$binarySize (Binary Size)

Last updated: 2026-10-09

$binarySize (Binary Size)

$binarySize returns the byte size of binary data (BinData); given a string it returns the UTF-8 byte length. It is used for attachment size monitoring, upload limits, or storage accounting. Inputs that are neither binary nor string throw. For strings the result matches $strLenBytes. An empty binary returns 0.

📝 Syntax

{ $binarySize: <expression> }

⚙️ Parameters

Returns：The size in bytes as an integer.

▶ Example

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

// $binarySize: attachment sizes
 db.files.aggregate([
  { $project: { size: { $binarySize: '$data' } } }
])

Output：

{ "_id": 1, "size": 4096 }
{ "_id": 2, "size": 0 }

📌 Notes

• BinData yields bytes; strings yield UTF-8 bytes
• Other types (numbers, documents) throw
• An empty binary returns 0

🏷️ Related Tags

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❓ FAQ

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