What is a MongoDB ObjectId?

A 12-byte identifier MongoDB uses by default for the _id field, usually shown as a 24-character hex string. It packs a 4-byte creation timestamp, a 5-byte random value, and a 3-byte incrementing counter.

Is the timestamp inside an ObjectId reliable?

It reflects the second the ObjectId was generated on the client or server that created it, so it is accurate to one second but depends on that machine's clock being correct.

Why is my ObjectId marked invalid?

An ObjectId must be exactly 24 hexadecimal characters (0-9, a-f). Extra spaces, quotes from a JSON dump, or a shorter/longer string will fail validation. Trim the value and try again.

Does this tool send my ObjectId anywhere?

No. Decoding happens entirely in your browser with plain JavaScript. Nothing is uploaded or logged.

Does the random value and counter mean anything besides randomness?

Since the 2016 MongoDB driver specification, bytes 4-8 are a single random value generated once per process and bytes 9-11 are a counter that increments for each ObjectId, so they mainly guarantee uniqueness rather than encode meaning.

MongoDB ObjectId Decoder

Paste a MongoDB ObjectId and see its embedded timestamp, random value, and counter decoded instantly.

Enter ObjectId

Byte layout

A 12-byte ObjectId splits into a 4-byte timestamp, a 5-byte random value, and a 3-byte counter, shown below as 24 hex characters.

Enter an ObjectId on the left to decode it

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Summary

This decoder splits a 24-character MongoDB ObjectId into its embedded Unix timestamp, five-byte random value, and three-byte counter. It validates pasted IDs locally and can generate a sample ObjectId for the current time.

How it works

Paste or type a 24-character hex ObjectId into the input field.
The tool validates the format and decodes it as you type.
View the embedded Unix timestamp and creation date/time.
View the 5-byte random value and 3-byte counter segments.
Click "Generate Random" to produce a valid sample ObjectId for the current time.

Use cases

Find out when a MongoDB document was created from its _id.
Debug a document by inspecting the ObjectId structure without a database client.
Verify that a string is a well-formed 24-character hex ObjectId.
Generate a sample ObjectId for a given moment for testing or fixtures.
Explain to a teammate how MongoDB ObjectIds encode a timestamp.
Check whether two ObjectIds were created around the same time.

Frequently Asked Questions

Last updated: 2026-09-09 · Reviewed by Nham Vu