The $add operator adds numbers together in MongoDB aggregation pipelines. It is one of the most common arithmetic operators — perfect for calculating totals, combining field values, or adding milliseconds to dates.
Sum two or more values.
Pass operands in an array.
Add document fields together.
Add milliseconds to dates.
Totals, tax, shipping.
Know how null inputs behave.
In MongoDB’s aggregation framework, the $add operator sums numeric expressions. For example, { $add: [ "$price", "$tax" ] } returns the combined total of two fields. You can also add a literal number to a field, or add multiple values in one expression.
Unlike $abs or $acos, $add always takes an array of at least two expressions. Think of it as MongoDB’s pipeline version of the + operator.
The $add operator takes an array of two or more expressions:
{ $add: [ <expression1>, <expression2>, ... ] }$add — returns the sum of all expressions in the array."$price"), literals (10), or nested expressions.$project, $addFields, or $set.null, the result is null.Date and another is a number, the number is treated as milliseconds.| Question | Answer |
|---|---|
| Operator type | Aggregation expression operator (arithmetic) |
| Syntax | { $add: [ expr1, expr2, ... ] } |
| Minimum operands | 2 (array of at least two expressions) |
| Date support | Yes — number operand = milliseconds |
| Common stages | $project, $addFields, $set |
{
$add: ["$a","$b"]
}Sum of two fields
{
$add: ["$qty", 1]
}Add a constant
{
$add: ["$a","$b","$c"]
}Sum multiple fields
{
$add: [null, 5]
}Returns null
Walk through sample orders, add field values together, and compute totals in aggregation pipelines.
Start with an orders collection and sum numeric fields using $project.
Suppose you have an orders collection with price and tax fields:
[
{ "_id": ObjectId("609c26812e9274a86871bc6a"), "price": 100, "tax": 18, "shipping": 5 },
{ "_id": ObjectId("609c26812e9274a86871bc6b"), "price": 250, "tax": 45, "shipping": 10 },
{ "_id": ObjectId("609c26812e9274a86871bc6c"), "price": 75, "tax": 14, "shipping": 5 }
]Add price and tax to get a subtotal:
db.orders.aggregate([
{
$project: {
price: 1,
tax: 1,
subtotal: { $add: [ "$price", "$tax" ] }
}
}
])100 + 18 = 118250 + 45 = 29575 + 14 = 89Sum multiple fields, compute order totals, and perform date arithmetic.
Include shipping in the total by adding three fields at once:
db.orders.aggregate([
{
$project: {
price: 1,
tax: 1,
shipping: 1,
orderTotal: {
$add: [ "$price", "$tax", "$shipping" ]
}
}
}
])$add accepts any number of operands in the array. MongoDB sums them left to right and returns the total.
Combine a field with a fixed literal value using $addFields:
db.employees.aggregate([
{
$addFields: {
bonusAmount: 5000,
totalPay: { $add: [ "$salary", 5000 ] }
}
},
{
$project: {
name: 1,
salary: 1,
bonusAmount: 1,
totalPay: 1
}
}
])Literals and field references can be mixed freely inside the $add array. This is useful for flat fees, bonuses, or fixed surcharges.
When one operand is a date, the number is treated as milliseconds. One day = 86,400,000 ms:
db.shipments.aggregate([
{
$project: {
orderDate: 1,
deliveryDate: {
$add: [ "$orderDate", 86400000 ]
},
deliveryAfter3Days: {
$add: [ "$orderDate", 259200000 ]
}
}
}
])$add on dates adds milliseconds. Multiply days by 86400000 (24 × 60 × 60 × 1000) to shift dates forward in pipelines.
$add WorksThe pipeline resolves every expression in the array — fields like "$price", literals, or nested operators.
For numbers, MongoDB adds them together. For dates, the numeric operand is treated as milliseconds.
The sum is written to the field you define in $project or $addFields.
Use the result for sorting, grouping, filtering, or displaying in reports.
The $add operator is one of the most frequently used tools in MongoDB aggregation pipelines. It lets you sum field values, add constants, combine multiple numbers, and perform basic date arithmetic — all without leaving the database.
For beginners, the key idea is simple: pass an array of expressions to { $add: [...] } inside a stage like $project, and MongoDB returns the sum.
{ $add: [ a, b ] }$ifNull to replace missing values before adding$round for currency totals$subtract for discounts before or after adding{ $add: "$a", "$b" } is invalid$add as a query filter outside expressions$add with the $sum accumulator in $group$addUse these points when writing your next aggregation pipeline.
Sums two or more values.
Purpose{ $add: [a, b] }
Add 3+ fields at once.
FlexibilityNumber = milliseconds.
Datesnull in → null out.
Edge caseMove on to $allElementsTrue or review $subtract for the opposite operation.
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