Invoke convertToCapped and convert an existing collection to capped in MongoDB

To convert an existing collection to capped in MongoDB, use the convertToCapped command with runCommand(). A capped collection has a fixed size and automatically removes old documents when the size limit is reached.

Syntax

db.runCommand({
    convertToCapped: "collectionName",
    size: sizeInBytes
});

Sample Data

Let us create a regular collection with some documents ?

db.demo260.insertMany([
    {"Name": "Chris"},
    {"Name": "Bob"},
    {"Name": "David"}
]);

{
    "acknowledged": true,
    "insertedIds": [
        ObjectId("5e47b1181627c0c63e7dba9b"),
        ObjectId("5e47b11c1627c0c63e7dba9c"),
        ObjectId("5e47b11f1627c0c63e7dba9d")
    ]
}

Display Documents Before Conversion

db.demo260.find();

{ "_id": ObjectId("5e47b1181627c0c63e7dba9b"), "Name": "Chris" }
{ "_id": ObjectId("5e47b11c1627c0c63e7dba9c"), "Name": "Bob" }
{ "_id": ObjectId("5e47b11f1627c0c63e7dba9d"), "Name": "David" }

Convert to Capped Collection

Now convert the existing collection to capped with a size of 8345 bytes ?

db.runCommand({
    convertToCapped: "demo260",
    size: 8345
});

{ "ok": 1 }

Verify Conversion

Check the collection stats to confirm it is now capped ?

db.demo260.stats();

The key fields in the output show the capped status ?

{
    "ns": "test.demo260",
    "size": 112,
    "count": 3,
    "avgObjSize": 37,
    "storageSize": 16384,
    "capped": true,
    "max": -1,
    "maxSize": 8448,
    "ok": 1
}

Key Points

• capped: true confirms the collection is now capped
• maxSize shows the maximum storage size in bytes
• max: -1 indicates no document count limit (only size limit)
• The operation preserves all existing data during conversion

Conclusion

Use db.runCommand({convertToCapped: "collectionName", size: bytes}) to convert any existing collection to capped. The converted collection will maintain FIFO behavior and automatically remove old documents when size limits are exceeded.