$avg (Average Accumulator)

Last updated: 2026-10-09

$avg (Average Accumulator)

$avg is an accumulator computing the average of a field across a group in $group, or the average of array elements in $project. It ignores null and missing values, averaging only numbers. Common in reporting; the averaged expression may be arbitrarily complex.

📝 Syntax

{ $group: { _id: <key>, avg: { $avg: "$field" } } }  // or average an array in $project

Returns：Numeric average (null and missing values ignored)

▶ Example

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

// $avg: average price per item
db.sales.aggregate([
  { $group: { _id: '$item', avg: { $avg: '$price' } } }
])

Output：

{ "_id": "Pen", "avg": 45.5 }
{ "_id": "Book", "avg": 12.0 }

📌 Notes

• Null, missing, and non-numeric values are ignored
• In $project it averages array elements
• Combine with $sum and $count for weighted averages

🏷️ Related Tags

💡 Related Cases

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❓ FAQ

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