Nominatim vs Postcodes.io

Nominatim provides worldwide forward and reverse geocoding built entirely on OpenStreetMap data, needing no API key — only a descriptive User-Agent or Referer header identifying the calling application. The public instance at nominatim.openstreetmap.org is a free courtesy service, not a commercial product: its usage policy caps requests at 1 per second, forbids autocomplete-style querying and bulk/parallel scripts, and requires caching results rather than re-fetching them. Heavier users are expected to self-host Nominatim against their own OSM extract instead of relying on the shared instance.

Postcodes.io is a free, open-source (MIT-licensed) UK postcode lookup and geocoding API: it validates postcodes, reverse-geocodes coordinates to the nearest postcode, and returns administrative geography — ward, constituency, parish, LSOA/MSOA, and more — sourced from the ONS Postcode Directory. It's built and maintained by Ideal Postcodes as a free public service, with a paid Ideal Postcodes product available for teams needing rooftop-level precision or fresher data.