56611be40a7b 100644
--- a/okhttp-tests/src/test/java/okhttp3/UrlComponentEncodingTester.java
+++ b/okhttp-tests/src/test/java/okhttp3/UrlComponentEncodingTester.java
@@ -20,6 +20,7 @@
 import java.util.Collections;
 import java.util.LinkedHashMap;
 import java.util.Map;
+import okhttp3.internal.Util;
 import okio.Buffer;
 import okio.ByteString;
 
@@ -231,7 +232,7 @@ private void testParseAlreadyEncoded(int codePoint, Encoding encoding, Component
     String urlString = component.urlString(encoded);
     HttpUrl url = HttpUrl.parse(urlString);
     if (!component.encodedValue(url).equals(encoded)) {
-      fail(String.format("Encoding %s %#x using %s", component, codePoint, encoding));
+      fail(Util.format("Encoding %s %#x using %s", component, codePoint, encoding));
     }
   }
 
@@ -242,7 +243,7 @@ private void testEncodeAndDecode(int codePoint, Component component) {
     HttpUrl url = builder.build();
     String actual = component.get(url);
     if (!expected.equals(actual)) {
-      fail(String.format("Roundtrip %s %#x %s", component, codePoint, url));
+      fail(Util.format("Roundtrip %s %#x %s", component, codePoint, url));
     }
   }
 
@@ -255,7 +256,7 @@ private void testParseOriginal(int codePoint, Encoding encoding, Component compo
 
     String s = component.encodedValue(url);
     if (!s.equals(encoded)) {
-      fail(String.format("Encoding %s %#02x using %s", component, codePoint, encoding));
+      fail(Util.format("Encoding %s %#02x using %s", component, codePoint, encoding));
     }
   }
 
@@ -264,7 +265,7 @@ private void testToUrl(int codePoint, Encoding encoding, Component component) {
     HttpUrl httpUrl = HttpUrl.parse(component.urlString(encoded));
     URL javaNetUrl = httpUrl.url();
     if (!javaNetUrl.toString().equals(javaNetUrl.toString())) {
-      fail(String.format("Encoding %s %#x using %s", component, codePoint, encoding));
+      fail(Util.format("Encoding %s %#x using %s", component, codePoint, encoding));
     }
   }
 
@@ -273,7 +274,7 @@ private void testFromUrl(int codePoint, Encoding encoding, Component component)
     HttpUrl httpUrl = HttpUrl.parse(component.urlString(encoded));
     HttpUrl toAndFromJavaNetUrl = HttpUrl.get(httpUrl.url());
     if (!toAndFromJavaNetUrl.equals(httpUrl)) {
-      fail(String.format("Encoding %s %#x using %s", component, codePoint, encoding));
+      fail(Util.format("Encoding %s %#x using %s", component, codePoint, encoding));
     }
   }
 
@@ -287,18 +288,18 @@ private void testUri(
     if (uriEscaped) {
       // The URI has more escaping than the HttpURL. Check that the decoded values still match.
       if (uri.toString().equals(httpUrl.toString())) {
-        fail(String.format("Encoding %s %#x using %s", component, codePoint, encoding));
+        fail(Util.format("Encoding %s %#x using %s", component, codePoint, encoding));
       }
       if (!component.get(toAndFromUri).equals(string)) {
-        fail(String.format("Encoding %s %#x using %s", component, codePoint, encoding));
+        fail(Util.format("Encoding %s %#x using %s", component, codePoint, encoding));
       }
     } else {
       // Check that the URI and HttpURL have the exact same escaping.
       if (!toAndFromUri.equals(httpUrl)) {
-        fail(String.format("Encoding %s %#x using %s", component, codePoint, encoding));
+        fail(Util.format("Encoding %s %#x using %s", component, codePoint, encoding));
       }
       if (!uri.toString().equals(httpUrl.toString())) {
-        fail(String.format("Encoding %s %#x using %s", component, codePoint, encoding));
+        fail(Util.format("Encoding %s %#x using %s", component, codePoint, encoding));
       }
     }
   }
@@ -315,7 +316,7 @@ public String encode(int codePoint) {
         ByteString utf8 = ByteString.encodeUtf8(IDENTITY.encode(codePoint));
         Buffer percentEncoded = new Buffer();
         for (int i = 0; i < utf8.size(); i++) {
-          percentEncoded.writeUtf8(String.format("%%%02X", utf8.getByte(i) & 0xff));
+          percentEncoded.writeUtf8(Util.format("%%%02X", utf8.getByte(i) & 0xff));
         }
         return percentEncoded.readUtf8();
       }
diff --git a/okhttp-tests/src/test/java/okhttp3/WebPlatformUrlTestData.java b/okhttp-tests/src/test/java/okhttp3/WebPlatformUrlTestData.java
index 881811139da7..72f92f9abf9a 100644
--- a/okhttp-tests/src/test/java/okhttp3/WebPlatformUrlTestData.java
+++ b/okhttp-tests/src/test/java/okhttp3/WebPlatformUrlTestData.java
@@ -19,6 +19,7 @@
 import java.io.IOException;
 import java.util.ArrayList;
 import java.util.List;
+import okhttp3.internal.Util;
 import okio.Buffer;
 import okio.BufferedSource;
 
