MongoDB $round Operator

Example 1 — Sample Data

Imagine three products with decimal prices. We save them in a collection named products.

db.products.insertMany([
  { _id: 1, name: "Mug", price: 12.346 },
  { _id: 2, name: "Pen", price: 2.5 },
  { _id: 3, name: "Book", price: 19.99 }
]);

How the Data Is Stored in MongoDB

After the insert, MongoDB keeps three documents in products. Run find() to see them:

db.products.find()

[
  { _id: 1, name: "Mug", price: 12.346 },
  { _id: 2, name: "Pen", price: 2.5 },
  { _id: 3, name: "Book", price: 19.99 }
]

Read those documents like this:

Mug   12.346  → round @2 → 12.35
Pen    2.5    → round @0 → 2     (half-to-even)
Book  19.99   → round @0 → 20

How It Works

1. Insert once. insertMany adds three product documents.
2. Prices are numbers. Not strings—$round needs a numeric type.
3. find() shows the same shape. What you inserted is what you get back—ready for $round next.

Example 2 — Round to 2 Decimals

Goal: add price_2dp rounded to cents (two decimal places).

db.products.aggregate([
  {
    $addFields: {
      price_2dp: { $round: [ "$price", 2 ] }
    }
  }
]);

[
  { _id: 1, name: "Mug", price: 12.346, price_2dp: 12.35 },
  { _id: 2, name: "Pen", price: 2.5, price_2dp: 2.5 },
  { _id: 3, name: "Book", price: 19.99, price_2dp: 19.99 }
]

How It Works

1. Read the stored price. "$price" is the number for each product.
2. Place = 2. Look two digits after the decimal, then round.
3. Mug. 12.346 → third decimal is 6 (≥ 5 and not a pure half) → 12.35.
4. Already fine. Pen and Book already fit in two decimals, so they stay the same.

Example 3 — Round to Integer

Same products, but round to a whole number (place: 0). Watch Pen: 2.5 becomes 2 because of half-to-even.

db.products.aggregate([
  {
    $addFields: {
      price_int: { $round: [ "$price", 0 ] }
    }
  }
]);

[
  { _id: 1, name: "Mug", price: 12.346, price_int: 12 },
  { _id: 2, name: "Pen", price: 2.5, price_int: 2 },
  { _id: 3, name: "Book", price: 19.99, price_int: 20 }
]

How It Works

1. Same stored documents. Still using the three products from Example 1.
2. Place = 0. Round using the first digit after the decimal.
3. Normal cases. Mug 12.346 → 12; Book 19.99 → 20.
4. Half-to-even. Pen 2.5 is exactly halfway; nearest even integer is 2, not 3.