# Mass add

`mass-add` · version 1.0.0 · Units & sizes · free, no key needed

Add mixed-unit canonical integer mass amounts and express the sum in a target unit as a reduced rational.

**Use when you need to: mass add · add mass quantities · sum masses in mixed units.**

## Supported

- mass add
- add mass quantities
- sum masses in mixed units

## Not supported

- mass multiply
- average mass
- density from mass and volume
- troy or stone units
- floating-point mass

## Behavior

- terms is an array of 1 to 32 objects, each with a canonical integer string value and a mass unit.
- Units are exactly ng, ug, mg, g, kg, t, oz, and lb; t is the metric tonne, lb is avoirdupois, oz is 1/16 lb.
- Each term is converted to nanograms as BigInt and the nanogram amounts are summed.
- The sum is converted to the to unit as the reduced rational sum_ng / to_ng.
- Negative term values are allowed; -0 canonicalizes to 0.
- Unknown fields, unknown units, empty or overlong terms arrays, and reduced results over 1000 digits are rejected.

## Input

- `terms` (array of object, required): min items 1; max items 32
- `to` (one of "ng", "ug", "mg", "g", "kg", "t", "oz", "lb", required)

## Output

- `numerator` (string, required): pattern `^(0|-?[1-9][0-9]*)$`
- `denominator` (string, required): pattern `^[1-9][0-9]*$`

## Limits

- max digits: 1000
- max terms: 32

## Example

Request input:

```json
{
  "terms": [
    {
      "value": "1",
      "unit": "kg"
    },
    {
      "value": "500",
      "unit": "g"
    }
  ],
  "to": "g"
}
```

Response:

```json
{
  "result": {
    "numerator": "1500",
    "denominator": "1"
  }
}
```

## How to call it

### MCP

Connect `https://computefirst.net/mcp` ([setup](/docs#connect)), then call `execute` with:

```json
{
  "id": "mass-add",
  "version": "1.0.0",
  "input": {
    "terms": [
      {
        "value": "1",
        "unit": "kg"
      },
      {
        "value": "500",
        "unit": "g"
      }
    ],
    "to": "g"
  }
}
```

### HTTP (no key)

```sh
curl -X POST https://computefirst.net/v1/tools/mass-add/versions/1.0.0/execute \
  -H "Content-Type: application/json" \
  -d '{"terms":[{"value":"1","unit":"kg"},{"value":"500","unit":"g"}],"to":"g"}'
```

The machine-readable contract is at [/v1/tools/mass-add/versions/1.0.0](/v1/tools/mass-add/versions/1.0.0).

### CLI

```sh
node cli.mjs run mass-add 1.0.0 --input input.json --base-url https://computefirst.net
```

Get the client at [/clients/cli/](/clients/cli/).

## Related tools

- [Byte size add](/tools/bytesize-add): Add non-negative integer byte-size terms in mixed IEC units and return the exact sum in a chosen unit.
- [Length add](/tools/length-add): Add integer length terms in mixed units and return the exact sum in a chosen unit.
- [Mass compare](/tools/mass-compare): Compare two mixed-unit canonical integer mass amounts using exact nanogram BigInt values.
- [Mass convert](/tools/mass-convert): Convert a canonical integer mass amount between exact nanogram-based units as a reduced rational.
- [Byte size convert](/tools/bytesize-convert): Convert a non-negative integer byte size between IEC binary units as an exact rational.
- [Length convert](/tools/length-convert): Convert a canonical integer length between exact SI and international customary units.