How to Use $mul Operator in MongoDB for Multiplying Field Values

In MongoDB, the $mul operator multiplies the value of a field by a specified number during an update. You use it inside an updateOne or updateMany command with the $mul key and the multiplication factor as the value.

Syntax

The $mul operator is used in an update statement to multiply the value of a field by a given number. It takes an object where the key is the field name and the value is the multiplier.

• Field: The name of the field to multiply.
• Multiplier: The number to multiply the field's current value by.

json

{
  $mul: { <field>: <number> }
}

Example

This example shows how to multiply the price field by 1.1 (increasing it by 10%) for a product with _id 1.

javascript

db.products.updateOne(
  { _id: 1 },
  { $mul: { price: 1.1 } }
);

Output

{ "acknowledged" : true, "matchedCount" : 1, "modifiedCount" : 1 }

Common Pitfalls

Common mistakes when using $mul include:

• Trying to multiply a field that does not exist or is not a number, which will cause no change or an error.
• Using $mul outside of an update operation.
• Confusing $mul with arithmetic operators in queries (it only works in updates).

javascript

/* Wrong: Using $mul in a find query (invalid) */
db.products.find({ $mul: { price: 2 } });

/* Right: Use $mul inside update */
db.products.updateOne({ _id: 1 }, { $mul: { price: 2 } });

Quick Reference

Operator	Purpose	Usage Example
$mul	Multiply a field by a number	{ $mul: { quantity: 3 } }
$inc	Increment a field by a number	{ $inc: { quantity: 3 } }
$set	Set a field to a value	{ $set: { quantity: 10 } }

Key Takeaways

Use $mul inside update commands to multiply a field's value by a number.

The field must exist and be numeric for $mul to work correctly.

$mul cannot be used in query filters, only in updates.

Always check the update result to confirm the operation succeeded.