MongoDB $multiply Operator

What Is $multiply?

$multiply is a MongoDB aggregation expression that multiplies two or more numbers and returns the product. Classic use: price × quantity for a line total.

$multiply = * for aggregation

{ $multiply: [ <a>, <b>, ... ] }

$multiply([10, 2]) → 20
$multiply([5, 10]) → 50
$multiply([2, 3, 4]) → 24

null / missing arg → null
needs 2+ numbers in the array

How to Use $multiply in MongoDB

Pass an array of numeric expressions—fields, literals, or nested math.

1. Choose an expression stage ($addFields or $project).
2. List at least two numbers in { $multiply: [ ... ] }.
3. Use field paths like "$price" and "$quantity" for line totals.
4. Add more factors when needed (tax rate, discount factor, dimensions).

Pseudocode

for each document:
    values = evaluate each expression in the array
    if any value is null or missing:
        result = null
    else:
        result = 1
        for each n in values:
            result = result * n
    emit { ..., field: result }

Cheat Sheet

Keep these facts handy while writing products.

Live Preview

Watch $multiply compute a product. Pick a pair (or trio), then run.

$multiply([10, 2])
10 × 2 = 20

{ total: 20 }

Worked Walkthrough — Line Total

An item costs 10 and the customer buys 2. The line total is price × quantity.

Document:
  { item: "abc", price: 10, quantity: 2 }

total = price × quantity
      = 10 × 2
      = 20

$multiply(["$price", "$quantity"]) → 20

Check: two units at $10 each = $20 ✓

MongoDB Examples

We use a small sales collection. First we compute line totals, then multiply three factors. Run these in mongosh after inserting the sample data once.

db.sales.insertMany([
  { _id: 1, item: "abc", price: 10, quantity: 2, date: ISODate("2014-03-01T08:00:00Z") },
  { _id: 2, item: "jkl", price: 20, quantity: 1, date: ISODate("2014-03-01T09:00:00Z") },
  { _id: 3, item: "xyz", price: 5, quantity: 10, date: ISODate("2014-03-15T09:00:00Z") }
]);

db.sales.find()

[
  { _id: 1, item: "abc", price: 10, quantity: 2, date: ISODate("2014-03-01T08:00:00Z") },
  { _id: 2, item: "jkl", price: 20, quantity: 1, date: ISODate("2014-03-01T09:00:00Z") },
  { _id: 3, item: "xyz", price: 5, quantity: 10, date: ISODate("2014-03-15T09:00:00Z") }
]

db.sales.aggregate([
  {
    $project: {
      item: 1,
      date: 1,
      total: { $multiply: ["$price", "$quantity"] }
    }
  }
]);

{ "_id": 1, "item": "abc", "date": ISODate("2014-03-01T08:00:00Z"), "total": 20 }
{ "_id": 2, "item": "jkl", "date": ISODate("2014-03-01T09:00:00Z"), "total": 20 }
{ "_id": 3, "item": "xyz", "date": ISODate("2014-03-15T09:00:00Z"), "total": 50 }

db.sales.aggregate([
  {
    $addFields: {
      withTax: {
        $multiply: ["$price", "$quantity", 1.1]
      }
    }
  }
]);

{
  _id: 1,
  item: "abc",
  price: 10,
  quantity: 2,
  date: ISODate("2014-03-01T08:00:00Z"),
  withTax: 22
}
{
  _id: 2,
  item: "jkl",
  price: 20,
  quantity: 1,
  date: ISODate("2014-03-01T09:00:00Z"),
  withTax: 22
}
{
  _id: 3,
  item: "xyz",
  price: 5,
  quantity: 10,
  date: ISODate("2014-03-15T09:00:00Z"),
  withTax: 55
}

Performance Notes

Multiplication is cheap compared with large sorts or joins. Filter early with $match when only some documents need a product. From MongoDB 6.1, putting constants before field references can help the optimizer.

Key Takeaways