Analyze HTTP Header Size
Paste raw request or response headers from browser DevTools, cURL, Postman, or server logs. The analyzer calculates byte size per header, total header bytes, and flags risky limits.
About This Tool
HTTP header size is easy to ignore until requests start failing with 400 Bad Request or
431 Request Header Fields Too Large. This analyzer helps you catch that problem early by
measuring your header block in bytes and showing which headers consume the most space.
Each line is measured as it is transmitted: name: value\r\n. That means the total includes
the header name, colon, space, value text, and line ending. The parser groups duplicate headers by a
normalized lowercase key, so repeated lines like multiple Set-Cookie entries are combined
in the summary while still counting all bytes.
In practice, cookie growth is one of the most common causes of oversized headers. Marketing tags, personalization state, and duplicated session data can quietly add kilobytes to every request. This page highlights cookie weight separately, surfaces oversized individual headers, and gives a sortable breakdown so you can trim the worst offenders first.
Use this tool during API debugging, reverse-proxy tuning, CDN rollout checks, and regression testing. Because everything runs client-side, you can paste sensitive internal headers safely without sending them anywhere. Keep your total small, keep your largest single header controlled, and verify changes before deployment.
Typical Operational Limits (May Vary)
| Rule of Thumb | Typical Threshold | Why It Matters |
|---|---|---|
| Total request headers | 8 KB | Many servers/proxies default near this range; exceeding it can cause 400/431 errors. |
| Single header line | 4 KB | Long values (especially cookies and tokens) may violate per-field limits. |
| Cookie header | 4 KB+ | Large cookies increase every request cost and commonly trigger header-size failures. |
| Total response headers | 8-16 KB | Large response metadata impacts bandwidth and can hit upstream gateway limits. |
FAQ
How are bytes calculated in this analyzer?
The tool uses UTF-8 byte length of header-name + ': ' + header-value + '\r\n' for each parsed header line.
Can I analyze both request and response headers?
Yes. Paste either block format. The first status/request line is ignored automatically when present.
Why do duplicate headers show one row?
Headers are normalized by lowercase key and grouped. Occurrence count shows how many lines were merged.
What warnings should I act on first?
Start with very large individual headers and cookie bloat, then reduce total bytes below your infrastructure limits.
Privacy & Limitations
- All calculations run entirely in your browser -- nothing is sent to any server.
- Results are computed locally and should be verified for critical applications.
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HTTP Header Size Analyzer FAQ
Why does HTTP header size matter?
Large headers can trigger 400 or 431 errors, exceed proxy limits, and increase request latency. Monitoring total header bytes helps prevent production issues.
Does this tool include CRLF in byte counts?
Yes. The analyzer measures each header as header-name + colon + space + header-value + CRLF, which reflects how headers are transmitted on the wire.
What header limits are common?
Limits vary by server and proxy. Many environments use around 8 KB per request header block by default, and some systems enforce per-header limits around 4 KB.
Are my headers uploaded?
No. All analysis runs in your browser. The tool does not send pasted header data to any server.