# Send a field the device type does not define (https://logable.app/docs/integrate/custom-data-points)

Add your own keys to a reading. They are stored and shown, but they never drive an alert or a forecast.



A device type declares a fixed set of fields. A custom key is any other field you add to the same `data` object.

## When to use a custom key [#when-to-use-a-custom-key]

Use a custom key when you have a value the device type does not declare. An example is a vibration reading on a device type with no vibration field. Logable stores the value and can display it. It cannot use the value for anything else.

## Rules [#rules]

A custom key must start with a letter. After that it may hold letters, digits or underscores, up to 64 characters in total.

| Limit | Value | Description |
| --- | --- | --- |
| custom keys | 20 | Maximum number of custom (unregistered) keys in one payload. |
| key length | 64 | Maximum characters in a custom key. |
| string value length | 256 | Maximum characters in a custom key's string value. |

## What happens to the value [#what-happens-to-the-value]

A custom value appears on the device's **Metrics** tab and can be plotted on a chart. It cannot be used inside a rule condition. It is never read by a forecast.

## What is rejected [#what-is-rejected]

A key that breaks the naming rule, a value of the wrong type, or a string over the length limit is rejected. So is a payload with more than 20 custom keys, with the message `Too many custom data keys ({n}); max is 20`. Every one of these failures returns HTTP 400 with `code: "VALIDATION_ERROR"`.

## Example [#example]

```json title="Custom data point"
{
  "data": {
    "weight": 18750.5,
    "status": "filling",
    "vibration_rms": 0.42
  }
}
```

`weight` and `status` are fields the device type declares. `vibration_rms` is the custom key.

## Next [#next]

* [Send readings over HTTP](/docs/integrate/rest-api)
* [Limits](/docs/reference/limits)
* [Device types](/docs/reference/device-types)