ST_Perimeter

What is ST_Perimeter?

ST_Perimeter returns the 2D perimeter — the boundary length — of a polygonal geometry. For POLYGON it is the length of the outer ring plus the lengths of any holes; for MULTIPOLYGON it is the sum across all parts.

1ST_Perimeter(geometry g1) → float
2ST_Perimeter(geography geog, boolean use_spheroid = true) → float

For non-polygonal input (points, lines, collections without polygons), the function returns 0. For geography, the result is in metres on the WGS84 spheroid by default.

When would you use ST_Perimeter?

Use ST_Perimeter to compute the boundary length of an area feature: fencing requirements around a property, shoreline length around an island, or sewer-line requirements along a subdivision boundary. It is also the denominator in compactness metrics such as (4π·area)/perimeter², useful for comparing polygon regularity.

1-- Perimeter in metres of each country boundary on lat/lon data
2SELECT iso_code, ST_Perimeter(geom::geography) AS metres
3FROM countries;

FAQs

What units does ST_Perimeter return?

For geometry, the units of the SRID — degrees for EPSG:4326, metres for projected CRSs. For geography, always metres on the WGS84 spheroid. Cast lat/lon geometry to geography when you need real-world metres.

Does it include holes?

Yes. A polygon with interior rings contributes its outer ring length plus all hole lengths. This matches OGC semantics: if you fenced the polygon, you would fence the inner boundaries too.

What does it return for non-polygonal input?

Zero. For linestring length use ST_Length; for collections extract polygons first with ST_CollectionExtract(geom, 3).

Is there a 3D version?

Yes — ST_3DPerimeter uses Z coordinates to return the slope perimeter, useful for polygons draped over terrain.