STLength (geometry Data Type)
Applies to: 
SQL Server
Azure SQL Database
Azure SQL Managed Instance
Returns the total length of the elements in a geometry instance.
Syntax
.STLength ( )
Return Types
SQL Server return type: float
CLR return type: SqlDouble
Remarks
If a geometry instance is closed, its length is calculated as the total length around the instance; the length of any polygon is its perimeter and the length of a point is 0. The length of any geometrycollection type is the sum of the lengths of its contained geometry instances.
STLength() works on both valid and invalid LineStrings. Typically a LineString is invalid due to overlapping segments, which may be caused by anomalies such as inaccurate GPS traces. STLength() does not remove overlapping or invalid segments. It includes overlapping and invalid segments in the length value that it returns. The MakeValid() method can remove overlapping segments from a LineString.
Examples
The following example creates a LineString instance and uses STLength() to find the length of the instance.
DECLARE @g geometry;
SET @g = geometry::STGeomFromText('LINESTRING(0 0, 2 2, 1 0)', 0);
SELECT @g.STLength();