@@ -81,7 +82,7 @@ private void set(String name, String value) {
   }
 
   @Override public String toString() {
-    return String.format("Parsing: <%s> against <%s>", input, base);
+    return Util.format("Parsing: <%s> against <%s>", input, base);
   }
 
   public static List<WebPlatformUrlTestData> load(BufferedSource source) throws IOException {
diff --git a/okhttp-tests/src/test/java/okhttp3/internal/http/RecordingProxySelector.java b/okhttp-tests/src/test/java/okhttp3/internal/http/RecordingProxySelector.java
index 5b4b630d32b5..33a51500e0a1 100644
--- a/okhttp-tests/src/test/java/okhttp3/internal/http/RecordingProxySelector.java
+++ b/okhttp-tests/src/test/java/okhttp3/internal/http/RecordingProxySelector.java
@@ -24,6 +24,7 @@
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.List;
+import okhttp3.internal.Util;
 
 import static org.junit.Assert.assertEquals;
 
@@ -45,7 +46,7 @@ public void assertRequests(URI... expectedUris) {
   @Override public void connectFailed(URI uri, SocketAddress sa, IOException ioe) {
     InetSocketAddress socketAddress = (InetSocketAddress) sa;
     failures.add(
-        String.format("%s %s:%d %s", uri, socketAddress.getHostName(), socketAddress.getPort(),
+        Util.format("%s %s:%d %s", uri, socketAddress.getHostName(), socketAddress.getPort(),
             ioe.getMessage()));
   }
 }
diff --git a/okhttp-ws-tests/src/test/java/okhttp3/internal/ws/WebSocketReaderTest.java b/okhttp-ws-tests/src/test/java/okhttp3/internal/ws/WebSocketReaderTest.java
index 358b35990a77..6bb7ef86d83a 100644
--- a/okhttp-ws-tests/src/test/java/okhttp3/internal/ws/WebSocketReaderTest.java
+++ b/okhttp-ws-tests/src/test/java/okhttp3/internal/ws/WebSocketReaderTest.java
@@ -22,6 +22,7 @@
 import java.util.concurrent.atomic.AtomicReference;
 import java.util.regex.Pattern;
 import okhttp3.ResponseBody;
+import okhttp3.internal.Util;
 import okhttp3.ws.WebSocketRecorder;
 import okhttp3.ws.WebSocketRecorder.MessageDelegate;
 import okio.Buffer;
@@ -385,7 +386,7 @@ public final class WebSocketReaderTest {
     data.write(ByteString.decodeHex("880203ed")); // Close with code 1005
     data.write(ByteString.decodeHex("880203ee")); // Close with code 1006
     for (int i = 1012; i <= 2999; i++) {
-      data.write(ByteString.decodeHex("8802" + String.format("%04X", i))); // Close with code 'i'
+      data.write(ByteString.decodeHex("8802" + Util.format("%04X", i))); // Close with code 'i'
     }
 
     int count = 0;
diff --git a/okhttp-ws-tests/src/test/java/okhttp3/internal/ws/WebSocketWriterTest.java b/okhttp-ws-tests/src/test/java/okhttp3/internal/ws/WebSocketWriterTest.java
index 66475fb7b03f..bdeff21c1538 100644
--- a/okhttp-ws-tests/src/test/java/okhttp3/internal/ws/WebSocketWriterTest.java
+++ b/okhttp-ws-tests/src/test/java/okhttp3/internal/ws/WebSocketWriterTest.java
@@ -19,6 +19,7 @@
 import java.io.IOException;
 import java.util.Random;
 import okhttp3.RequestBody;
+import okhttp3.internal.Util;
 import okio.Buffer;
 import okio.BufferedSink;
 import okio.ByteString;
@@ -99,7 +100,7 @@ public final class WebSocketWriterTest {
     sink.close();
 
