diff --git a/transport/src/main/java/io/netty/channel/ChannelOutboundBuffer.java b/transport/src/main/java/io/netty/channel/ChannelOutboundBuffer.java
index b3e2e7a708..e3533c489d 100644
--- a/transport/src/main/java/io/netty/channel/ChannelOutboundBuffer.java
+++ b/transport/src/main/java/io/netty/channel/ChannelOutboundBuffer.java
@@ -175,7 +175,7 @@ public final class ChannelOutboundBuffer {
         }
 
         long newWriteBufferSize = TOTAL_PENDING_SIZE_UPDATER.addAndGet(this, size);
-        if (newWriteBufferSize >= channel.config().getWriteBufferHighWaterMark()) {
+        if (newWriteBufferSize > channel.config().getWriteBufferHighWaterMark()) {
             setUnwritable(invokeLater);
         }
     }
@@ -194,8 +194,7 @@ public final class ChannelOutboundBuffer {
         }
 
         long newWriteBufferSize = TOTAL_PENDING_SIZE_UPDATER.addAndGet(this, -size);
-        if (notifyWritability && (newWriteBufferSize == 0
-            || newWriteBufferSize <= channel.config().getWriteBufferLowWaterMark())) {
+        if (notifyWritability && newWriteBufferSize < channel.config().getWriteBufferLowWaterMark()) {
             setWritable(invokeLater);
         }
     }
diff --git a/transport/src/test/java/io/netty/channel/ChannelOutboundBufferTest.java b/transport/src/test/java/io/netty/channel/ChannelOutboundBufferTest.java
index 4a510db067..e65dbd4599 100644
--- a/transport/src/test/java/io/netty/channel/ChannelOutboundBufferTest.java
+++ b/transport/src/test/java/io/netty/channel/ChannelOutboundBufferTest.java
@@ -220,19 +220,21 @@ public class ChannelOutboundBufferTest {
         ch.config().setWriteBufferLowWaterMark(128);
         ch.config().setWriteBufferHighWaterMark(256);
 
-        // Ensure exceeding the low watermark does not make channel unwritable.
         ch.write(buffer().writeZero(128));
+        // Ensure exceeding the low watermark does not make channel unwritable.
+        ch.write(buffer().writeZero(2));
         assertThat(buf.toString(), is(""));
 
         ch.unsafe().outboundBuffer().addFlush();
 
         // Ensure exceeding the high watermark makes channel unwritable.
-        ch.write(buffer().writeZero(128));
+        ch.write(buffer().writeZero(127));
         assertThat(buf.toString(), is("false "));
 
         // Ensure going down to the low watermark makes channel writable again by flushing the first write.
         assertThat(ch.unsafe().outboundBuffer().remove(), is(true));
-        assertThat(ch.unsafe().outboundBuffer().totalPendingWriteBytes(), is(128L));
+        assertThat(ch.unsafe().outboundBuffer().remove(), is(true));
+        assertThat(ch.unsafe().outboundBuffer().totalPendingWriteBytes(), is(127L));
         assertThat(buf.toString(), is("false true "));
 
         safeClose(ch);
@@ -329,7 +331,7 @@ public class ChannelOutboundBufferTest {
         ChannelOutboundBuffer cob = ch.unsafe().outboundBuffer();
 
         // Trigger channelWritabilityChanged() by writing a lot.
-        ch.write(buffer().writeZero(256));
+        ch.write(buffer().writeZero(257));
         assertThat(buf.toString(), is("false "));
 
         // Ensure that setting a user-defined writability flag to false does not trigger channelWritabilityChanged()