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# extrapolated_delta()

> Calculate the extrapolated change from a gauge aggregate

<Icon icon="flask" /> Early access <Icon icon="tag" iconType="duotone" /> Since [1.6.0][toolkit-1.6.0]

Calculate the change in a gauge during the time period specified by the bounds
in the gauge aggregate. The bounds must be specified for the `extrapolated_delta`
function to work. You can provide them as part of the original [`gauge_agg`][gauge-agg]
call, or by using the [`with_bounds`][with-bounds] function on an existing
gauge aggregate.

## Samples

Extrapolate the change in a gauge over every 15-minute interval.

```sql theme={"dark"}
SELECT
    id,
    bucket,
    extrapolated_delta(
        with_bounds(
            summary,
            toolkit_experimental.time_bucket_range('15 min'::interval, bucket)
        )
    )
FROM (
    SELECT
        id,
        time_bucket('15 min'::interval, ts) AS bucket,
        gauge_agg(ts, val) AS summary
    FROM foo
    GROUP BY id, time_bucket('15 min'::interval, ts)
) t;
```

## Arguments

The syntax is:

```sql theme={"dark"}
extrapolated_delta(
    summary GaugeSummary
) RETURNS DOUBLE PRECISION
```

| Name      | Type           | Default | Required | Description                                              |
| --------- | -------------- | ------- | -------- | -------------------------------------------------------- |
| `summary` | `GaugeSummary` | -       | ✔        | A gauge aggregate created using [`gauge_agg`][gauge-agg] |

## Returns

| Column              | Type             | Description                                                                       |
| ------------------- | ---------------- | --------------------------------------------------------------------------------- |
| extrapolated\_delta | DOUBLE PRECISION | The extrapolated change in the gauge over the time period of the gauge aggregate. |

[gauge-agg]: /api-reference/timescaledb-toolkit/counters-and-gauges/gauge_agg/gauge_agg

[toolkit-1.6.0]: https://github.com/timescale/timescaledb-toolkit/releases/tag/1.6.0

[with-bounds]: /api-reference/timescaledb-toolkit/counters-and-gauges/gauge_agg/with_bounds