$strLenBytes (Byte Length)

Last updated: 2026-10-09

$strLenBytes (Byte Length)

$strLenBytes returns the UTF-8 byte length of a string — a staple for storage estimation and field-size limits. For pure ASCII, bytes equal characters; Chinese takes 3 bytes per character and emoji usually 4. It accepts strings only; other types throw, so convert with $toString first. Results can differ from the code-point-based $strLenCP.

📝 Syntax

{ $strLenBytes: <string-expression> }

⚙️ Parameters

Returns：The UTF-8 byte length as an integer.

▶ Example

Run the commands below in your local mongosh (requires a running mongod):

// $strLenBytes: byte size of article titles
 db.articles.aggregate([
  { $project: { sizeBytes: { $strLenBytes: '$title' } } }
])

Output：

{ "_id": 1, "sizeBytes": 42 }
{ "_id": 2, "sizeBytes": 36 }

📌 Notes

• A Chinese character is 3 bytes — not the character count
• Non-string input throws — convert with $toString
• Results differ from $strLenCP when multi-byte characters exist

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❓ FAQ

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