CheckSum (FIX tag 10)

StringFIX 4.4FIX 4.2session envelope

CheckSum (tag 10) is a String 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: 10
Name: CheckSum
Datatype: String
Dialects: FIX 4.4 and FIX 4.2
Messages: 93

What the specification says

FIX 4.4

Three byte, simple checksum (see Volume 2: "Checksum Calculation" for description). ALWAYS LAST FIELD IN MESSAGE; i.e. serves, with the trailing <SOH>, as the end-of-message delimiter. Always defined as three characters. (Always unencrypted)

FIX 4.2

Three byte, simple checksum (see Appendix B: CheckSum Calculation for description). ALWAYS LAST FIELD IN MESSAGE; i.e. serves, with the trailing <SOH>, as the end-of-message delimiter. Always defined as three characters. (Always unencrypted)

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

A real message using tag 10

MarketDataSnapshotFullRefresh · FIX 4.4

Tag	Field	Wire value	Meaning
8	BeginString	FIX.4.4
9	BodyLength	75
35	MsgType	W	Market Data-Snapshot/Full Refresh
49	SenderCompID	BOARTEAM
56	TargetCompID	COUNTERPARTY
34	MsgSeqNum	2
52	SendingTime	20240101-12:00:00.000
268	NoMDEntries1 entry	1	repeating group
269	MDEntryType	0	Bid
10	CheckSum	246

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 10 appears

Session envelope field

Tag 10 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

Free-form text. Any byte except the SOH separator is legal, so values are not trimmed or case-folded on the wire. @boarteam/fix keeps the value as a string; format rules are checked by validate(), which reports validate/invalid-value rather than throwing.

Read tag 10 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[10];field?.name;   // "CheckSum"field?.raw;    // the verbatim wire stringfield?.value;  // coerced to a string

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