BodyLength (FIX tag 9)

LengthFIX 4.4FIX 4.2session envelope

BodyLength (tag 9) is a Length field defined in FIX 4.4 and FIX 4.2. It belongs to the session envelope: every FIX 4.4 message carries it, so it identifies the session rather than describing the business event.

At a glance

Tag: 9
Name: BodyLength
Datatype: Length (int)
Dialects: FIX 4.4 and FIX 4.2
Messages: 93

What the specification says

Message length, in bytes, forward to the CheckSum field. ALWAYS SECOND FIELD IN MESSAGE. (Always unencrypted)

Descriptions are quoted from the FIX Orchestra sources under the Apache 2.0 licence.

A real message using tag 9

Logon · FIX 4.4

Tag	Field	Wire value	Meaning
8	BeginString	FIX.4.4
9	BodyLength	74
35	MsgType	A	Logon
49	SenderCompID	BOARTEAM
56	TargetCompID	COUNTERPARTY
34	MsgSeqNum	2
52	SendingTime	20240101-12:00:00.000
98	EncryptMethod	0	None / other
108	HeartBtInt	1
10	CheckSum	168

Edit and decode it yourself

Shown with | separators for readability. On the wire FIX uses SOH (0x01), an invisible control byte — and because BodyLength and CheckSum are computed over the actual bytes, the two forms have different checksums. Both are valid and both decode here.

Open in the full decoder →

Generated from the FIX 4.4 dictionary and verified at build time: it parses and validates with no issues. Open the decoder to try your own message.

Where tag 9 appears

Session envelope field

Tag 9 appears in every one of the 93 FIX 4.4 message types, because it is part of the standard header or trailer. Listing them all would say nothing useful — it is present on anything you will ever decode.

Wire format

A byte count for the length-prefixed field that follows it. Count bytes, not characters — a multi-byte UTF-8 value makes the two differ. @boarteam/fix parses the value as a number and reports parse/invalid-int when it does not lex as one; the raw string is kept either way.

Read tag 9 in TypeScript

decode.tsts

import { createFixEngine } from "@boarteam/fix";import { dictionary } from "@boarteam/fix-dict-fix44";const fix = createFixEngine(dictionary);const { message, issues } = fix.parse(raw);const field = message.fields[9];field?.name;   // "BodyLength"field?.raw;    // the verbatim wire stringfield?.value;  // coerced to a number

Other Length fields

Decode this in your own code

The same engine that produced the decoded example above is an Apache-2.0 npm package with zero runtime dependencies. It runs in Node and in the browser, and parse() returns problems as data instead of throwing.

npm i @boarteam/fix @boarteam/fix-dict-fix44

Try the decoder Source on GitHub