Find sum of fields inside array in MongoDB?

To find sum of fields inside array in MongoDB, use the $sum operator with the aggregation pipeline. The $sum operator can directly reference array fields using dot notation to calculate the total of numeric values.

Syntax

db.collection.aggregate([
    {
        $project: {
            "fieldName": 1,
            "totalSum": {
                $sum: "$arrayName.fieldToSum"
            }
        }
    }
]);

Sample Data

Let us create a collection with documents ?

db.demo96.insertOne({
    "Name": "Chris",
    "Details": [
        { "Marks": 67 },
        { "Marks": 33 },
        { "Marks": 50 }
    ]
});

{
    "acknowledged": true,
    "insertedId": ObjectId("5e2d6aadb8903cdd865577ad")
}

Display Documents

db.demo96.find();

{
    "_id": ObjectId("5e2d6aadb8903cdd865577ad"),
    "Name": "Chris",
    "Details": [
        { "Marks": 67 },
        { "Marks": 33 },
        { "Marks": 50 }
    ]
}

Example: Calculate Total Marks

Following is the query to find sum of fields inside array ?

db.demo96.aggregate([
    {
        $project: {
            "Name": 1,
            "TotalMarks": {
                $sum: "$Details.Marks"
            }
        }
    }
]);

{
    "_id": ObjectId("5e2d6aadb8903cdd865577ad"),
    "Name": "Chris",
    "TotalMarks": 150
}

How It Works

The $sum operator with "$Details.Marks" automatically iterates through the Details array and sums all Marks values (67 + 33 + 50 = 150).

Conclusion

Use $sum with dot notation in the aggregation pipeline to calculate totals of numeric fields within arrays. The operator automatically processes all array elements and returns the cumulative sum.