https://grafana.com/docs/grafana/latest/variables/variable-types/global-variables/

$__range

Currently only supported for Prometheus and Loki data sources. This variable represents the range for the current dashboard. It is calculated by to - from. It has a millisecond and a second representation called $__range_ms and $__range_s.

$__interval

You can use the $__interval variable as a parameter to group by time (for InfluxDB, MySQL, Postgres, MSSQL), Date histogram interval (for Elasticsearch), or as a summarize function parameter (for Graphite).

Grafana automatically calculates an interval that can be used to group by time in queries. When there are more data points than can be shown on a graph then queries can be made more efficient by grouping by a larger interval. It is more efficient to group by 1 day than by 10s when looking at 3 months of data and the graph will look the same and the query will be faster. The $__interval is calculated using the time range and the width of the graph (the number of pixels).

Approximate Calculation: (to - from) / resolution

For example, when the time range is 1 hour and the graph is full screen, then the interval might be calculated to 2m - points are grouped in 2 minute intervals. If the time range is 6 months and the graph is full screen, then the interval might be 1d (1 day) - points are grouped by day.

In the InfluxDB data source, the legacy variable $interval is the same variable. $__interval should be used instead.

The InfluxDB and Elasticsearch data sources have Group by time interval fields that are used to hard code the interval or to set the minimum limit for the $__interval variable (by using the > syntax -> >10m).