How to Check Collection Size in MongoDB Quickly

To check the size of a collection in MongoDB, use the db.collection.countDocuments() method to get the number of documents. For storage size details, use db.collection.stats() which returns the collection size in bytes and other useful info.

Syntax

The main commands to check collection size are:

• db.collection.countDocuments(): Returns the number of documents in the collection.
• db.collection.stats(): Returns detailed statistics including size (data size in bytes) and storageSize (allocated storage size).

mongodb

db.collection.countDocuments()
db.collection.stats()

Example

This example shows how to get the document count and the storage size of a collection named users.

mongodb

use myDatabase

// Get number of documents in 'users' collection
const count = db.users.countDocuments()
print('Document count:', count)

// Get collection stats including size
const stats = db.users.stats()
print('Collection size in bytes:', stats.size)
print('Storage size in bytes:', stats.storageSize)

Output

Document count: 1500
Collection size in bytes: 245760
Storage size in bytes: 262144

Common Pitfalls

Common mistakes when checking collection size include:

• Using count() which is deprecated and may give inaccurate results with filters.
• Confusing size (actual data size) with storageSize (allocated space including padding).
• Not switching to the correct database before running commands.

mongodb

/* Wrong: deprecated count() method */
db.users.count()

/* Right: use countDocuments() for accurate count */
db.users.countDocuments()

Quick Reference

Command	Description
db.collection.countDocuments()	Returns the number of documents in the collection.
db.collection.stats().size	Returns the total size of data in bytes.
db.collection.stats().storageSize	Returns the allocated storage size in bytes.
use databaseName	Switch to the database containing the collection.

Key Takeaways

Use db.collection.countDocuments() to get the exact number of documents.

Use db.collection.stats() to get detailed size information including storage size.

Avoid using deprecated db.collection.count() for counting documents.

Remember to switch to the correct database before running commands.

Understand the difference between data size and storage size in stats.