     assertData("817e");
-    assertData(String.format("%04x", length));
+    assertData(Util.format("%04x", length));
     assertData(bytes);
     assertTrue(data.exhausted());
   }
@@ -199,7 +200,7 @@ public final class WebSocketWriterTest {
     // Write directly to the unbuffered sink. This ensures it will become single frame.
     sink.write(payload.clone(), byteCount);
     assertData("027e"); // 'e' == 4-byte follow-up length.
-    assertData(String.format("%04X", payload.completeSegmentByteCount()));
+    assertData(Util.format("%04X", payload.completeSegmentByteCount()));
     assertData(payload.readByteArray());
 
     sink.close();
@@ -219,7 +220,7 @@ public final class WebSocketWriterTest {
     // Write directly to the unbuffered sink. This ensures it will become single frame.
     sink.write(payload.clone(), byteCount);
     assertData("027f"); // 'f' == 16-byte follow-up length.
-    assertData(String.format("%016X", byteCount));
+    assertData(Util.format("%016X", byteCount));
     assertData(payload.readByteArray(byteCount));
 
     sink.close();
diff --git a/okhttp-ws/src/main/java/okhttp3/ws/WebSocketCall.java b/okhttp-ws/src/main/java/okhttp3/ws/WebSocketCall.java
index 8e5c5ce0559e..46f8181036e4 100644
--- a/okhttp-ws/src/main/java/okhttp3/ws/WebSocketCall.java
+++ b/okhttp-ws/src/main/java/okhttp3/ws/WebSocketCall.java
@@ -160,7 +160,7 @@ static RealWebSocket create(StreamAllocation streamAllocation, Response response
       String url = response.request().url().toString();
       ThreadPoolExecutor replyExecutor =
           new ThreadPoolExecutor(1, 1, 1, SECONDS, new LinkedBlockingDeque<Runnable>(),
-              Util.threadFactory(String.format("OkHttp %s WebSocket", url), true));
+              Util.threadFactory(Util.format("OkHttp %s WebSocket", url), true));
       replyExecutor.allowCoreThreadTimeOut(true);
 
       return new StreamWebSocket(streamAllocation, random, replyExecutor, listener, url);
diff --git a/okhttp/src/main/java/okhttp3/Headers.java b/okhttp/src/main/java/okhttp3/Headers.java
index 992569a50100..840a272d3434 100644
--- a/okhttp/src/main/java/okhttp3/Headers.java
+++ b/okhttp/src/main/java/okhttp3/Headers.java
@@ -25,6 +25,7 @@
 import java.util.Map;
 import java.util.Set;
 import java.util.TreeSet;
+import okhttp3.internal.Util;
 import okhttp3.internal.http.HttpDate;
 
 /**
@@ -271,7 +272,7 @@ private void checkNameAndValue(String name, String value) {
       for (int i = 0, length = name.length(); i < length; i++) {
         char c = name.charAt(i);
         if (c <= '\u001f' || c >= '\u007f') {
-          throw new IllegalArgumentException(String.format(
+          throw new IllegalArgumentException(Util.format(
               "Unexpected char %#04x at %d in header name: %s", (int) c, i, name));
         }
       }
@@ -279,7 +280,7 @@ private void checkNameAndValue(String name, String value) {
       for (int i = 0, length = value.length(); i < length; i++) {
         char c = value.charAt(i);
         if (c <= '\u001f' || c >= '\u007f') {
-          throw new IllegalArgumentException(String.format(
+          throw new IllegalArgumentException(Util.format(
               "Unexpected char %#04x at %d in %s value: %s", (int) c, i, name, value));
         }
       }
diff --git a/okhttp/src/main/java/okhttp3/internal/NamedRunnable.java b/okhttp/src/main/java/okhttp3/internal/NamedRunnable.java
index b5a41036044f..ff9729e2af52 100644
--- a/okhttp/src/main/java/okhttp3/internal/NamedRunnable.java
+++ b/okhttp/src/main/java/okhttp3/internal/NamedRunnable.java
@@ -22,7 +22,7 @@ public abstract class NamedRunnable implements Runnable {
   protected final String name;
 
   public NamedRunnable(String format, Object... args) {
-    this.name = String.format(format, args);
+    this.name = Util.format(format, args);
   }
 
