# Performance Metrics | Sentry for React Native

Sentry's SDKs support sending performance metrics data to Sentry. These are numeric values attached to transactions that are aggregated and displayed in Sentry.

The React Native SDK automatically collects the following performance metrics:

* [Cold and warm app start time](https://docs.sentry.io/platforms/react-native/tracing/instrumentation/automatic-instrumentation.md#app-start-instrumentation)
* [Slow and frozen frame rendering](https://docs.sentry.io/platforms/react-native/tracing/instrumentation/automatic-instrumentation.md#slow-and-frozen-frames)

## [Custom Measurements](https://docs.sentry.io/.md#custom-measurements)

In addition to automatic performance metrics, the SDK supports setting [custom performance measurements](https://docs.sentry.io/product/insights/overview/metrics.md#custom-performance-measurements) on transactions. This allows you to define measurements that are important to your application and send them to Sentry.

To set a performance measurement, you need to supply the following:

* name (`string`)
* value (any numeric type - `float`, `integer`, etc.)
* unit (`string`, defaults to the string `none` if omitted)

Sentry supports adding arbitrary custom units, but we recommend using one of the [supported units listed below](https://docs.sentry.io/.md#supported-measurement-units).

```javascript
// Record amount of memory used
Sentry.setMeasurement("memoryUsed", 123, "byte");

// Record time when Footer component renders on page
Sentry.setMeasurement("ui.footerComponent.render", 1.3, "second");

// Record amount of times localStorage was read
Sentry.setMeasurement("localStorageRead", 4);
```

Currently, unit conversion is only supported once the data has already been stored. This means that, for example, `('myMeasurement', 60, 'second')` and `('myMeasurement', 3, 'minute')` would not be aggregated, but rather stored as two separate measurements. To avoid this, make sure to use a consistent unit when recording a custom measurement.

## [Supported Measurement Units](https://docs.sentry.io/.md#supported-measurement-units)

Units augment measurement values by giving meaning to what otherwise might be abstract numbers. Adding units also allows Sentry to offer controls - unit conversions, filters, and so on - based on those units. For values that are unitless, you can supply an empty string or `none`.

### [Duration Units](https://docs.sentry.io/.md#duration-units)

* `nanosecond`
* `microsecond`
* `millisecond`
* `second`
* `minute`
* `hour`
* `day`
* `week`

### [Information Units](https://docs.sentry.io/.md#information-units)

* `bit`
* `byte`
* `kilobyte`
* `kibibyte`
* `megabyte`
* `mebibyte`
* `gigabyte`
* `gibibyte`
* `terabyte`
* `tebibyte`
* `petabyte`
* `pebibyte`
* `exabyte`
* `exbibyte`

### [Fraction Units](https://docs.sentry.io/.md#fraction-units)

* `ratio`
* `percent`

If you want to explore further, you can find details about supported units in our [event ingestion documentation](https://getsentry.github.io/relay/relay_metrics/enum.MetricUnit.html).
