diff --git a/capture_metrics.go b/capture_metrics.go
index 8f9191f..6874282 100644
--- a/capture_metrics.go
+++ b/capture_metrics.go
@@ -69,6 +69,16 @@ func (m *Metrics) CaptureMetrics(w http.ResponseWriter, fn func(http.ResponseWri
 				}
 			},
 
+			WriteString: func(next WriteStringFunc) WriteStringFunc {
+				return func(s string) (int, error) {
+					n, err := next(s)
+
+					m.Written += int64(n)
+					headerWritten = true
+					return n, err
+				}
+			},
+
 			ReadFrom: func(next ReadFromFunc) ReadFromFunc {
 				return func(src io.Reader) (int64, error) {
 					n, err := next(src)
diff --git a/capture_metrics_test.go b/capture_metrics_test.go
index 238ed02..222c8cb 100644
--- a/capture_metrics_test.go
+++ b/capture_metrics_test.go
@@ -61,6 +61,14 @@ func TestCaptureMetrics(t *testing.T) {
 			WantWritten: 17,
 			WantCode:    http.StatusOK,
 		},
+		{
+			Name: "string writer",
+			Handler: http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+				_, _ = io.WriteString(w, "write string")
+			}),
+			WantWritten: int64(len("write string")),
+			WantCode:    http.StatusOK,
+		},
 		{
 			Name: "empty panic",
 			Handler: http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {