How to Use $count in Aggregation in MongoDB

In MongoDB aggregation, use the $count stage to count the number of documents passing through the pipeline and return the count as a field. It outputs a single document with the count value under the specified field name.

Syntax

The $count stage in an aggregation pipeline counts the number of documents and outputs a document with a single field containing that count.

Syntax:

{ $count: "fieldName" }

Here, fieldName is the name of the field in the output document that will hold the count number.

json

{ $count: "total" }

Example

This example counts how many documents are in the orders collection using an aggregation pipeline with $count.

mongodb

db.orders.aggregate([
  { $count: "orderCount" }
])

Output

[ { "orderCount" : 5 } ]

Common Pitfalls

Common mistakes when using $count include:

• Using $count without specifying a field name, which causes an error.
• Placing $count before filtering stages, which counts all documents instead of filtered ones.
• Expecting $count to return multiple documents; it always returns a single document with the count.

mongodb

/* Wrong: missing field name */
db.orders.aggregate([
  { $count: "" }
])

/* Correct: specify field name */
db.orders.aggregate([
  { $count: "totalOrders" }
])

Quick Reference

Stage	Description	Example
$count	Counts documents and outputs a single document with the count	{ $count: "countField" }
Usage	Place after filtering stages to count filtered documents	db.collection.aggregate([ { $match: {...} }, { $count: "count" } ])
Output	Single document with count field	{ "countField": 10 }

Key Takeaways

Use $count with a field name to get the number of documents in aggregation.

$count outputs a single document with the count under the specified field.

Place $count after filtering stages to count only matching documents.

Always specify a field name inside $count to avoid errors.

$count does not return multiple documents, only one with the total count.