Key:diameter

Use diameter=* for the nominal diameter of a feature.

Pipelines and hydrants

In mechanical technology (which applies to pipes, hydrants) nominal diameter is related but not equal to the inner diameter of the feature.
The European and international designation is DN (diamètre nominal/nominal diameter/Durchmesser nach Norm), in which sizes are measured in millimetres, see ISO 6708.

Trees

This key can be applied to other features, where it makes sense to use a nominal diameter.
For trees, it can be intended as the diameter of the tree trunk measured at 1.30 meters from the ground. This is conventionally done in arboricolture where it is called diameter at breast height (DBH).

turning circle / turning loop

For highway=turning_circle use diameter=*;

For highway=turning_loop, use diameter=* for whole loop and inner_diameter=* for the inner part.

The diameter of these features is typically measured in meters. Because of the situation described in #Unit of measure, these units should be made explicit, e.g. 10 m or 10.5 m.

Unit of measure

Unless otherwise specified, the unit of measure is mm.
This is a consequence of the original use of diameter=* for pipelines.

Using mm as the default is an exception from the standard OSM conventions for default units, which assume metre for units of length – see width=*, height=*, length=* and other similar keys. Even closely related keys such as diameter_crown=* use metre as the default unit. Because of this unfortunate inconsistency, it can avoid misunderstandings to explicitly specify the unit in the value, such as diameter=150 mm.

Inner and outer diameter

For features that have inner and outer diameters, and for which a nominal diameter is not defined, it would be appropriate to use:
inner_diameter=*
outer_diameter=*

Use in connection with

• man_made=pipeline
• emergency=fire_hydrant
• natural=tree

Also used as subkey

• fire_hydrant:diameter=*