diff --git a/devtools/client/netmonitor/src/components/CustomRequestPanel.js b/devtools/client/netmonitor/src/components/CustomRequestPanel.js
index 7af69bae327a..d521d67bf196 100644
--- a/devtools/client/netmonitor/src/components/CustomRequestPanel.js
+++ b/devtools/client/netmonitor/src/components/CustomRequestPanel.js
@@ -125,10 +125,26 @@ class CustomRequestPanel extends Component {
             : { method: val.trim() };
         break;
       case "custom-postdata-value":
+        // Update "content-length" header value to reflect change
+        // in post data field.
+        const { requestHeaders } = request;
+        const newHeaders = requestHeaders.headers.map(header => {
+          if (header.name.toLowerCase() == "content-length") {
+            return {
+              name: header.name,
+              value: val.length,
+            };
+          }
+          return header;
+        });
+
         data = {
           requestPostData: {
             postData: { text: val },
           },
+          requestHeaders: {
+            headers: newHeaders,
+          },
         };
         break;
       case "custom-query-value":