MongoDB $multiply Operator

What You’ll Learn

The $multiply operator multiplies numbers together in MongoDB aggregation pipelines. It is a core arithmetic operator for line totals, tax amounts, scaled metrics, and any product-based calculation.

Definition and Usage

In MongoDB’s aggregation framework, the $multiply operator returns the product of numeric expressions. For example, { $multiply: [ "$price", "$quantity" ] } computes a line total, and { $multiply: [ 10, 3 ] } returns 30.

Like $add and $divide, $multiply always takes an array of at least two operands. You can multiply field values, literals, or nested expressions in a single call.

📝 Syntax

The $multiply operator takes an array of two or more expressions:

{ $multiply: [ <expression1>, <expression2>, ... ] }

Syntax Rules

• $multiply — returns the product of all expressions in the array.
• Array required — always wrap operands in square brackets, even for two values.
• Operands can be field paths ("$price"), literals (1.08), or nested expressions.
• Use inside stages like $project, $addFields, or $set.
• If any operand is null, the result is null.
• Multiplying by 0 returns 0; multiplying by 1 leaves the other value unchanged.

Examples Gallery

Compute line totals, apply tax rates, multiply three dimensions for volume, and convert ratios to percentages.

[
  { "_id": ObjectId("609c26812e9274a86871bc6a"), "product": "Notebook", "price": 5,  "quantity": 3 },
  { "_id": ObjectId("609c26812e9274a86871bc6b"), "product": "Pen",      "price": 2,  "quantity": 10 },
  { "_id": ObjectId("609c26812e9274a86871bc6c"), "product": "Desk",     "price": 250, "quantity": 1 }
]

db.lineItems.aggregate([
  {
    $project: {
      product: 1,
      price: 1,
      quantity: 1,
      lineTotal: {
        $multiply: [ "$price", "$quantity" ]
      }
    }
  }
])

{ "product": "Notebook", "lineTotal": 15 }
{ "product": "Pen",      "lineTotal": 20 }
{ "product": "Desk",     "lineTotal": 250 }

db.orders.aggregate([
  {
    $project: {
      orderId: 1,
      subtotal: 1,
      taxAmount: {
        $multiply: [ "$subtotal", 0.08 ]
      },
      grandTotal: {
        $add: [
          "$subtotal",
          { $multiply: [ "$subtotal", 0.08 ] }
        ]
      }
    }
  }
])

db.packages.aggregate([
  {
    $project: {
      sku: 1,
      volume: {
        $multiply: [
          "$length",
          "$width",
          "$height"
        ]
      }
    }
  }
])

// length: 10, width: 5, height: 2  → volume: 100
// length: 3,  width: 4, height: 6  → volume: 72

db.projects.aggregate([
  {
    $project: {
      name: 1,
      completionPct: {
        $multiply: [
          {
            $divide: [ "$completed", "$total" ]
          },
          100
        ]
      }
    }
  }
])

db.lineItems.aggregate([
  {
    $project: {
      product: 1,
      finalTotal: {
        $multiply: [
          "$price",
          "$quantity",
          "$discountFactor"
        ]
      }
    }
  }
])

// price: 100, quantity: 2, discountFactor: 0.9
// finalTotal: 180 (10% off)

🚀 Use Cases

• Line totals — compute price × quantity for invoices and carts.
• Tax and fees — multiply subtotals by rate factors (0.08, 1.05, etc.).
• Scaling values — convert units, currencies, or normalize scores by a factor.
• Percentage display — multiply ratios from $divide by 100.

💡 Best Practices