MongoDB $floor Operator

What Is $floor?

$floor is a MongoDB aggregation expression that rounds a number down to the nearest integer—toward negative infinity. Positive decimals drop the fraction; negative decimals move further left on the number line.

$floor = Math.floor for aggregation

{ $floor: <expression> }

 23.8 → 23
 23.1 → 23
 -3.2 → -4   // toward −∞
 10   → 10

null / missing → null
NaN → NaN

How to Use $floor in MongoDB

Pass a number (or an expression that produces one) and emit the floored integer.

1. Choose an expression stage ($addFields or $project).
2. Pass a field, literal, or nested math expression to $floor.
3. Remember negatives go toward −∞ (-3.2 → -4).
4. Use $ceil, $trunc, or $round when you need a different rounding rule.

Pseudocode

for each document:
    x = evaluate(expression)
    if x is null or missing:
        result = null
    else if x is NaN:
        result = NaN
    else:
        result = greatest integer ≤ x
    emit { ..., field: result }

Cheat Sheet

Keep these facts handy while rounding down in pipelines.

Live Preview

Watch $floor round values down. Try a negative number to see −∞ behavior.

x = 23.8
$floor(x) → 23

{ floored: 23 }

Worked Walkthrough — Whole Dollars

A notebook costs 23.8. We want the whole-dollar amount by rounding down.

Document:
  { item: "Notebook", price: 23.8 }

Number line (positive):
  ... 22  23  23.8  24 ...
           ^
         floor

$floor(23.8) → 23 ✓

Negative check:
  $floor(-3.2) → -4  (not -3)

MongoDB Examples

We use one product with a price and a negative adjustment. First floor the price, then floor the negative value. Run these in mongosh after inserting the sample data once.

db.products.insertOne({
  _id: 1,
  item: "Notebook",
  price: 23.8,
  adjustment: -3.2
});

db.products.find()

[
  {
    _id: 1,
    item: "Notebook",
    price: 23.8,
    adjustment: -3.2
  }
]

price 23.8
  floor → 23   (drop the .8)

adjustment -3.2
  floor → -4   (toward −∞, not -3)

db.products.aggregate([
  {
    $addFields: {
      wholeDollars: {
        $floor: "$price"
      }
    }
  }
]);

{
  _id: 1,
  item: "Notebook",
  price: 23.8,
  adjustment: -3.2,
  wholeDollars: 23
}

db.products.aggregate([
  {
    $addFields: {
      flooredAdj: {
        $floor: "$adjustment"
      }
    }
  }
]);

{
  _id: 1,
  item: "Notebook",
  price: 23.8,
  adjustment: -3.2,
  flooredAdj: -4
}

Performance Notes

Flooring is cheap compared with large sorts or joins. Filter early with $match when only some documents need rounding.

Key Takeaways