TCP Header Length Calculator

Calculator Form

Calculation Method

Data Offset Field

Use 5 to 15 words. One word equals 4 bytes.

Option Bytes

Used when option byte mode is selected.

TCP Segment Length

Optional. Enter bytes if known.

IP Total Length

Optional. Used to estimate TCP segment length.

IP Header Length

Usually 20 bytes for IPv4 without options.

Raw TCP Header Hex

For raw mode, paste at least the first 13 TCP header bytes.

Formula Used

TCP header length in bytes = Data Offset × 4

TCP header length in bits = TCP header bytes × 8

TCP option bytes = TCP header bytes − 20

Estimated payload bytes = TCP segment length − TCP header bytes

The Data Offset field is four bits wide. It stores the header length in 32-bit words. A value of 5 means 20 bytes. A value of 15 means 60 bytes.

How to Use This Calculator

Select a calculation method.
Enter the Data Offset field, option bytes, or raw TCP header hex.
Add TCP segment length if you want payload estimation.
Use IP total length and IP header length if TCP segment length is not known.
Press the calculate button.
Review header bytes, option bytes, padding, and payload.
Use the CSV or PDF buttons to save the result.

Example Data Table

Case	Data Offset	Header Bytes	Option Bytes	TCP Segment Bytes	Payload Bytes
Basic TCP Header	5	20	0	60	40
Timestamp Option	8	32	12	100	68
Maximum Header	15	60	40	120	60

About TCP Header Length

TCP carries application data between hosts. Each segment starts with a header. The base header is twenty bytes. Extra options can increase it. The Data Offset field tells how long the header is. It uses 32 bit words. So a value of five means twenty bytes. A value of fifteen means sixty bytes.

Why Header Length Matters

Header length affects payload size. A larger header leaves fewer bytes for useful data inside the same segment. Analysts check it when studying packet captures, latency, fragmentation, and performance. Students also use it to connect binary fields with real byte counts. The number is small, yet it shapes many networking decisions.

What This Calculator Does

This calculator accepts three practical input styles. You can enter the Data Offset value directly. You can enter option bytes and let the tool add required padding. You can also paste raw TCP header bytes in hexadecimal form. The tool reads the high nibble of byte twelve. That nibble contains the Data Offset field.

Results Explained

The result shows header length in bytes, bits, and 32 bit words. It also shows option bytes, padding bytes, and an estimated payload. When segment length is available, the calculator reports the header share as a percentage. This helps compare ordinary headers with option heavy headers.

Practical Notes

A normal TCP header without options is twenty bytes. Common options include MSS, window scale, timestamps, and selective acknowledgment. Options must align on a four byte boundary. Padding fills unused space, often with no operation bytes. If the computed header is below twenty bytes, the header is invalid. If it is above sixty bytes, it is also invalid.

Using Values Safely

Packet tools may show total IP length, IP header length, and TCP segment length. If TCP segment length is missing, subtract the IP header length from total IP length. This gives the TCP segment length. Then subtract TCP header bytes to estimate payload bytes.

Best Use Cases

Use this tool for lessons, capture reviews, lab reports, and troubleshooting notes. It is not a packet sniffer. It helps verify fields already collected from captures or documentation. Always compare results with trusted capture software when investigating production networks. During class exercises.

FAQs

What is TCP header length?

TCP header length is the size of the TCP header before payload data starts. The minimum size is 20 bytes. Options can increase it up to 60 bytes.

What is the Data Offset field?

The Data Offset field is a four-bit TCP field. It stores header length in 32-bit words. Multiply it by 4 to get bytes.

Why is the minimum value 5?

A basic TCP header is 20 bytes. Since the field uses 32-bit words, 20 divided by 4 equals 5.

Why can the header be larger than 20 bytes?

TCP options add extra information. Examples include MSS, timestamps, window scaling, and selective acknowledgment support. These options increase header length.

What is the maximum TCP header length?

The maximum TCP header length is 60 bytes. The Data Offset field has four bits, so its largest value is 15. Then 15 × 4 equals 60.

How does raw hex parsing work?

The calculator reads byte 12 of the TCP header. It takes the high nibble from that byte. That nibble is the Data Offset value.

How do I estimate payload length?

Enter the TCP segment length. The calculator subtracts TCP header bytes from it. The remaining bytes are the estimated TCP payload.

Is this calculator a packet analyzer?

No. It calculates header length from values you provide. Use trusted packet capture software when you need full protocol inspection.