   @Override public final void run() {
diff --git a/okhttp/src/main/java/okhttp3/internal/Util.java b/okhttp/src/main/java/okhttp3/internal/Util.java
index 4dc139f4fb08..f2b1e70d46e3 100644
--- a/okhttp/src/main/java/okhttp3/internal/Util.java
+++ b/okhttp/src/main/java/okhttp3/internal/Util.java
@@ -446,4 +446,9 @@ private static boolean containsInvalidHostnameAsciiCodes(String hostnameAscii) {
   public static boolean verifyAsIpAddress(String host) {
     return VERIFY_AS_IP_ADDRESS.matcher(host).matches();
   }
+
+  /** Returns a {@link Locale#US} formatted {@link String}. */
+  public static String format(String format, Object... args) {
+    return String.format(Locale.US, format, args);
+  }
 }
diff --git a/okhttp/src/main/java/okhttp3/internal/framed/FramedConnection.java b/okhttp/src/main/java/okhttp3/internal/framed/FramedConnection.java
index 4a6a8e7d0943..362d63ad393f 100644
--- a/okhttp/src/main/java/okhttp3/internal/framed/FramedConnection.java
+++ b/okhttp/src/main/java/okhttp3/internal/framed/FramedConnection.java
@@ -155,7 +155,7 @@ private FramedConnection(Builder builder) throws IOException {
       // Like newSingleThreadExecutor, except lazy creates the thread.
       pushExecutor = new ThreadPoolExecutor(0, 1, 60, TimeUnit.SECONDS,
           new LinkedBlockingQueue<Runnable>(),
-          Util.threadFactory(String.format("OkHttp %s Push Observer", hostname), true));
+          Util.threadFactory(Util.format("OkHttp %s Push Observer", hostname), true));
       // 1 less than SPDY http://tools.ietf.org/html/draft-ietf-httpbis-http2-17#section-6.9.2
       peerSettings.set(Settings.INITIAL_WINDOW_SIZE, 0, 65535);
       peerSettings.set(Settings.MAX_FRAME_SIZE, 0, Http2.INITIAL_MAX_FRAME_SIZE);
diff --git a/okhttp/src/main/java/okhttp3/internal/framed/Header.java b/okhttp/src/main/java/okhttp3/internal/framed/Header.java
index 953671eb3281..866edc2591f0 100644
--- a/okhttp/src/main/java/okhttp3/internal/framed/Header.java
+++ b/okhttp/src/main/java/okhttp3/internal/framed/Header.java
@@ -15,6 +15,7 @@
  */
 package okhttp3.internal.framed;
 
+import okhttp3.internal.Util;
 import okio.ByteString;
 
 /** HTTP header: the name is an ASCII string, but the value can be UTF-8. */
@@ -66,6 +67,6 @@ public Header(ByteString name, ByteString value) {
   }
 
   @Override public String toString() {
-    return String.format("%s: %s", name.utf8(), value.utf8());
+    return Util.format("%s: %s", name.utf8(), value.utf8());
   }
 }
diff --git a/okhttp/src/main/java/okhttp3/internal/framed/Http2.java b/okhttp/src/main/java/okhttp3/internal/framed/Http2.java
index a6c177cc608b..8ad6c5224e7f 100644
--- a/okhttp/src/main/java/okhttp3/internal/framed/Http2.java
+++ b/okhttp/src/main/java/okhttp3/internal/framed/Http2.java
@@ -26,8 +26,8 @@
 import okio.Source;
 import okio.Timeout;
 
-import static java.lang.String.format;
 import static java.util.logging.Level.FINE;
+import static okhttp3.internal.Util.format;
 import static okhttp3.internal.framed.Http2.FrameLogger.formatHeader;
 import static okio.ByteString.EMPTY;
 
diff --git a/okhttp/src/main/java/okhttp3/internal/framed/Spdy3.java b/okhttp/src/main/java/okhttp3/internal/framed/Spdy3.java
index 72053e006638..37e1796c5c2a 100644
--- a/okhttp/src/main/java/okhttp3/internal/framed/Spdy3.java
+++ b/okhttp/src/main/java/okhttp3/internal/framed/Spdy3.java
@@ -275,7 +275,7 @@ private void readSettings(Handler handler, int flags, int length) throws IOExcep
     }
 
     private static IOException ioException(String message, Object... args) throws IOException {
-      throw new IOException(String.format(message, args));
+      throw new IOException(Util.format(message, args));
     }
 
     @Override public void close() throws IOException {