diff --git a/middleware/metrics/README.md b/middleware/metrics/README.md
index 973d257e1..99b5a9f27 100644
--- a/middleware/metrics/README.md
+++ b/middleware/metrics/README.md
@@ -17,6 +17,7 @@ Each counter has a label `zone` which is the zonename used for the request/respo
 holds the transport of the response ("udp" or "tcp") and the address family of the transport (1
 = IP (IP version 4), 2 = IP6 (IP version 6)).
 The `response_rcode_count_total` has an extra label `rcode` which holds the rcode of the response.
+The `*_size_bytes` counters also hold the protocol in the `proto` label ("udp" or "tcp").
 
 If monitoring is enabled queries that do not enter the middleware chain are exported under the fake
 domain "dropped" (without a closing dot).
diff --git a/middleware/metrics/handler.go b/middleware/metrics/handler.go
index 6d0eb1f84..019a02c3e 100644
--- a/middleware/metrics/handler.go
+++ b/middleware/metrics/handler.go
@@ -49,6 +49,6 @@ func Report(state middleware.State, zone, rcode string, size int, start time.Tim
 		requestDo.WithLabelValues(zone).Inc()
 	}
 
-	responseSize.WithLabelValues(zone).Observe(float64(size))
+	responseSize.WithLabelValues(zone, net).Observe(float64(size))
 	responseRcode.WithLabelValues(zone, rcode).Inc()
 }
diff --git a/middleware/metrics/metrics.go b/middleware/metrics/metrics.go
index 5c2c37ff8..093f2a7dd 100644
--- a/middleware/metrics/metrics.go
+++ b/middleware/metrics/metrics.go
@@ -103,7 +103,7 @@ func define() {
 		Name:      "response_size_bytes",
 		Help:      "Size of the returns response in bytes.",
 		Buckets:   []float64{0, 100, 200, 300, 400, 511, 1023, 2047, 4095, 8291, 16e3, 32e3, 48e3, 64e3},
-	}, []string{"zone"})
+	}, []string{"zone", "proto"})
 
 	responseRcode = prometheus.NewCounterVec(prometheus.CounterOpts{
 		Namespace: middleware.Namespace,