How to Analyze a HAR File to Debug Network Issues
Export a HAR file from browser DevTools and use the HAR Viewer to find slow requests, failed API calls, and network bottlenecks.
Tool Used
HAR File Viewer
Export a HAR file from your browser
Open browser DevTools (F12) and click the Network tab. Reproduce the issue you want to investigate — load the page, trigger the flow, or perform the action that is slow or broken. Once captured, right-click anywhere in the request list and select Save all as HAR with content (Chrome/Edge) or click the gear icon and Save All As HAR (Firefox). Save the .har file to your desktop.
Open the HAR file in the viewer
Open the HAR Viewer tool. Click the upload button and select the .har file, or drag and drop it onto the tool. The viewer parses the file and displays all network requests in a sortable table. A single page load typically produces dozens to hundreds of entries.
Filter for failed or slow requests
Use the filter controls to narrow the request list. Filter by status code 4xx or 5xx to find failed requests. Filter by URL substring to focus on a specific domain or endpoint. Sort by time descending to put the slowest requests at the top. This quickly surfaces the requests that most deserve attention.
Read the timing waterfall
The timing waterfall shows each request as a horizontal bar on a timeline. Sequential requests (one starts after another finishes) show as stacked bars. Parallel requests overlap. Large gaps between bars indicate the browser is waiting — often for DNS, a TCP handshake, or a blocking script. Requests with a very long bar and most of the time in the waiting (TTFB) phase indicate a slow server response.
Inspect individual request details
Click any request to open its detail panel. Review the request headers, request body, response headers, and response body (if included in the HAR). The timings breakdown shows DNS, connect, TLS, send, wait (TTFB), and receive times separately. High TTFB means the server is slow. High receive time means the response payload is large. Use this to pinpoint whether the problem is the server, the network, or the payload size.
All done!
You are ready to use HAR File Viewer like a pro.