diff --git a/prober/derp.go b/prober/derp.go
index f405549ff..870460d96 100644
--- a/prober/derp.go
+++ b/prober/derp.go
@@ -301,13 +301,14 @@ func (d *derpProber) probeBandwidth(from, to string, size int64) ProbeClass {
 		derpPath = "single"
 	}
 	var transferTimeSeconds expvar.Float
+	var totalBytesTransferred expvar.Float
 	return ProbeClass{
 		Probe: func(ctx context.Context) error {
 			fromN, toN, err := d.getNodePair(from, to)
 			if err != nil {
 				return err
 			}
-			return derpProbeBandwidth(ctx, d.lastDERPMap, fromN, toN, size, &transferTimeSeconds, d.bwTUNIPv4Prefix)
+			return derpProbeBandwidth(ctx, d.lastDERPMap, fromN, toN, size, &transferTimeSeconds, &totalBytesTransferred, d.bwTUNIPv4Prefix)
 		},
 		Class: "derp_bw",
 		Labels: Labels{
@@ -315,11 +316,15 @@ func (d *derpProber) probeBandwidth(from, to string, size int64) ProbeClass {
 			"tcp_in_tcp": strconv.FormatBool(d.bwTUNIPv4Prefix != nil),
 		},
 		Metrics: func(l prometheus.Labels) []prometheus.Metric {
-			return []prometheus.Metric{
+			metrics := []prometheus.Metric{
 				prometheus.MustNewConstMetric(prometheus.NewDesc("derp_bw_probe_size_bytes", "Payload size of the bandwidth prober", nil, l), prometheus.GaugeValue, float64(size)),
 				prometheus.MustNewConstMetric(prometheus.NewDesc("derp_bw_transfer_time_seconds_total", "Time it took to transfer data", nil, l), prometheus.CounterValue, transferTimeSeconds.Value()),
-				prometheus.MustNewConstMetric(prometheus.NewDesc("derp_bw_bytes_total", "Amount of data transferred", nil, l), prometheus.CounterValue, float64(size)),
 			}
+			if d.bwTUNIPv4Prefix != nil {
+				// For TCP-in-TCP probes, also record cumulative bytes transferred.
+				metrics = append(metrics, prometheus.MustNewConstMetric(prometheus.NewDesc("derp_bw_bytes_total", "Amount of data transferred", nil, l), prometheus.CounterValue, totalBytesTransferred.Value()))
+			}
+			return metrics
 		},
 	}
 }
@@ -655,7 +660,7 @@ func derpProbeUDP(ctx context.Context, ipStr string, port int) error {
 // DERP clients connected to two DERP servers.If tunIPv4Address is specified,
 // probes will use a TCP connection over a TUN device at this address in order
 // to exercise TCP-in-TCP in similar fashion to TCP over Tailscale via DERP.
-func derpProbeBandwidth(ctx context.Context, dm *tailcfg.DERPMap, from, to *tailcfg.DERPNode, size int64, transferTimeSeconds *expvar.Float, tunIPv4Prefix *netip.Prefix) (err error) {
+func derpProbeBandwidth(ctx context.Context, dm *tailcfg.DERPMap, from, to *tailcfg.DERPNode, size int64, transferTimeSeconds, totalBytesTransferred *expvar.Float, tunIPv4Prefix *netip.Prefix) (err error) {
 	// This probe uses clients with isProber=false to avoid spamming the derper logs with every packet
 	// sent by the bandwidth probe.
 	fromc, err := newConn(ctx, dm, from, false)
@@ -677,7 +682,7 @@ func derpProbeBandwidth(ctx context.Context, dm *tailcfg.DERPMap, from, to *tail
 	}
 
 	if tunIPv4Prefix != nil {
-		err = derpProbeBandwidthTUN(ctx, transferTimeSeconds, from, to, fromc, toc, size, tunIPv4Prefix)
+		err = derpProbeBandwidthTUN(ctx, transferTimeSeconds, totalBytesTransferred, from, to, fromc, toc, size, tunIPv4Prefix)
 	} else {
 		err = derpProbeBandwidthDirect(ctx, transferTimeSeconds, from, to, fromc, toc, size)
 	}
@@ -848,7 +853,7 @@ var derpProbeBandwidthTUNMu sync.Mutex
 // to another over a TUN device at an address at the start of the usable host IP
 // range that the given tunAddress lives in. The time taken to finish the transfer
 // is recorded in `transferTimeSeconds`.
-func derpProbeBandwidthTUN(ctx context.Context, transferTimeSeconds *expvar.Float, from, to *tailcfg.DERPNode, fromc, toc *derphttp.Client, size int64, prefix *netip.Prefix) error {
+func derpProbeBandwidthTUN(ctx context.Context, transferTimeSeconds, totalBytesTransferred *expvar.Float, from, to *tailcfg.DERPNode, fromc, toc *derphttp.Client, size int64, prefix *netip.Prefix) error {
 	// Make sure all goroutines have finished.
 	var wg sync.WaitGroup
 	defer wg.Wait()
@@ -1046,9 +1051,10 @@ func derpProbeBandwidthTUN(ctx context.Context, transferTimeSeconds *expvar.Floa
 				readFinishedC <- fmt.Errorf("unable to set read deadline: %w", err)
 			}
 		}
-		_, err = io.CopyN(io.Discard, readConn, size)
-		// Measure transfer time irrespective of whether it succeeded or failed.
+		n, err := io.CopyN(io.Discard, readConn, size)
+		// Measure transfer time and bytes transferred irrespective of whether it succeeded or failed.
 		transferTimeSeconds.Add(time.Since(start).Seconds())
+		totalBytesTransferred.Add(float64(n))
 		readFinishedC <- err
 	}()